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BRITISH STANDARD
Code of practice for
Earth retaining structures
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BS 8002 : 1994
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.BS 8002 : 1994
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Committees responsible for this British Standard The preparation of t h s Brltlsh Standard was entrusted by the Techrucal Sector Board for BLuldlng and Cml Engmeenng (BI-) to Techlucal Cornm~tteeBlb26, upon whlch the followmg bodles were represented Assoc~at~on of Consulting Engmeers Assoc~atlonof Geotechrucal Speaahsts Department of the Environment (Construct~onD~rectorate) Department of Transport Federatlon of Clwl Engmeenng Contractors Federatlon of P h ~ Speclalets g Inst~tut~on of Clvd Engineers Inst~tutionof Structural Engmeers
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Tms Bntish Standard, hav~ng been prepared under the directmn of the Technical Sector Board for Bmldmng and Clvll Ensneenng (B1 ), was pubhhed under the authonty of the Standards Board and comes Into effect on 15 Apnl1994
Amendments issued since publication
Amd No
Rxt affected
Date
BSI 1994
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The followmg BSI references relate to the work on thls standard Committee reference 91526 Draft for comment 87114383 DC
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ISBN 0 580 22826 6
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Contents
Page Inslde front cover Committees responsible 5 Foreword Code of -practice Section 1 Introduction 1.1 Scope 1.2 References 1.3 Defin~t~ons 1.4 Major symbols 1.5 Selection and types of structure 1.5.1 General 9 1 5.2 Selection of type Section 2. Data for design 2.1 S ~ t eand geotechrucal data 2.1.1 General 2.1.2 Site lnvestlgations 2.1.3 Ground water 2.1.4 Flood tldes and waves 2.1 5 Chmate 2.1.6 Trees 2.2 So11properties 2.2.1 General 2.2.2 Select~onand evaluation of soil parameter values 2.2.3 Clay soils 2.2.4 Cohes~onlesssolls 2.2.5 Silts 2.2.6 Rock 2 2.7 I W 2.2.8 Wall fnct~on,base fnct~onand undramed wall adheson 2.3 Externally applied loads Section 3. Design philosophy, design method and earth pressures Desgn ph~losophy General Lun~tstate d e s w Ultimate hmlt states S e m c e a b ~ t y1m1t states Lunlt states and compat~b~hty of deformations Desgn values of parameters Applied loads Desgn sod strength Desgn earth pressures Design method Equ~l~bnnm calculat~ons Desgn s~tuations Calculations based on total and effectwe stress parameters Deslgn usmg total stress parameters
3.2.5 Design usmg effectwe stress parameters 3.2 6 Design values of wall fnct~on,base fnctlon and undra~nedwall
adhesion Des@ to structural codes Dlsturbmg forces General At-rest earth pressures Active earth pressures Surcharge loads Water pressure Resistance to movement General Passive earth resistance Weak rocks Layered solls Water pressures and seepage forces Section 4. Design of spec~ficearth retaining structures Interrelation of sectlon 3 and sectlon 4 4.1 4.1.1 General 4.1.2 Deslgn 4.2 Gravlty walls 4.2.1 General 4.2.2 Foundations 4.2.3 Mass concrete retamng walls 4.2.4 Unremforced masonry retanlng walls 4.2.5 Remforced so11 4.2.6 Gab~ons 4.2.7 Cnbwork Reinforced concrete and reinforced masonry walls on spread 4.3 foundations 4.3.1 Remforced concrete walls (other than basement walls) 4.3.2 Basement walls, excavation, support and retent~onsystems 4.3.3 Remforced and prestressed masonry retalnmg walls 4.4 Embedded walls 4.4.1 General 4.4 2 Types of wall and apphcabdity 4.4.3 Design 4.4.4 Steel sheet phng 4.4.5 T~mbersheet pdes 4.4.6 Remforced and prestressed concrete sheet plles 4.4.7 in sltu concrete plle walls 4.4.8 Diaphragm walls 4.4.9 Soldier/lung p~les 4.5 Strutted excavations and cofferdams 4.5.1 General 4.5.2 Struts, tles, waling3 and anchorages 4.5.3 Cellular cofferdams
3 2.7 33 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.4 3.4.1 3.4 2 3.4.3 3.4.4 3.4.5
Anchorages General Equllibnum Ground anchorages Tens~onp~les Deadman anchorages Waterfront structures General Concrete and remforcement Des~gn Construction Annexes A (normatwe) Graphs for K, and Kp B (~nformative)Trad~t~onal des~gnmethods for embedded walls
4.6 4.6.1 4.6 2 4.6.3 4.6.4 4.6.5 4.7 4.7.1 4.7 2 4.7.3 4.7.4
U ~ u weghts t of solls (and s~mllarmaterials) p',,, for clay sods p' for s~l~ceous sands and gravels p' for rock 5 Select~onof p ~ l eslze to suit dnvmg cond~t~ons m granular soils usmg lmpact hammers 6 Select~onof ode slze to sult d n v m ~cond~t~ons m cohes~vesods Figures Strength envelopes for a gwen pre-consohdat~on Denvat~onof N' from SPT value N Llm~tstates for earth retammg structures Pressure d~agrams Graphcal detenrunat~onof actlve earth pressure for cohes~onless solls Graph~caldetermmatlon of actwe earth pressure for coheswe soils Construct~onof earth pressure d~agramsfor earth retalrung structures m multi-layered sod Flow net determ~natlonof pore water pressure Lmear vanahon m hydraul~chead Graded filter d m n Foundat~onsof gravlty walls Bas~cforms of mass concrete walls Masonry clad mass concrete wall wlth cavlty Stepped and buttressed retamng walls in unremforced masonry Hexagonal woven mesh gab~oncage (typical) Welded mesh gabion cage (typ~cal) Examples of gab~onreta~rungwalls Sect~onand elevat~onof typ~calcnb wall Examples of timber cnbwork Examples of remforced concrete cnbwork Further examples of reinforced concrete cnbwork Basic forms of remforced concrete cantilever or stem wall 1 2 3 4
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11 13 15 15 69 70
Basic forms of re~nforcedconcrete counterfort and buttressed walls Temporary support agamst central dumpmg Temporary support by fully braced trench Long flylug shores across excavations Fully braced temporary support Concurrent upward and downward constructlon Floors cast on ground w ~ t hexcavation continuing below Open cut Remforced masonry grouted-cavlty constructlon Remforced masonry Quetta bond constructlon Reinforced masonry pocket-type constructlon Remforced hollow blockwork constructlon Post-tens~onedmasonry d~aphragmwall constructlon Types of embedded retainmg wall Actwe pressure dlagrams relatmg to mmmum strut loads rn braced earth retammg structures (Terzagh~& Peck) Illustrat~onof method of calculation of bendmg moments and frame loads by successwe stage analysls in cofferdams Typ~calsections of t~mbersheet piles Detad of dnvmg edge Honzontal sheetmg (laggmg) Vert~calsheetmg (lagg~ng) Compos~testeel sold~erpiles Vanous methods of locat~ngthe sheetlng (laggng) Cofferdam for nver crossmg Cofferdam m water Types of cellular cofferdams Types of anchorage Non-mterference of zones for anchored wall Double wall constructlon where zones mterfere Actwe pressure - Honzontal ground surface behmd wall Values of K, (honzontal component) Passwe pressure - Honzontal ground surface behind wall Values of K p (honzontal component) Actwe pressure - Sloping ground surface behmd wall Values of K, (honzontal component) (based on Kensel and Absl, 1990) Actwe pressure - Sloplug ground surface behmd wall Values of K, (horuontal component) (based on Kensel and Absl, 1990) Actwe pressure - Slopmg ground surface behmd wall Values of K, (hor~zontalcomponent) (based on Kensel and Abs~,1990) Passwe reslstance - Sloping ground surface behmd wall Values of Kp (honzontal component) Passive reslstance - Sloping ground surface behind wall Values of Kp (honzontal component) Passlve reslstance - Slopmg ground surface behmd wall Values of Kp (honzontal component) Different methods of assessing the ratlo of restonng moments to overturning moments Index List of references
54 57 57 58 58 59 60 61 62 62 62 62 63 65 66 67 72 72 76 76 77 78 79 81 85 87 88 89 94 95 96 97 98 99 LOO
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BS 8002 : 1994
.. Foreword
BS 8002 has been prepared under the d~rectlonof the Techn~calSector Board for Budding and Cwll Engmeenng T h ~ code s of practlce IS a complete revwon of the C~vdEngmeenng Code of Practice No 2, whch was ~ssuedby the Institution of Structural Engmeers m 1951 on behalf of the Clv11Engmeenng Codes of Pract~ceJomt Committee A draft of t h ~ code s of practlce, was ~ssuedin 1988 for publ~ccomment and In 1992 a new commlttee rewewed and rewsed the text
The m a n changes m the deslgn of earth retanmg structures m t h ~ code s of practlce are a) the recognltlon that effective stress analys~s1s the mam bas~sfor the assessment of earth pressures w ~ t htotal (undraned) stress analysls bemg important for some walls dunug or ~mmedlatelyfollowing construction, b) the need to take account of the effect of movement (or lack of it) upon the resultmg earth pressures on the wall The largest earth pressures wh~chact on a retanung wall occur dunng workmg cond~t~ons These earth pressures do not Increase d the wall deforms suffic~entlyto approach falure cond~t~ons T h ~ code s of practlce takes Into account that for small movements of a wall the shear strength developed m the so111s less than the mammum shear strength measured in a convent~onaltnax~altest and furthermore that when large strans occur m the soil, the shear strength may reduce to the res~dualshear strength value It has been assumed m t h ~ code s of practlce that desgn of retammg walls IS entrusted to chartered structural or chartered c ~ wel n p e e r s who have sufficient knowledge of the pnnc~plesand practlce of so11mechan~csas well as the principles and practlce for the use of the appropnate structural matenals, 1 e masonry, concrete, steel or t~mber T h ~ code s of practlce does not restnct des~gnersfrom applymg the results of research nor from takmg advantage of special sltuatlons or prevlous expenence m the des~gnof retaming structures In t h s code of practlce references have been made to non-BSI pubhcat~onsThe t~tlesof these pubhcat~onsare aven m annex C The llst of those engmeers who have partlc~patedm the preparation of the ~ m t ~ a l draft, m the spec~allyconvened panel and m the more recently formed commlttee mcludes the majonty of engmeers who have a speclal mterest m retalnmg walls The Cha~rmanthroughout the long process of draftmg, revlewmg and complete red~aftmg,has been Mr Thomas Akroyd, M Sc Tech, LL B (Hons), C Eng , a former Pres~dentof the lnst~tutionof Structural Engmeers
BSI Comm~tteeEl526 whose constltutlon IS shown in thls Bntrsh Standard, takes collectwe respons~bllltyfor rts preparation under the authonty of the Standards Board The Comm~tteewlshes to acknowledge the personal contnbutlon of Mr T N WAkroyd M Sc Tech, LL B (Hons) (Chairman) DrMBoltonPhD,MSc,MA C E n g , M I C E DrWGKFlemmgBSc,PhD,CEng,MICE DrBS~mpsonPhD , C Eng , FI C E MrKWVlckery BSc , F G S 1)
Mr D Walte C Eng , M I C E , F I Struct E
As a code of practrce, thls Brltlsh Standard takes the form of guldance and recommendat~ons It should not be quoted as if it were a speclficat~onand particular care should be taken to ensure that clalms of compliance are not m~sleadrng Compliance with a British Standard does not of itself confer immunity from legal obligations.
.. Section 1. Introduction 1.1 Scope
1.3 Definitions
The subject of thls code of practlce IS the des~gn and constructlon of structures to retam soils and matenals w ~ t hsunilar engtneenng propertles, at slopes steeper than those wh~chthey would naturally assume The code of practice provides gu~dancefor a desgner, conversant w ~ t htheoretical and apphed so11 mechamcs and expenenced in structural design and construction The code ls pnmanly apphcable to small and medium walls \nth a retamed he~ghtof up to about 8 m, although many of the recommendat~onsare more generally appl~cableSpec~alrstadvlce should be sought w11h regard to the demgn and constructlon of larger structures and for those where movement of the retaned soils requlres close control The code 1s divlded Into four sectlons Sect~on1 explans the terms used m the document and summarizes the factors ~nfluencmgthe choice of a retamng wall Sect~on2 descnbes the slte and geotechn~caldata that IS reqwed together w ~ t hmaterial propertles It gwes gu~danceon the determmation of the values of representatwe soil strength necessary for des~gnpurposes Sect~on3 identifies the des~gnpmosophy and the des~gnmethods for earth r e t m m g structures, mcluding the determmation of earth pressures and the analysis of overall stable equ~l~bnum It defines des~gnso11 strength and cons~dersthe loads on retauung walls and the forces avdable to atlam equhbnum with tolerable displacements Gu~dance 1s gwen on methods of slmple practical des~gnand on the mfluence of ground conditions Sect~on4 cons~dersm detml vanous md~vldual types of structure and apphcat~onof earth pressure theory together ulth matters of constructlon and mamtenance
For the purposes of this Bntish Standard the followmg defimtlons apply and are l ~ i n ~ t etod words used w t h spec~almeanmg in t h ~ sdocument Normal so11 mechanics terminology is not defined
1.2 References 1.2.1 Normative references T h ~ sBnt~shStandard incorporates, by dated or undated reference, provlslons from other publications These normative references are made at the appropnate places in the text and the c~ted pubhcat~onsare hsted on page 110 For dated references, only the e d ~ t ~ oc~ted n apphes, any subsequent amendments to or revmons of the c~ted pubhcation apply to this Bntlsh Standard only when mcorporated in the reference by amendment or revxion For undated references, the latest e d ~ t ~ oofnthe c~tedpubhcat~onapphes, together w ~ t hany amendments 1.2.2 Informative references This Bnt~shStandard refers to other pubhcations that pmv~demformation or guidance Edit~onsof these publ~cationscurrent at the time of issue of thls standard are listed on the ins~deback cover, but reference should be made to the latest editions
1.3.1 active earth pressure The earth pressure exerted on the retauung wall by the retamed soil It may be greater than the fully actlve earth pressure (see 1.3.11 and 3.1.9) 1.3.2 conservative values Values of sol1 parameters wh~chare more adverse than the most hkely values They may be less (or greater) than the most hkely values They tend towards the lunit of the cred~blerange of values 1.3 3 design situation A set of phys~calcond~aonsfor w h ~ c hit should be demonstrated that a h t state (see 1.3.13 and 3.2.2) wdl not occur 1.3.4 design soil strength Sod strengths wh~chare assumed will be mobhzed at the occurrence of a h ~ state t (see 1.3.13) The desgn value of sod strength is the lower of either the peak sod strength reduced by a mob~l~zat~on factor (see 1.3.14) or the cnt~calstate strength 1.3.5 design surcharge load Loadmg wluch 1s assumed to occur at some tune dunng the Me of the structure and for w h c b the des~gnshould provide The m m u m requlred value IS 10 kN/m2 See 3.3.4 1.3.6 design value of a parameter The value of the parameter entered mto eqwhbnum calculatrons 1.3.7 design value of wall friction The smaller of elther the actual wall fncuon or adhes~onmeasured by test or 75 % of desgn sod strength (see 1.3.4) See 2.2.8 and 3.2.6 1.3.8 disturbing force The force exerted by retained soil on a r e t m m g wall, tendmg to cause the wall to move It mcludes the surcharge loads, external loads and water pressure The mnnmum value is the fully actwe earth pressure (see 1.3.11) 1.3.9 earth pressure coefficients Ratlo of honzontal effective stress to vert~cal effective stress K, IS the fully active earth pressure (see 1 3.11) coefficient, Kp is the fully passive earth resistance (see 1.3.12) coeffic~entBoth are based on the des~gnsod strength (see 1.3.4) Des~gnvalues are determined from design values of soil parameters Graphs are provlded in annex A for values of honzontal component of K, and KP The values aven m the varlous graphs m annex A are for vanous ratlos of 9' and wall fnct~on6
Section 1
BS 8002 : 1994
1.3 10 embedded walls Formerly known as sheet plle walls, this term embraces walls of s m ~ l a structural r behav~our whether constructed of steel sheet piles, concrete piles, concrete diaphragms or t~mberThey are supported, at least m part, by passwe earth resistance (see 1.3.15) 1.3.11 fully active earth pressure The minimum value of the actwe earth pressure (see 1.3.1), which occurs after suffment movement or deflect~onof the retailung wall, the necessary movement 1s usually w~thinthe semlceability h m ~ t state (see 1.3.18) of the wall 1.3.12 fully passive earth resistance The maxlmum value of the passwe earth reslstance (see 1.3.15), wluch occurs after suffmeut movement or deflect~onof the retmmg wall The necessary movement 1s often outs~dethe semceabihty h i t state (see 1.3.18) of the wall 1.3.13 limit state Any state of stabhty beyond which the retainmg wall no longer sat~sfiesthe design performance requirements A h ~ state t is not assoc~atedwlth any part~cularmethod of structural des~gnSee ultmate h i t state (1.3.19) and serviceablhty hmit state (13.18) 1.3.14 mobilizat~onfactor A factor M of 1 2 or 1 5 (or more, see 3.2.4 and 3.2.5) appl~edto the representatwe so11 shear strength to produce the desgn sod strength (see 1.3.4) M deternunes the proportion of the representative strength whch may be mob~hzedat a lmut state (see 1.3.13) 1.3.15 passive earth resistance The earth pressure generated by the sod when it reslsts movement of a retanmg wall 1.3.16 rapid shearing In the context of total stress a n a l y s ~the , sheanng of a sod at a rate suffic~entto prevent or l n h ~ b ~ t any s~gnlficantpore water pressure dffis~pat~on so that c, is the operatwe shear strength 1.3.17 representative soil streugth Consematwe est~mateof the mass strength of the so11 The value e determmed from reliable site mvestigation and so11 test data In the absence of such data, see tables 1, 2, 3 and 4 1.3.18 serviceabll~tylimit state State of deformation of a retamng wall such that ~ t use s is affected, its durab~htyis mpaued, its maintenance requirements are substant~ally mcreased or damage 1s caused to non-structural elements Alternatively such movement of the earth retaming structure which may affect adjacent structures or semces in a hke manner
1.3.19 ultimate limit state State of collapse, instability or forms of falure that may endanger property or people or cause major econormc loss 1.3.20 unplanned excavation The m i m u m depth, below the nommal finished surface in front of the wall, wluch it is assumed, for design purposes, w~llbe excavated at some tlme durmg the Me of the retmmg wall See 3.2.2
1.4 Major symbols effectwe cohes~on base adhesion u n d m e d shear strength undrruned wall adhes~on effective gram slze effective gram sue Young's modulus moment of mertia flow-net parameter (see figure 9) fully actwe earth pressure coeffment actwe pressure coefficient for cohesion ratlo of horizontal to vert~caleffective stress for sol1 at rest (no stram) coeffic~entof earth pressure a t rest (Ko KJ fully passwe earth reslstance coeffment mobiluation factor result of standard penetration test modlf~edvalue of N (see figure 2) beanng capacity factor beanng capacity factor beartng capaclty factor total active thrust normal to the wall total passwe thrust normal to the wall pore water pressure rad~us surcharge pressure water pressure load depth depth to water table inchnation of the wall mclinat~onof the surface of the retained so11
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u ~ u welght t of so11 (kNIm3) unlt welght of water angle of wall fnct~on angle of base fnctlon active pressure normal to the wall passwe pressure normal to the wall total vert~calpressure effectwe vertical pressure effectwe angle of shearmg reslstance c r ~ t ~ cstate a l angle of shearmg reslstance maxlmum value of 9' determmed from conventional tr~axlaltest res~dualfnct~onangle base reslstance
1.5 Selection and types of structure 1.5.1 General There is a w d e vanety of different forms of earth retainmg structure Many structures mclude a combinat~onof wall and support system
1.5.2 Selection of type The select~onof a particular form of earth retanlng structure w ~ ldepend l on a) the locat~onof the wall, its posltlon relat~veto other structures and the amount of space awlable, lncludmg the necessity or othennse to confine the support system witlun the slte boundanes, b) the proposed helght of the wall and the topography of the ground, both before and after construcbon, c) the ground conditions, d) the ground water and t ~ d a cond~t~ons, l e) the extent of ground movement acceptable dunng construction and in semce and the effect of movement of the earth retmlng structure on e x ~ s t ~ norg supported structures and services, f) external h e loadmg, g) the avallabhty of materials, h) appearance, I) reqwed Me and mamtenance, Where several alternatwes are su~tablethen an economc companson should be made
BS 8002 : 1994
Section 2
Section 2. Data for design c) where the type of wall or method of constructlon 1s uncertain at the t m e of 2.1 1 General mvestigatlon the borehole depth, below excavatlon level, should be at least three tlmes The deslgn of an earth retmmg structure requlres the proposed retamed helght mformation on the physlcal condlt~onsin the v~cmltyof the structure, mcluding the topography If ground anchorages are proposed the mvestigatlon and layout of the slte, detalls of adjacent should be of sufficlent extent and depth to provide foundat~onsand services, the nature of the ground data for the strata m wh~chthe anchorages will and the ground water condit~onsmcludmg, where attam their bond length apphcable, the t~daland seasonal vanations An The essent~dpropertles of the so~ls,in the adequate site investigation should be carned out to lrnmed~atevlcmlty of the retammg structure, prov~dethe necessary informat~onWhen the site should be ascertained together w ~ t hthe details of investlgatlon has been carried out and so11test foundations of any adjacent structures The results are obtamed, these are processed to prov~de relat~onsh~p of the slte to the overall geology values for the representatwe soil parameters should be establ~shedmcludmg the exlstence of (see 2.2.2) Once representatwe values have been any specla1 condit~onssuch as geologcal faults, established, dewgn values should be denved for use movement jomts, areas of Iandshp or any tendency m the equihbrlum and structural design of the slte to shift, creep or settle, as for example calculat~ons in areas of mmng subsidence The poss~bhtyof NOTE The denvatmn of deslgn values is explaned In 3 1 8 externally generated v~brat~ons and their effect upon earth pressures should be ascertained 2.1.2 Site investigations The process of slte mvestlgation contmues during Sufflclent informat~onshould be obtamed on the constructlon Inspections should be carr~edout ground and ground water conditions and the from tlme to tlrne, durmg constructlon, to strength and deformation propertles of the so~ls determme that the conditions revealed are m wh~chw ~ lbe l retamed and the soils whlch will accordance wrth the design assumptions If the support the earth retam~ngstructure Major earth cond~t~ons differ then the desgn should be checked retamlng structures require an extenslve Site agamst the changed cond~tions lnvestlgation Mmor earth retammg structures requlre sufficlent mformat~onabout the site 2.1.3 Ground water together w ~ t hso11 data to permit the selection of An adequate design requres knowledge of the representative values and des~gnvalues of the sod ground water levels and seepage pressures at the parameters to permlt a sat~sfactoryd e s w to be site, together with ~nformat~on as to the exlstence prepared Geolo@calmaps, memoirs and of any hydrostatic uphft pressures Information on handbooks should be consulted together wlth any ground water cond~tionsmay be avalable from other source of local knowledge records of the slte, geologcal maps or memoirs, or The code of practlce for site lnvestlgatlon BS 5930 from knowledge of other simllar sltes in the describes the general cons~derat~ons to be taken local~tyGround water conditions may be into account and deta~lsthe methods of slte pred~ctablefrom a knowledge of the local geology investlgatlon avdable Informat~onon methods of The poss~b~My of flooding should be ascertamed m situ and laboratory testlng 1s glven In BS 1377 together w ~ t hits effect on the ground water PartltoPart9 conditions The number of boreholes, or other form of Standp~pesor piezometers should be mstalled mvestigatlon, should be adequate to establ~shthe where necessary to determine the ground water ground cond~t~ons along the length of the wall and cond~tions,they should be mstalled in accordance to ascertam the var~abilityin those conditions The w ~ t hBS 5930 Where layers or strata of markedly centres between boreholes wlll v a q from site to different permeabhty emst, then the hydrostat~c slte but should generally be at Intervals of 10 m to levels w t h m each stratum should be obtamed 50 m along the length of the wall The depth of NOTE Water levels encountered dunng bonng operations are mvestlgation w ~ lbe l related to the geology of the unrelrable, they seldom -present equlhbrlum condmons slte and to the type of wall Poss~blechanges in ground water levels due to the a) for a backfilled gravlty or reinforced stem wall presence of the retammg wall and seasonal or the borehole depth below foundmg level should other causes, ~nclnd~ng future trends and acc~dent be at least twce the proposed retamed helght, circumstances, should be mvestigated Future works, in the vicm~tyof the wall, may @ve nse to b) where excavation will be carried out in front of the waU the borehole depth, below excavatlon changes in the long-term ground water cond~t~ons, level, should be at least three tlmes the proposed where such future works can be reasonably anticipated, the potentlal changes in ground retamed he~ght, condlt~onsshould be assessed
2.1 Site and geotechnical data
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BS 8002 : 1994
Section 2
The presence of deletenous chem~calsm the ground water and sod should be estabhshed m accordance w ~ t hBS 1377 Part 3 and the effect of such deletenous chemcals upon the corrosion of the proposed structure should be assessed in accordance w ~ t hBS 8110 Part 1 and Part 2 and BS 5493 2.1.4 Flood tides and waves Ground water cond~t~ons, both for waterfront structures and also for structures a short distance mland, may be mfluenced by tidal cond~t~ons The maxlmum t ~ d a range l to waterfront structure should be established includmg potentlal or poss~blesurge t ~ d e sand flood cond~t~ons The he~ght,length and angle of approach of waves and the resultmg forces on the structure should be determmed
2.1.5 Climate The clunat~cvanatlons and their effect on the structure should be determmed, including a) diurnal and seasonal temperature changes and the effect on earth pressures of temperature changes, particularly ground freenng, b) short-term and long-term rainfall vanations and the effect on earth pressures of the resulting molsture content changes, c) artifmally mduced clunat~cchanges such as those produced in boiler houses or cold stores and their effect on earth pressures and stabhty 2.1 6 Trees Retammg walls budt adjacent to exlstlng trees may suffer deletenous effects from the penetration of root-systems Dunug the course of the site investlgatlou, the presence of trees and large shrubs should be noted so that dec~s~ons can be taken at the desgn stage concermng the retention or removal of such trees or shrubs Trees and large shrubs In general, should not be p e n t t e d nor planted wrthm a distance from the retammg wall equal to half of thew expected mature heght and dec~duousforest trees such as alder, beech, oak, poplar and d o w should not be p e m t t e d w~thlna d~stanceequal to the mature helght of the tree BS 5837 prov~desuseful adv~ce Where ~tis requwed to plant or retam trees or large shrubs close to the retamng wall after its construction, the locat~onand choice of the tree or shrub specles should be such as to munmlze or elmunate the adverse effects of root penetratlon and the changes m the molsture content of the sod and any assoc~ateddesiccation and shnnkage of the sod The adverse effects of trees and root penetratlon mcludes mcreased loading on the structure and penetration of roots Into joints or dramage systems
2.2 Soil properties 2.2.1 General The desgn of earth retalnmg structures usually involves an effective stress analysis, although m some circumstances a total stress des~gnmay be appropnate, accordingly data on the so11propertles m respect of both strength (see 1.3.17)and stiffness under both dramed and undramed conditions should be obtamed Sod propertles are determmed as part of the slte lnvestlgatlon process but may be amphfied by data from back analysis of comparable retainmg structures In smular ground conditions The u n ~ we~ghts t of materials, m table 1, prov~de reasonable values for umt we~ghtsof sods m the absence of reliable test results
a b l e 1. Unit weights of soils (and similar materials) Materm1
y m morst bulk welght (kN/m3)
Loose
A - Granular
1
Gravel 16 0 Well graded sand and gravel 1 9 0 Coarse or medium sand 16 5 Well graded sand 18 0 F ~ n eor sdty sand 17 0 Rock fill 15 0 Bnck hardcore 13 0 Slag fill 12 0 Ash fill 65 B - Cohesive Peat (very vanable) Orgamc clay Soft clay F~rmclay St~ffclay Hard clay Stlff or hard glac~alclay
Dense
y, saturated bulk wezght (k~lm~) Loose Dense
2.2.2 Selection and evaluation of soil parameter values The so11test results, obtamed from the slte mvestigatlon, requlre two stages of analys~sand mterpretatlon m order to denve sat~sfactorydeslgn parameters from the raw geotechn~caldata In the first stage, values for the representatwe sod parameters are chosen (see 1.3.6) These should be comervatwe estnnates (see 1.3.2) of the propertles of the sod as it exists m situ Care should be taken that the representatlve value is properly applicable to the part of the des~gnfor whlch it IS intended The second stage, the denvat~onof sat~sfactory des~gnparameters from representatwe so11 parameters, 1s considered m 3.1.8 The first step in obtaming representative values of the measured so11 parameters, is to make a cntical examination of the raw data assisted by estabhshed calibration factors between different types of so11 tests Cons~stencymd~ces,denved from molsture content and hquid and plastic limlt tests, provlde a useful correlat~onw t h sol1 strength and stiffness ind~cesData from d~fferentsamples and different locations wlLl spread over a range of values Isolated low or high values should be scrutin~zedto determ~nethew accuracy, where such values are attnbutable to errors they should be rejected, where they are due to extreme local varlatlons t h e ~ relevance r requlres further cons~derat~on For soil parameters, such as dens~ty,for which field values can be determmed with confidence from test results wh~chshow httle vanation, the representatwe value should be the mean value of the test results Where greater vanatlons occur or where values cannot be fixed with confidence then the representative value should be a cautious assessment of the lower h m ~ (or t the upper h m ~ ift that IS the relevant bound) of the acceptable data In the absence of detaled test information, representatwe values should be selected by the apphcation of conservative bounds to generally avalable parameters The selection of representative values of soil parameters should take the followng matters into account a) geologxal and other background information, b) differences between the in situ condit~onsand the propertles measured by field or laboratory tests, c) the effect of construction activities on the properties of the ground, d) changes wkch may occur in the field due to vanations in the environment or weather, e) relevant data from prevlous projects and the performance of exlstmg faclllties Careful assessment of the so11 parameter values 1s necessary to ensure select~onof those values which are pertment to the behav~ourof retaming
structures The assessment of the proper parameter value IS often dependent on the mechan~smor mode of deformat~onbemg considered for the retalrung structure, for example, different representatlve strengths wdl be reqwed for a shear fallure m a fissured matenal dependmg upon whether the shear surface 1s free to follow the fissures or is constramed to Intersect intact matenal A range of values should be considered part~cularly,lfthe sol1 parameter values are hkely to change dunng the hfetime of the retammg structure Under serv~ceab~l~ty condlt~ons,where deformations are comparatively small, the so11 wll operate at below peak strength conditions The appropriate strength and stiffness values may be obtamed by examlnmg the stress-stram hehamour of the sod, as gwen for example by laboratory tnaxlal tests Under ultimate hmlt state cond~t~ons where deformations are comparatwely large, the so11 will operate at beyond peak strength condlt~ons and may d~lateto approach the cnt~calstate values conslstent with the strength envelope for loose or normally consohdated solls Tables 2, 3 and 4 promde gu~danceon the empmcal relat~onshpbetween class~ficat~on and mdex tests and representatwe values of the angle of sheanng resistance and the denslty of various materials 2.2.3 Clay soils The construction of a reta~nmgwall may result in changes in the strength of the ground m the viclnlty of the wall Where the mass permeab~lrty of the ground 1s low the changes of strength take place over some tlme and therefore a m necessary to determine parameter values apphcable to both short-term and to long-term cond~tlons,1 e undmned and drained conditions The undrained shear strength of a clay so11 is not a fundamental so11 property D~fferentvalues may be recorded m tnaxlal compression and extension, in dlrect shear and m pressuremeter tests in situ Although convent~onalpractlce has been based on tnaxlal compression tests, w h c h are conslstent with active so11 conditions, extenslon tests may be required d the behav~ourof a passive zone IS of particular concern The undmned strength of a soft clay with a small overconsohdated ratio (less than 3) Increases when the posltlve pore pressures d~ss~pate, but the negative pore pressures Induced by sheanng a stlff clay, w t h a high overconsolidation ratio, cause it to swell and soften m the long-term Lf the undramed strength of a shff clay is to be rehed upon dunng temporary works construction then care IS necessary to ensure that there are no sand or sdt partings contaming free ground water wh~chwould affect the undrained shear strength, such permeable zones are common m clays
fnctlon angles p', Fmt tlme shdes due to new In assessing the strength of clay so~ls,part~cularly constructlon have been found to moblllze mass from undmned tests in accordance w t h BS 1377 strengths no lower than pc i, Part 7, the procedures used for sampl~ngand testing should be taken Into account For example, Two approaches may be adopted for the UlOO sampllng of stlff clays leads to partla1 convent~onalhneanzat~onof the peak so11envelope remouldmg and the creatlon of excess negatlve over some deslred range of stress, see figure 1 A pore pressures, these m turn cause excesslve ~ n ~ t ~ secant al p' value can be selected as a funct~onof effectlve stresses whlch can lead to unconsolldated stress level If a smgle value 1s chosen, the resultmg tests reglstenng erroneously lugh undramed envelope is h e a r to the on@n and falls safely strengths, even when the water content has been ms~dethe envelope of tests caxned out from preserved T h ~ is s due to the mode of fa~lureof ~dent~cal m ~ t ~condit~ons al Slnce sod samples from heavlly overconsohdated clays, whlch, by stram the field are not ident~cal,the method should softenmg, lead to shear rupture Such fa~lures normally be apphed by select~ngthe lowest secant occur at strengths lower than those apphcable at p' for any sample tested wlthm the target range of the same water content but lower stress Alternatwely, the tangent parameters overconsohdat~onratlo More consistent results are (c', p') may be used, where each IS a funct~onof obtalned if samples are consohdated to a best stress level for ident~calsamples Sample vaxlatlon estimate of in s ~ t ueffect~vestresses pnor to causes scatter m the tangent parameter values and shearlng Representatlve values for undralned conservatwe values are best selected by fitting a strength parameters should be assessed for the lower bound to the relevant data, tak~ngcare to peak strength and for the remoulded strength of conslder the range of effect~vestress requxed the sod The values for the representatlve peak In the absence of rehable laboratory test data, the strength should make due allowance for the conservatwe values of p f o t gwen m table 2 may be Influence of samphng and the method of testmg, as used, m t h c' = O well as for hkely softenmg on excavation If samples of clay contmmg vems or seams of sand To determme the strength of clay sods, for an or silt are remoulded for the plastic~tyIndex tests effective stress analysis, t n m a l tests may be the test results gwe lower plastlc~tymd~cesthan carned out e ~ t h e fully r dramed or undramed w ~ t h the clay Itself Care should be taken to carry out pore pressure measurement, provlded the samples the tests on the clay alone if there are doubts as are fully saturated m accordance m t h BS 1377 to the mclus~onof sand or sdt then, in table 2 use Part 7 and Part 8 The tests are carned out the next value of the plast~cltymdex h~gherthan sufficiently slowly to ensure equahzatlon of pore recorded m the tests pressures The Mohr-Coulomb falure envelope for overconsohdated clays, of lnltlally Identical 'hble 2. pIcnt for clay soils samples, IS generally curved, see flgure 1 At effect~vepressures close to the preconsolldatlon pressure, the sod mobll~zesrts cnt~calstate angle of sheanng ,p ,' At lower ~ n ~ teffect~ve ~al stresses, I e at hgher overconsohdated rat~os,the so11 e x h ~ b ~at sddatant peak at failure before a s strength drops to, and poss~blybeyond, the cnt~cal state value Representatlve values should be assessed separately for the peak strength and for the cnt~calstate strength of the sod The In all tests a non-hear so11 response should be representatlve peak strength should be appropnate ant~c~pated, so that stress-strm curves form to the antmpated stress state of the so11in the hysteres~sloops on load-unload-reload cycles In ground Where the stress-strm curve never assessmg the deformat~onproperties of so~ls,the reaches a peak, dunng the maximum stram range stlffness measured m convent~onallaboratory tests ach~evabledunng test, the peak strength should be m accordance wlth BS 1377 Part 5 and Part 6 assumed to be the largest strength mobhzed dunng generally underestimates m s ~ t uvalues denved the test It may be represented by values of c' and from back analys~sof instrumented field structures p' or by secant values of p' The representatlve Appropnate stiffness values can be measured in crltlcal state strength 1s represented by the cntlcal the laboratory by laboratones expenenced in thls state angle of sheanng resistance, p'c,lt Cohes~ve spec~ahstwork provlded part~cularcare IS taken m sods w ~ t hhlgh clay contents e x h b ~the t greatest sample preparation and local s t m n measurement fall from peak to resldual strength, formmg a Sample d~sturbanceIS corrected by flrst t a h g the pohshed rupture surface Prev~ousshear surfaces, sample through its most recent effectwe stress m plast~cclays, may be reactwated a t low res~dual cycle so that its m situ state IS properly recreated
I
BS 8002 : 1994
Section 2
-.
Stress range a1 Peak and u l t ~ m a t estrength most sods
c Secant parameters
A
0'
f (0'1
C
0'
L
0'
bl Peak and ultimate strength s o i s with > 50% t h y minerals
d ) Tangent parameters s,,,=r'+g' tan 0'
Figure 1. Strength envelopes for a gwen pre-consolidation St~ffnessparameters can be determmed from certam field tests which cause httle d~sturbancem accordance with BS 1377 Part 9 and CIRIG Ground engmeenng report, 1987
2.2.4 Cohesionless soils The strength and st~ffnessof cohes~onlessso~lsare determmed md~rectlyby m situ statlc or dynanuc penetration tests Detals of three types of penetration tests as well as plate loadmg tests are Sven m BS 1377 Part 9 The peak and crit~cal state angles of sheanng resistance for siliceous sands and gravels may be estimated from the following equations The est~matedpeak effectwe angle of sheanng reslstance 1s gwen by ,,,p ' 30 + A + B + C (1)
-
The estmated cnt~calstate angle of sheanng reslstance is gwen by pfCnt= 30 + A + B The values of A = angularity of the part~cles B gradlng of the sandlgravel C results of standard penetratlon tests are gwen in table 3 The standard penetratlon test (SPT) values should be corrected for the effect of overburden pressure m accordance with figure 2 (see Thorburn, 1963), other correction effects may be necessary See ClRIA Report FRICP17Ll Bolton (1986) has mtroduced emp~ncalrelations between,,,,p ' ,p ,' ~ n i t ~sol1 a l relatwe density and mean
-
effect~vestress at fa~lureto reflect the change in the secant value of peak angle of sheanng resistance wlth the change m the mean effective stress m the ground 2.2.5 Silts It 1s d~fficultand often mpract~cableto obtam und~sturbedsamples of s~ltsand fine sands, even employmg speclal samphng techn~quesLoose slits are read~lyhqu~fledby vlbrat~on,both dunng prob~ngand dunng the life of the retaining wall, accordmgly excess pore pressures should be taken into account lnorgan~cs~hceousSINS can generate as much dllatancy as sands, at the same relative denslty, but they more easlly soften to cntlcal states in thm rupture bands In the absence of other data and where disturbed samples have shown the s ~ l 1s t a rock flour wlth neghgble organlc or clay mineral content, the representatwe effectwe angle of sheanng may be conservatwely taken as q',,,, m table 3 a b l e 3. q' for siliceous sands and gravels A -A~ularitvl) 1A (degrees) 10 Rounded Sub-angular 12 Angular B - Grading of soil2)
/
Moderate gradmg Well graded C - h"3) (blows 300 mm) < 10 20 40 60
10 2 6 9
'I Angularity ie emmated fwm vrsunl descnptlon of soil " ~ r a d m gcan be determmed from grading CUNe by use of Unlfonnzty coefficient = D,,lD1, where Dlo and D,,, are part~cleslzes such that m the sample. 10 X of the matenal is finer than D,o and 60 ih is finer than Dhii
Gradmg Unlformlty e o e f f m e n t Uniform <2 Moderate grading 2 to 6 >6 Well graded A step-graded sol1 should be treated as umform or moderately graded so11 according to the gradmg of the fmer fractmn iV' from results of standard penetration test modified where necessary by figure 2 lntermed~atevaules of A. B and C bv mterwolahon
2 2 6 Rock The engneermg properties of rock relevant in deslgn are controlled by the extent and onentatlon of the beddlng planes andjomts wlthm the rock mass together with the water pressures on the dlscont~nu~ty planes The slte mvestgation should estabhsh the strength and onentatlon of the discontinuity planes Weak rocks, part~cnlarly weakly cemented sandstones, f~ssuredshales and chalk, are often difficult matenals to sample and test Some correlat~onhas been obtamed between the standard penetration test in accordance w ~ t h BS 1377 Part 9 and the strength and stiffness properties for c e r t m weak rock masses In addition the mass rock properties may be derived from compression wave and shear wave veloc~ty measurements The f o l l o m g lud~cativevalues of the effect~ve angle of fr~ctlonin table 4 relate to rocks which can conservatively be treated as composed of granular fragments, i e they are closely and randomly jolnted or othenv~sefractured, havmg an RQD (rock quahty des~gnation)value close to zero
a b l e 4. 9' for rock Stratum
B'
(degrees)
Chalk Clayey marl Sandy marl Weak sandstone Weak s~lstone Weak mudstone
35 28 33 42 3 5
NCTE 1 The presence of a prefemd onentatmn of jomts, bedding or cleavage in a dlrectlon near that of a poss~hle failure plane may requre a reduction m the above values, especially d the dlscontinuitles are filled w t h weaker materlals hOTE 2 Chalk is defined here as unweathered medlum to hard, rubbly to blocky chalk, grade I11 (see Clayton, 1990)
2 2 7 Fill A wide range of matenals may be used as fill behind retamng walls Selected cohesionless granular fill placed m a controlled manner such as well graded small rockfds, gravels and sands, are smtable as fill Cohes~vematenals. sub~ectto the further recommendat~onsbelow, may 6e suitable but other materlals such as mdustnal, chem~caland domestic wastes should not be used All fill matenals should be properly invest~gatedand class~fied l involve The use of coheswe soil as f ~ l may problems durmg deslgn and construction additional to those whch occur with granular fill, but the use of coheswe soil may result m sigmf~canteconomies by avoidmg the need to Import granular matenals
Section 2
BS 8002 : 1994
Figure 2. Derivation of N'from SPT value N The cohesive sod should be wthm a range su~table for adequate compactmn, for guidance on the selection of such fill see the Transport Research Laboratory pubhcations LR406, LR750, SR522 and RR90, the proceedmgs of the conference on clay fills, ICE 1979, the DOTSpecification for h~ghwayworks, 1991 and DOTStandard BD30187 The placement moisture content of cohewve fill should be close to the final equdbnum value to prevent elther the swelling of clays placed too dry or the consohdat~onof clays placed too wet Volume changes in clay soils wdl affect the pressure d~stnbut~on on the wall in the medlum to long-term Compaction pressures should also be taken into account, see 3.3.3.6 Problems associated w t h swelling and consolidation wdl be m~nimizedd clay fill IS limited to clays with a hquid l~rnitnot exceeding 45 % and a plast~cityindex not exceeding 25 !& (DOTSpec~ficat~on for highway works, 1991)
Chalk wlth a saturation moisture content of 20 % or less IS acceptable as fill and may be compacted as a well graded granular soil The saturation molsture content of chalk is evaluated from the dry denslty of individual lumps, determined m accordance with 7.3 of BS 1377 Part 2 1990 Saturation moisture content =
where yd = dry density in mg/m3 Exceptionally, some granltes are found whch detenorate by weathering of the feldspars If it is proposed to use such granltlc rocks, due allowance should be made for deterioration in estlmatmg the angle of fnctlon Conditioned pulverized fuel ash (PFA) from a single source may be used as fill it should be supphed at a moisture content of 80 X to 100 % of the optlrnum molsture content
..
Section 2
BS 8002 : 1994
No effect~veadhesmn c' should be taken for walls or bases in contact with sod The effects of wall construction on the mterface fnction between the soil and the wall should be taken in account The undramed shear strength mobillzed on a wall surface may be ~rrelevantdue 2.2.8 Wall friction, base friction and undrained to the presence of drainage mater~alcreating wall adhesion effectwe fnct~onconditlons on the boundary Representatwe values of the strength of the soil Craclung and arr entry against the wall also tend to shdmg as a mass against the wall can be produce fnctlon conditlons \nth zero (atrnosphenc) determined from appropriate drained and pore pressures against the wall, m contrast to the undmned shear box tests The wall matenal should possibly negatlve pore pressures mobfized be placed m the bottom half of the box w ~ t hits temporanly w~thinthe clay mass Under these ~nterfaceon the plane of shdmg The so11 is then circumstances the representative coefficient of placed in the upper part of the box m the requ~red effective fnction on the boundary 1s tan 6 and the state With large scale surface roughnesses (1 e normal effective stress at the boundary IS equal to concrete formed on or agalnst coarse granular sods) the normal total stress a, in the soil so that the side and end effects of the small shear box representatwe wall fnction is a, tan 6 Where the (60 mm x 60 mm) w~llaffect the laboratory test undramed sod strength agamst a surface 1s results and large shear boxes should be used Tests relevant, and in the absence of appropnate tests, should be carned out over the range of normal the representatwe value should not exceed the stresses hkely to exlst on the wall dunng its hfe remoulded undramed strength of the soil Testing should be contmued to determine any reduction in strength \nth continued sllding 2.3 Externally applied loads In the absence of large shear box test results the representatwe strength, m terms of effect~ve All necessary details should be obtamed of statlc, stress, should not exceed values calculated usmg translent and dynam~cloads that may be applied externally to the earth retainmg structure a) 6 = P',,,~ for the sod, for rough surfaces \nth a texture coarser than that of the medlan particle size, b) 6 20°, for smooth surfaces w t h a texture finer than that of the median part~clesue Shale, mudstone and steel slag swell when they absorb water These matenals should not be used as fdl, except at some distance from the retaining wall Peaty or highly organic sol1 should not be used as fill
-
Section 3
BS 8002 : 1994
-.
Section 3. Design philosophy, design method and earth pressures 3.1 Design philosophy 3 1 1 General The design of earth retammg structures requlres cons~derat~on of the mteractlon between the ground and the structure It requlres the performance of two sets of calculatlons 1) a set of equ~hbnumcalculat~onsto determme the overall proportions and the geometry of the structure necessary to acheve equ~hbnumunder the relevant earth pressures and forces, 2) structural design calculat~onsto determme the sue and properties of the structural sectlons necessary to reslst the bendmg moments and shear forces determmed from the equ~hbnumcalculat~ons Both sets of calculat~onsare camed out for spec~fic design s~tuations(see 3.2.2) m accordance wlth the pnnc~plesof hmrt state deslgn The selected des~gn s~tuat~ons should be suffic~entlysevere and vaned so as to encompass all reasonable cond~t~ons whch can be foreseen dunng the penod of construction and the l ~ f eof the retalnmg wall 3.1.2 Limit state design This code of practlce adopts the phlosophy of h ~ state des~gn This ph~losophydoes not Impose upon the des~gnerany specla1 requirements as to the manner in whch the safety and stab~htyof the retammg wall may be ach~eved,whether by overall factors of safety, or partial factors of safety, or by other measures Llm~tstates (see 1.3.13) are class~fiedInto a) ult~mateh i t states (see 3.1 31, b) servlceabhty hmit states (see 3.1.4)
Typ~calult~mateh m ~ states t are dep~ctedin figure 3 Rupture states whch are reached before collapse occurs are, for s~mphc~ty, also class~f~ed and treated as ult~mate11m1tstates Ultlmate hmlt states mclude a) mstab~htyof the structure or any part of it, mcludmq supports and foundat~ons,cons~dered as a n@d body, b) fa~lureby rupture of the structure or any part of it, mclud~ngsupports and foundat~ons 3.1.3 Ultimate limit states 3.1.3.1 General The followmg ultimate 11m1tstates should be considered b l u r e of a retan~ngwall as a result of a) mstability of the earth mass, e g a shp fadure, overturnmg or a rotat~onalfafiure where the disturbmg moments on the structure exceed the restonng moments, a translat~onalfa~lurewhere the d~sturbulgforces (see 1.3.8) exceed the restonng forces and a beanng fadure Instability of the earth mass mvolving a shp fa~luremay occur where 1) the wall IS bulk on sloping ground wh~ch t ~tself1s close to l m t m g equil~bnum,or 2) the structure IS underlam by a slgmficant depth of clay whose undramed strength mcreases only gradually wlth depth, or 3) the structure 1s founded on a relatively strong stratum underlam by weaker strata, or 4) the structure 1s underlain by strata w~bhin wh~chhigh pore water pressures may develop from natural or artificial sources b) fallure of structural members lncludmg the wall ~tselfm bendmg or shear, c) excesswe deformat~onof the wall or ground such that adjacent structures or servlces reach theu ultimate lumt state
..
Section 3
Figure 3. Limit states for earth retaining structures
BS 8002 : 1994
BS 8002 : 1994
Section 3
F a ~ l u r eof gravlty wall due t o slldmg
Grav~ty hall with shear key Fallure may be on A B or along B C sloplng plane
Bear~ngcapacity fallure of gravity wall on r e s t r ~ c t e ddepth of soft SOIL
Figure 3. Limit states for earth retaining structures (contznued)
Bend~ng moment fa~lureof embedded wall
Embedded wall cantiever fallurc by forward rotahon
13-
fivwx
Fa~lureby y ~ e l dof anchor or t ~ e(or prop 1
Fa~lureof embedded wall by rotahon about anchor ( o r prop I
Figure 3. Limit states for earth retaining structures (concluded) I
3.1.3.2 Analysis method
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The so11 deformat~ons,whch accompany the full mob~hzat~on of shear strength in the surroundmg Where the mode of fadure mvolves a shp fa~lure sod, are large m companson w t h the normally the methods of analysis, for stab~l~ty of slopes, are acceptable strams in semce Accordmgly, for most descnbed in BS 6031 and in BS 8081 Where the reta~nmgstructures the semceab~htyh m ~ t earth mode of fadure mvolves a beanng capaclty fa~lure, state of d~splacementwlll be the govenung the calculat~onsshould establ~shan effectwe w ~ d t h of foundat~onThe beanng pressures as determmed cntenon for a sat~sfactolyequ~hbnumand not the ultmate h m ~ state t of overall stabd~tyHowever, from 4.2.2 should not exceed the ult~matebeanng although it IS generally nnposs~bleor ~mpract~cal to capaclty In accordance w ~ t hBS 8004 calculate dls~lacementsdlrectlv. serwceab~htvcan Where the mode of fa~lure1s by translat~onal be suffic~enti~ assured by hm&g the propor& of movement, w t h passwe resistance excluded, stable avadable strength actually mobll~zedm semce, by equ~l~bnum should be ach~evedusmg the desgn the method gwen in 3.2.4 and 3.2.5 shear strength of the sod m contact w ~ t hthe base The des~gnearth pressures used for serv~ceab~l~ty of the earth retammg structure h m t state calculat~onswlll d~fferfrom those used Where the mode of fadure mvolves a rotat~onalor for ult~mateh m ~ state t calculat~onsonly where translat~onalmovement, the stable equ~hbr~um of structures are to be subjected to drffenng des~gn the earth retalmg structure depends on the values of external loads (generally surcharge and mob~l~zat~on of shear stresses w t h m the sol1 The hve loads) for the ult~mateh m ~ state t and for the full mob~hzatlonof the so11shear strength gwes nse semceablhty h m ~ state t to hm~tmgactwe and passive thrusts These hm~tingthrusts act m concert on the structure only 3.1.5 Limit states and compatibility of deformations at the pomt of collapse, I e ultrmate l ~ r n state ~t The deformatlon of an earth retalnlng structure 1s 3.1.4 ServiceabiLity limit states important because it has a du-ect effect upon the The f o l l o m g semceablbty h m ~ states t should be forces on the structure, the forces from the cons~dered retamed sod and the forces whlch result when the a) substantla1 deformat~onof the structure, structure moves agalnst the sol1 The structural forces and bendmg moments due to earth pressures b) substantial movement of the ground reduce as deformat~onof the structure increases
The mawmum earth pressures on a retanmg structure occur dunng worlang condit~onsand the necessary equ~hbnumcalculations (see 3.2.1) are based on the assumption that earth pressures greater than fully active pressure (see 1.3.11) and less than fully passlve \rill act on the retaming structure dunng semce As ultimate h i t state with respect to so11 pressures is approached, with sufficient deformat~onof the structure, the active earth pressure (see 1.3.1) m the retamed so11 reduces to the fully actwe pressure and the passwe resistance (see 1.3.15) tends to mcrease to the full available passive remtance (see 1.3 12) The compatibil~tyof deformation of the structure and the correspondmg earth pressures 1s Important where the form of structure, for example a propped cantilever wall, prevents the occurrence of fully active pressure at the prop it is also particularly mportant where the structure behaves as a bnttle matenal and loses strength as deformat~onincreases, such as an unremforced mass gravity structure or where the soil is hable to stram softenmg as deformat~onincreases 3 1.6 Design values of parameters These are applicable at the specified lirmt states in the speclf~eddeslgn situations All elements of safety and uncertainty should be incorporated into the deslgn values The selection of design values for soil parameters should take account of a) the poss~b~hty of unfavourable vanations in the values of the parameters, b) the independence or mterdependence of the var~ousparameters mvolved in the calculat~on, c) the quality of workmanshlp and level of control spec~f~ed for the constructlon 3.1.7 Applied loads The des~gnvalue for the dens~tyof fill materials, should be a pessmistic or unfavourable assessment of actual density For surcharges and live loadmgs different values may be appropriate for the differing conditions of serviceabihty and ult~matelimlt states and for different load combmations The mtention of thls code of pract~ceIS to determine those earth pressures w h ~ c hw~llnot be exceeded in a hmit state, if external loads are correctly predicted External loads, such as structural dead loads or veh~clesurcharge loads may be spec~fiedin other codes as nominal or charactenstic values Some of the structural codes, m t h wluch this code interfaces, spec~fydifferent load factors to be apphed for semceab~htyor ult~matehmlt state checks and for different load combmat~ons, see 3.2.7 Design values of loads, denved by factonng or otherw~se,are intended, here, to be the most pesslmlstlc or unfavourable loads whlch should he used in the calculat~onsfor the structure Similarly, when external loads act on the
active or retamed s ~ d eof the wall these same external loads should be derwed m the same way The sod IS then treated as forming part of the whole structural system 3.1.8 Design soil strength (see 1.3.4) Assessment of the design values depends on the requlred or ant~cipatedl f e of the structure, but account should be taken also of the short-term conditions which apply dunng and unmed~ately followng the penod of constructlon Single des~gn values of so11 strength should be obtamed from a considerat~onof the representatwe values for peak and ultmate strength The value so selected w l l satisfy, simultaneously, the cons~derat~ons of The des~gn t ultimate and semceabhty h m ~ states value should be the lower of a) that value of sod strength, on the stress-strain relat~onleadmg to peak strength, which is mob~l~zed at so11 strains acceptable for semceabhty Tlus can be expressed a s the peak strength reduced by a mob~hzat~on factor M as glven in 3.2.4 or 3.2.5, or b) that value which would be mobihzed a t collapse, after sign~ficantground movements Tlus can generally be taken to be the cntical state strength Des~gnvalues selected m t h way should be checked to ensure that they conform to 3.1.6 Design values should not exceed representatwe values of the fully softened cnt~calstate so11 strength 3.1.9 Design earth pressures The deslgn values of lateral earth pressure are mtended to gwe an overestunate of the earth pressure on the actwe or retained s ~ d and e an underestunate of the earth remtance on the passive slde for small deformations of the structure as a whole, in the worlung state Earth pressures reduce as fully active conditions are mobhzed a t peak soil strength in the retamed sod, under deformations larger than can be tolerated for serv~ceabihtyAs collapse threatens, the retamed soil approaches a cnt~calstate, m which its strength reduces to that of loose matenal and the earth pressures consequently tend to mcrease once more to active values based on cntlcal state strength The ~ n ~ tpresumption ~al should be that the des~gn earth pressure wdl correspond to that ansmg from the des~gnsod strength, see 3.1.8 But the mobhzed earth pressure in semce, for some walls, w~llexceed these values m s enhanced earth pressure wdl control the design, for example a) Where clays may swell in the retained sod zone, or be subject to the effects of compaction m layers, larger earth pressures may occur m that zone, causmg correspondmg res~stancefrom the ground, proppmg forces, or anchor tenslons to mcrease so as to mamntam overall equihbnum
Section 3
b) Where clays may have lateral earth pressures In excess of the assessed values takmg account of earth pressures pnor to construction and the effects of wall ~nstallat~on and sol1 excavation or fillmg, the earth pressure in retamed so11zones wlll be Increased to maintam overall equrl~bnum C) Where both the wall and backfill are placed on compressible solls, d~fferentlalsettlement due to consolidation may lead to rotatlon of the wall Into the backfill Thls Increases the earth pressures m the retamed zone d) Where the structure 1s part~cularlystlff, for example fully plled box-shaped bndge abutments, h~gherearth pressures, caused, for example by compaction, may be preserved, notw~thstandlngthat the degree of wall displacement or f l e x ~ b ~ lrequred ~ty to reduce retamed earth pressures to them fully actwe values m cohes~onlessmatenals is only of the order of a rotatlon of 10-3 radians In each of these cases, mobilized so11 strengths w~ll mcrease as deformat~onscontinue, so the unfavourable earth pressure cond~t~ons WIU not perslst as collapse approaches The des~gnearth pressures are denved from des~gn soil strengths usmg the usual methods of plast~c analys~s,w ~ t hearth pressure coeffments (see 1 3.9) p e n in ttus code of practlce bemg based on Kensel & Absi (1990) The same des~gn earth pressures are used in the default cond~t~on for the deslgn of structural sections, see 3.2.7
Figure 4. Pressure diagrams
BS 8002 : 1994
3.2 Design method 3.2.1 Equilibrium calculations In order to determme the geometry of the retammg wall, for example the depth of penetration of an embedded wall (see 1.3.10), equil~bnum calculat~onsshould be earned out for carefully formulated design sltuatlons The des~gn calculatlons relate to a free-body diagram of forces and stresses for the whole retalmg wall The des~gncalculat~onsshould demonstrate that there 1s global equ~hbr~um of vert~caland honzontal forces, and of moments Separate calculat~ons should be made for d~fferentdes~gnsltuat~ons The structural geometry of the retamlng wall and the equllibnum calculations should be determ~ned from the design earth pressures denved from the design sol1 strength using the appropnate earth pressure coefficients Des~gnearth pressures wdl lead to actwe and passwe pressure d~agramsof the type shown in figure 4 The earth pressure distnbut~onshould be checked for global equ~hbnumof the structure Honzontal forces equhbnum and moment e q h b n u m WIU @ve the prop force in figure 4a and the locat~onof the point of reversed stress cond~tlonsnear the toe in figure 4b Vertlcal forces e q u ~ l ~ b n ushould n also be checked
BS 8002 : 1994
3.2.2 Design situations 3.2.2.1 General The spec~ficat~on of des~gnsituations should mclude the d ~ s p o s ~ t ~and o n class~ficat~on of the var~ouszones of so11 and rock and the elements of constructlon wh~chcould be mvolved m a h m ~ t state event The spec~ficat~on of des~gns~tnat~ons should follow a cons~derat~on of all uncertamtles and the r~skfactors mvolved, mcludmg the followng a) the loads and them combmat~ons,e g surcharge and/or external loads on the actlve or retamed s ~ d eof the wall, b) the geometry of the structure, and the ne~ghbounngsod bod~es,representmg the worst credlble cond~tlons,for example over-excavation durmg or after constructlon, c ) the material charactenstlcs of the structure, e g followmg corroslon, d) effects due to the envlronment w~thmwhlch the des~gnIS set, such as - ground water levels, mcludlng thelr vanatlons due to the effects of dewatenng, possible floodmg or falure of any dramage system, - scour, eroslon and excavatlon, leading to changes m the geometry of the ground surface, - chemlcal corroslon, - weathenng, - freezmg, - the presence of gases emergmg from the ground, - other effects of tlme and envlronment on the strength and other properties of matenals, e) earthquakes, f) subs~dencedue to mlnlng or other causes, g) the tolerance of the structure to deformations, h) the effect of the new structure on exlstmg structures or serwces and the effect of exlstlng structures or semces on the new structure, I) for structures restrng on or near rock, the considerat~onof - mterbedded hard and soft strata, - faults, jolnts and fissures, - solutlon cavltles such a s swallow holes or fissures, filled w ~ t hsoft matenal, and contlnumg solutlon processes 3.2 2.2 Minimum surcharge and minimum unplanned excauataon In checkmg the stable equ~l~bnum and sod deformat~onall walls should be des~gnedfor a min~mumdes~gnsurcharge loadmg of 10 kN/m2 and a mlmmum depth of add~t~onal unplanned excavatlon in front of the wall, wh~chshould be a) not less than O 5 m, and b) not less than 10 % of the total heght retamed for cantilever walls, or of the he~ghtretamed below the lowest support level for propped or anchored walls
Section 3
These mmmum values should be rewewed for each deslgn and more adverse values adopted in particularly cntlcal or uncertain clrcumstances The requirement for an addit~onalor unplanned excavatlon as a des~gncrltenon IS to provlde for unforeseen and accidental events Foreseeable excavations such as servlce or dramage trenches m front of a retalnlng wall, wh~chmay be requ~redat some stage in the hfe of the structure, should be treated as a planned excavatlon Actual excavatlon beyond the planned depth IS outs~dethe des~gn cons~derat~ons of t h ~ code s 3.2.2.3 Water pressure regsme The water pressure regme used m the design should be the most onerous that IS cons~deredto be reasonably possible 3.2 3 Calculations based on total and effectwe stress parameters The changes In loaduig assoc~atedw t h the constructlon of a retammg wall may result m changes m the strength of the ground m the vlcmty of the wall Where the mass permeabhty of the ground is low these changes of strength take place over some tlme and therefore the des~gn should conslder cond~t~ons m both the short- and long-term Whch cond~t~on w ~ lbe l cntical depends on whether the changes m load apphed to the sod mass cause an mcrease or decrease m sod strength The long-term cond~tlon1s ltkely to be cntical where the so11 mass undergoes a net reduct~on~n load as a result of excavatlon, such as adjacent to a cant~leverwall Conversely where the so11mass IS subject to a net Increase m loadmg, such as beneath the foundat~onof a gravity or remforced stem wall at ground level, the short-term cond~t~on 1s l~kelyto be cnt~calfor stab~l~ty When cons~dennglong-term earth pressures and equhbnum, allowance should be made for changes In ground water cond~t~ons and pore water pressure reame w h ~ c hmay result from the construction of the works or from other agencies Calculat~onsfor long-term cond~tlonsrequlre shear strength parameters to be m terms of effectwe stress and should take account of a range of water of poss~ble pressures based on cons~derat~ons seepage flow cond~t~ons w t h the earth mass Effectwe stress methods can also be used to assess the short-term cond~t~ons prov~dedthe pore water pressures developed dunng construction are known A total stress method of analys~smay be used to assess the short-term condlt~onsm clays and sods of low permeab~lity,but an Inherent assumption of t h ~ method s IS that there w ~ lbe l no change m the sod strength as a result of the changes in load caused by the construction For granular materials and solls of hlgh permeab~lltyall excess pore water pressure w~lld~sslpaterapidly so that the relevant strength IS always the dramed strength and the earth pressures and equhbnum calculat~onsare always m terms of effectlve stresses
3.2.4 Design using total stress parameters The retammg wall should be designed to be m equhbr~umwhen based on a mob~hzedundramed design clay strength (deslgn c,) whlch does not exceed the representatwe undramed strength d~vldedby a mob~hzat~on factor M The value of M should not be less than 1 5 if waU displacements are requ~redto be less than 0 5 % of wall he~ght The value of M should be larger than 1 5 for clays whlch requlre large strams to m o b h e t h e ~ peak r strength 3.2.5 Design using effective stress parameters The retanung wall should be des~gnedto be m equ~hbnummob~lmnga. so11strength the lesser of a) the representat~vepeak strength of the so11 d ~ v ~ d ebyd a factor M = 1 2 that 1s representatlve tan q',, des~gntan 9' = (3) M representatlve c' design c' = M or b) the representatlve cnt~calstate strength of the so11 Thls WIU ensure that for so~lswh~chare med~um dense or firm the wall dlsplacements m servlce WIU be lumted to 0 5 % of the wall he~ghtThe rnobhzat~onfactor of 1 2 should be used m conjunctmn w ~ t hthe 'unplanned' excavation ~n front of the wall, the rmmmum surcharge loadmg and the water pressure regme, see 3.2.2.2 and 3.2.2.3 A more d e t d e d analys~sof d~splacementshould be performed where tlghter cntena are to be apphed or for soft or loose so~lsThe cntena a) and b), taken together, should provlde a suffic~entreserve of safety a g m t small unforeseen loads and adverse cond~tions In stlff clays subject to cycles of stram, such as through seasonal vanatlon of pore water pressure, the long-term peak strength may detenorate to the cntical state strength The requirements of a) and b) above are suffmently cautlous to accommodate this poss~b~lity 3.2.6 Design values of wall friction, base friction and undrained wall adheston These should be denved from the representatlve strength determmed in accordance w ~ t h2.2.8, usmg the same mobduat~onfactors as for the adjacent so11 The des~gnvalue of the fnct~onor adheslon to be mob~lizeda t an mterface wlth the structure should be the lesser of a) the representat~vevalue determmed by test as descnbed m 2.2.8 d such test results are available, or
b) 75 % of the deslgn shear strength to be mob~l~zed m the so11~tself,that 1s uslng
-
0 75 x des~gntan 9' des~gntan 6 des~gn,c 0 75 x deslgn c, Smce for the so11mass representatlve tan 9' design tan (of 12 tlus IS equwalent to 2 deslgn 6 representatlve 9' 3 sundarly, in total stress analysis des~gnc, 0 5, aftertalongM= 1 5 representatlve c,
(5) (6)
(7)
%
-
(9)
The fnction or adheson, whch can be mob~hzedm practice, 1s generally less than the value deduced on the basis of sou shdmg agamst the relevant surface It is unkkely for example, that a cant~lever wall WIU remaln at constant elevat~onwh~lethe actlve sod zone subs~descreatlng full downward wall fnctlon on the retamed s ~ d eand , the passwe zone heaves creatlng full upward wall frlct~onon the excavated s ~ d eIt IS more hkely that the wall would move vert~callyw ~ t hone or other sod zone, reducmg fnct~onon that s ~ d eand , thereby attanlng vert~calforce equ~hbnum The 25 % reduction m the des~gnshear strength in b) above makes an allowance for t h ~ poss~b~l~ty s Further reduct~ons,and even the ehmmatlon of wall frlct~onor its reversal, may be necessary when sod structure Interaction IS taken Into account Wall fnct~onon the retamed or actwe s ~ d should e be excluded when the wall IS capable of penetratmg deeper, due to the vert~calthrust Imparted by ~nchnedanchors on an embedded wall, by structural loads on a basement wall, or where a clay so11 may heave due to swellmg dunng outward movement of the wall Wall fnctlon on the passlve s ~ d should e be excluded when the wall 1s prevented from s~nkmgbut the adjacent so11 may fail to heave, due for example to settlement of loose granular solls mduced by cycl~cloads, or when the wall is free to move upwards w ~ t hthe passwe sol1 zone, as may happen w t h buned anchor blocks 3.2.7 Design to structural codes The earth pressures to be used m structural des~gn calculations are the most severe earth pressures determmed for serv~ceablhtyh m ~ state, t see 3.1.9 These are the most severe that can cred~blyoccur under the desgn s~tuatlons,see 3.2.2 Accordmgly the apphcat~onof partial load factors to the bendlng moments and mternal forces denved from these earth pressures, IS not normally reqnlred Hamng determined the earth pressures usmg des~gn the structure Increases it should be assumed that
Section 3
BS 8002 : 1994
loads and desgn sod strengths, the structural load effects (bendmg moments, and shears) can be calculated usmg equ~l~bnum pnnc~plesin the usual way w~thoutapplying any further factors Fmally, the materlal properties and sectlons should be denved from the load effects accord~ngto the structural codes Reference should be made to the documentary source for the loadmgs, such as BS 5400 Part 4 for guidance on the respective design values Structural design cslculat~onsbased upon ultlmate h m ~ state t assume that the moments and forces appl~cableat ultunate hmit state are sigmficantly larger than at serviceabil~tylmut state BS 8110 Part 1 and Part 2, BS 5400 Part 4 and BS 5950 Part 1 and Part 5 make this assumption At ultunate lim~tstate, the earth pressures on the actwe or retamed s ~ d are e not a maxrmum Because the structural forces and bendmg moments due to earth pressures reduce as deformation of the most severe earth pressures, whlch are usually determined for the serwceabihty 11mt state, also apply to the ultlmate h m ~ state t structural des~gn calculat~onsThe des~gnat serviceab~l~ty hmlt state for relatwely flexlble structures such as steel or reinforced and prestressed concrete may be undertaken in a hke manner to the analysls in 3.1 to 3.4 of BS 8110 Part 2 1985 For gravity mass walls such as masonry structures, wh~chare relatively n g ~ d the , earth pressures on the retamed or actlve srde are hkely to be higher than the fully active values in the work~ngstate The earth pressures at serviceabdlty and ultlmate hmit states w ~ lbe l s~mdar,because the d~splacementcrltena wrU be sim~lar
3.3 Disturbing forces 3.3.1 General The disturbing forces to be taken Into account m the eqwhbnum calculat~onsare the earth pressures on the actwe or retamed slde of the wall, together with loads due to the compaction of the fill (lf any) belund the wall, surcharge loads, external loads and last, but by no means least, the water pressure 3.3.2 At-rest earth pressures The earth pressures whch act on retainmg walls, or parts of retaming walls, below exstmg ground, depend on the lnitial or at-rest state of stress in the ground For an undisturbed soil at a state of rest, the ratlo of the honzontal to vertical stress depends on the type of sod, its geolog~calongm, the temporary loads wh~chmay have acted on the surface of the sod and the topography For so11 m a state of rest, the ratlo of honzontal to vert~caleffective stress (K,)can be estunated by a vanety of means includmg self-bonng pressuremeter tests, laboratory determmat~onof
so11 suctlon and emp~ncalcorrelat~onswlth in sltu tests includmg statlc cone and ddatometer The value of K, depends on the type of sod, its geoloacal hlstory, the topography, the temporaxy loads w h c h may have acted on the ground surface and changes in ground s t r m or ground water reglme due to natural or art~ficlalcauses Where there has been no lateral stram ~ 7 t the h ground, K, can be equated w t h KOthe coefficient determmable from one-dimensional consohdat~on and swehng tests conducted in a stress-path tnaxlal test uslng appropnate stress cycles For normally consolidated sods, both granular and cohesive KO = l - sm fp' (10) For overconsohdated so~ls,K, 1s larger and may approach the passlve value at shallow depths in a heavlly overconsohdated clay, (see for example Lambe and Whitman, quotlng Hendron and Wroth 1975) K, is not used d~rectlyin earth retmmg structure design because the construct~onprocess always The value of K, is modifies this i n ~ t ~value al however, important m assessing the degree of deformat~onwhich wdl be induced as the earth pressure tends towards actwe or passwe states In normally consohdated sod the ground deformat~on necessary to mobdze the active cond~t~on w ~ lbe l small m relat~onto that requ~redto mobhze the full passwe resistance, wlule m h e a d y overconsol~datedso11 the required ground deformat~onwdl be of sun~larmagn~tude Additional ground deformat~onis necessary for the structure to approach a fa~lurecond~tlonwith the earth pressures movlng further towards their lim~t~ng actwe and passwe values Where a stressed support system 1s employed (e g ground anchorage) then the part~almobd~zationof the active state on the retamed side 1s reversed dunng mstallation of the system and, m the zone of support, the effectwe stress ratlo in the soil may pass through the ongmal value of KO and tend toward the value of K p 3.3 3 Active earth pressures 3.3.3.1 General Active earth pressures are generally assumed to Increase hnearly w t h mcreaslng depth However there may be vanations from a h e a r relationslup as a consequence, for example, of wall flexure This can result in reduced bendmg moments m the structure, where the structure is flexlble Where deformat~onsof the retanmg structure are caused by translent loads, as encountered m hlghway structures, locked-~nmoments may remaln after the load has been removed These locked-in stresses WIU accumulate under repeated loadmg Tlus effect will lim~tthe apphcat~onof reduced bending moments In such structures
Section 3
BS 8002 : 1994
..
The des~gnsod strength, denved in accordance with 3.1.8 should be used m evaluatmg the active earth pressure 3.3.3.2 Cohesionless soils The baslc formula for active pressure is applicable in the followmg sunple sltuatlon - uniform cohesionless sod, - no water pressure, - mode of deformation such that earth pressure increases linearly w t h depth, - uu~formlydistnbuted surcharge only In these restricted clrcumstances, the active pressure at depth z is @ven by an = K ~ ( Y+z Q) (11) where the earth pressure coefficient K, IS based on des~gnvalues of soil parameters The total active thrust normal to the wall between ground level and depth z IS then
Pan
22
=
KayF
+
Kagz
(12)
If there is statlc ground water beneath a water table at depth zw, then for z > zw oan= K,a,,' + u (13) where u = YW(~ (14) then
T h ~ equatlon s is general, it is not hmlted to uruform soils or hydrostat~cwater pressures or to modes of deformation such that earth pressure Increases linearly w t h depth More than one surcharge can be accommodated, but each must be un~formly dlstnbuted Using the desgn soil strength, the value of K, should be d e t e m e d from the graphs in annex A Ka In these graphs 1s the honzontal component In the special case of a smooth vertical wall and horizontally retamed sod surface ( /3 = 0, a = 90°, S = 0), Ranlane's formula may be used
The desgn value of the angle of wall fnct~onto be used in the graphs In annex A should be determmed in accordance w ~ t h3.2.5 Where the ground surface 1s irregular the actlve thrust may be deternuned by the graplucal procedure shown in figure 5 A shp plane 1s chosen and the thrust on the wall is determined from the tnangle of forces The procedure IS repeated w t h other shp planes uutd suffic~entvalues have been obtained to enable the maxlmurn thrust to be found by graphical lnterpolatlon Not less than three planes should be used, but it is not usually necessary to have more than five The posltlon of the centre of pressure on the back of the wall may be taken a s the pomt of intersection ulth the back of the wall of a hne drawn through the centre of gravity of the wedge parallel to the slip plane of the wedge
Figure 5. Graphical determination of active earth pressure for cohesionless soils
Section 3
BS 8002 : 1994
An alternatlve approach is to consider the additional soil mass above a honzontal retamed surface as a surcharge load, see 3.3.4 Where there IS a supenmposed hne load for a considerable dlstance along and parallel to the wall, the weight per unlt.length of this load may be included in the force W in the diagram If there are several dlfferent strata of cohesionless soils behind the wall, the foregoing procedure can be used for the uppermost stratum in contact with the wall and, unless the wall is appreciably m c h e d from the vertical, the active pressures exerted by the lower strata can be calculated from equatlon 1.5 usmg an assumed average ground surface level for the estimation of the effective overburden pressure 3.3.3 3 Clay soils Clays in the long term behave as granular solls exhtbiting friction and dilation If a secant q' value is selected, the procedures descnbed in 3.3.3.2 wlLl apply If tangent parametem (c', q') are to be used, then the active thrust between gmund level and depth z 1s @ven by
Pan
- ji
+
(0'"
u)&
(17)
0
where o n K a - 2cr JK, (18) Equation 17 can be apphed generally for layered solls and irrespectwe of the mode of deformation, provided fully active earth pressures are relevant When a clay soil is subjected to rapid shearmg (see 1.3.16) then it may be assumed to behave m an undramed condition A total stress analysis may then be carned out using design values of the undrained shear strength c, and the undrained wall adhesion c,
-
pan
=
jz(0"
-
~accu)&
(19)
0
where
and c, is the design value of undramed wall adhesion, see 3 2.6 In some circumstances tension cracks may develop in the retained clay sol1 These may become water-filled immediately following a ram storm If there is a tension crack care is necessary in the use of c, The expresson o, - K,, c, cannot be taken as negative Where an Increase in soil volume is possible, e g following deflection of the wall or shrinkage of clay durlng penods of dry weather, any free water will, in time, be absorbed by the clay w t h consequent swelhng and reduction of shear strength towards the crltical state value
[t is usually convenient and practical to consider the loads on retammg walls as unposed by one of two types of clay, normally consolidated clays and overconsolidated clays These have different stress hstones and therefore exlubit dlfferent pore water pressure characteristics dunng shear 3.3.3.4 Normally and lightly overconsolidated clay For normally consohdated and hghtly overconsohdated clay it is satisfactory to use the design undramed shear strength cu, because in a total stress analysis, the short-term condition 1s usually cntlcal, except for excavation in front of embedded walls The strength of normally consolidated clays 1s often sensitive to disturbance Where there is a possibihty of excessive disturbance to the retamed materlal (e g by construction procedures such as nearby pllu~goperations) then the design should be based on the nndramed shear strength of soil remoulded at its natural water content Where the possibility of a tension crack filled wlth water to ground level 1s precluded the design should be checked for the estunated equilibrium conditions using effective shear strength soil parameters in the retamed so11with C' = 0 3 3 3.5 Overconsolidated clay Dunng sheanng of an overconsohdated clay negatlve excess pore water pressures are Induced by dilatlon These gradually reach equfibnum but in so doing the clay will draw in water, swell and soften The short-term stability of an overconsohdated clay whose mass permeabhty is low, of the order of mls or less, and where the consequences of failure are not severe, the pressures apphed by a clay may be based on the design undramed shear strength (c,,) The possible influence of a water-filled tenslon crack should be taken into account Where the mass permeabhty is not of a low order and particularly where the overconsohdated clay is fissured or weathered, the design should assume that negatwe pore water pressures, in the retamed soil, will reach equillbnum ~na short tune, the active earth pressure should be calculated nsmg an effective stress analysls Where the ground surface is irregular or the wall is not vertical, an estunate of the actual earth pressure may be obtamed by the procedure used for cohesionless solls, see 3.3.3 2 The graphical construction 1s shown m figure 6 The position of the centre of pressure on the back of the wall may be taken as the point of intersection, w t h the back of the wall, of a line drawn through the centre of gravlty of the wedge parallel to the fallure surface of the wedge
-
-
Section 3
BS 8002 : 1994
Figure 6. Graphical determination of active earth pressure for cohesive soils 3.3.3.6 Compaction earth pressures A substant~aloverconsohdat~onratlo can be Imposed on a backfill by compactlon Such compaction may lead to an m s ~ t ustress ratlo over the upper part of the wall w h ~ c he s~gnificantly greater than the value of KOfor a normally consol~datedclay and can even lead to values nearly as h ~ g has Kp These pressures can cause deformat~onsand movement of a structure des~gnedfor actlve pressures and, where heavy compaction of a backfill is essent~al,account should be taken of these pressures in des~gn Gudance on the pressure assoc~atedw t h the compactlon of backfill, is given by Broms (1971), Ingold (1979), Symons and Murray (1988), Clayton and Symons (1992) and TRRL pubhcat~onsLR766, LR946 and RR192 3.3.3.7 Weak rocks The active thrust pressures from weak rocks can vary over a w d e range Knowledge of the relat~on between the rock geometry, m part~cularthe d~scontmu~t~es, and the excavation geometry IS essent~alAn analys~sin terms of effect~vestress 1s generally apphcable, slnce most weak or weathered rocks possess a relatwely h ~ g hmass permeabd~ty, but the mfluence of water pressure m d w o n t m u ~ t ~ should es be taken Into account and part~cularlywhere fine gouge m beddmg planes may lead to a perched water table The rock should be cons~deredto have the potentla1 to fall e ~ t h e as r a rock mass or on planes of d~scontmu~ty General gudance cannot be @ven because of the wdely varylng nature of weak rocks Specla1 field and analyt~callnvestlgatlons w~llbe requ~redm the des~gnof any major structure
For mmor structures it IS generally adequate to take a consewatwe approach and treat weak rocks as bemg composed of mterlockmg granular fragments, w t h an effect~veangle of fnction The angle of fnct~ondepends upon the mter-fragment fr~ct~o and n upon the part~clesize of the grams and the mmeralogy of the rock lhble 4 glves effectwe angles of fnct~onfor rocks wh~chcan be treated as composed of granular fragments For major structures an exammat~onof exposures of the rock type should be camed out to determme m part~cularthe stable slope angle and propensity to weather~ngor degradation A non-zero desgn value of effect~ve cohesion, c', may be used where there IS evidence from earth structures in the same geolo@cal formatton m the locahty, not only that it e m t s but that weather~ng or the method of wall construction does not lead to a breakdown of such cohes~veproperties or that the proposed retammg structure WIU effect~vely prevent such degradation Where such mformat~ou w not avalable it 1s adv~sableto use a conservatwe des~gnw ~ t hc' = 0 3.3.3 8 Layered sock Layered so~lsare commonly encountered in the UK The assessment of the resultmg so11pressures may be determmed a s s u m g that the so11 pressures Increase lmearly w ~ t hdepth The sod pressure at the mterface of each stratum change IS calculated by modify~ngthe common overburden pressure at the ~nterfaceby the sod pressure coeffic~entrelative to the soil ~mmed~ately above and below the interface, so as to produce a change in the sod pressures on the wall at the mterface of each stratum, as shown m figure 7 In fact, such sudden jumps in lateral pressures, wh~lst produced by calculat~ons,are unhkely to exist m practlce 29
BS 8002 : 1994
Section 3
Water pressure in tens~on T r a c k s
<, <
= .
Figure 7. Construction of earth pressure dmgrams for earth retaining structures in multi-layered soil 3.3.4 Surcharge loads
3 3.4.3 Concentrated vertical loads and line loads 3.3.4.1 Mcnimum surcharge The lateral thrust mduced bv vomt or h e loads An obhgatory m~mmumsurcharge of 10 kN/m2 actmg on the retained sol1 should be estimated should be applied to the surface of the retamed so11 preferably by plast~canalysis, using d e s ~ n soil In the des~gnof all retamng walls Add~t~onal strengths The influence of hne loads can be found surcharge loading should be used m the design to e ~ t h e by r analyslng the plast~cequhbnum of plane take account of madental loadmg arlslng from wedges (Pappm, 1986) or by consldenng safe constructlon plant, stacklng of matenals and d~stnbutionsof plane stress (Bolton, 1991) The movement of traffic both dunng constructlon and average mfluence of point loads, on a sectlon of subsequently unless the nature or layout of the slte wall, can be estimated safely by consldenng an precludes the need for such addlt~onalsurcharge equ~valentline load actmg on that plane section, The vanous surcharges imposed on a structure may lgnonng support from nelghbounng sectlons be claswfied as Elast~cstress dlstributlons may be used to estmate a ) un~formlydlstnbuted load, conslstlng of a the vertlcal stress Increments due to concentrated contmuous load on the surface of the haclung loads, but the vertlcal stresses should then be (e g roadways, goods stored on quays behmd mcluded m a plast~canalys~susing desgn earth dock walls), pressure coeff~clents b) concentrated loads (e g column footmgs), The extra thrust Induced by a concentrated load may not disappear when the load is removed more C)lme loads (e g stnp footings), than 50 96 of the effect may remaln This hysteresis d) dynamic loads (e g traffic, Impact loads) is the cause of the compaction-mduced stresses, 3.3.4.2 Uniformly distrcbuted loads see 3 3.3.6 Locked-ln stresses, such as compaction stresses, wdl relax if the wall moves away from the The lateral thrust due to uu~formlyd~stnbuted compacted fa, but otherwise they should be taken surcharge 1s assumed to act at the same angle to mto account in determirung load effects on the the back of the wall a? the thrust due to the earth The surcharge pressure IS cons~deredas an l n ~ t ~ a l structure The locked-IUlateral stress is a funct~on of the greatest vertical stress previously induced at overburden pressure at ground level
*
BS 8002 : 1994
Section 3
.
.
that pomt in the sod, Bolton (1991) offers further gu~danceA reasonable estlmate can be obtaned by separately calculat,lng the poss~blesurcharge loadmg cases and deslgnlng for the worst case of a) compaction machme in place, together with locked-ln stresses ~nthe fill beneath, b) future concentrated loads In place, in the absence of compaction-mduced stresses
3.3.4.4 Dynamic loads Dynamlc loads may be due to natural phenomena, for example by earthquakes, or may be man-made by traffic or machmery v~brationsThis dwers~ty precludes the l~stmgof numencal cntena for such types of loadmg It 1s necessary first to assess quahtatwely the vulnerability of the earth retammg structure to the dynamic loadmgs T h ~ sshould establ~shwhether the dynam~cresponse 1s ltkely to be excesswe m terms of e ~ t h e stress r or motlon Factors requmng examination include resonance (or high dynam~c amphficat~on)and undue flex~bhtyof the system H~ghdynam~camphf~catlonmay occur when there 1s a close match between the forcmg frequency and a dominant structural frequency, e ~ t h e for r the whole system or locally w ~ t h ~the n system It 1s undes~rableto have closely matchmg frequenc~es for linklng parts of the retanung structure system Loose sands, sllty clays, saturated silts and flex~ble structures can all result m excesswe d~splacements, even in the absence of h ~ g hamphf~cat~on effects Where dynam~ceffects are not sign~ficant, estimates of peak lateral earth pressures may be made usmg the Mononobe-Okabe approach, see Seed and Wh~tman(1970) Flexible structures may Incur excesswe non-structural damage because of lateral dnft dunng an earthquake Where a retanmg wall, slted on soft or loose soil, may be subjected to significant ground v~brations,then the structure should be provlded w ~ t ha s t ~ f f foundat~onso as to prevent excesswe dlfferent~al movement in the upper part of the structure S~ltsand loose- to rned~um-densesands may undergo hquefactlon dunng an earthquake The depth of potent~alhquefactlon should be assessed for the earthquake cond~tionsappropnate to the slte It may be necessary to carry the foundatmns of the retainmg wall below the llquefactmn zone, or compact the sod wlthm the zone See Seed H B et a1 (1983) and lsh~hara(1993) In dense cohes~onlessso~lsor overconsohdated clay so~ls, severe problems w ~ t hdynam~cloads occur only wlth severe machme vlbrations or earthquakes Numencal desgn cntena are reasonably weU developed for earthquakes and are mcluded in the codes of practice applicable m seismic reaons such as Califorma and New Zealand, see Earthquake reststant regulatzons - A world lzst, IAEE (1992)
Defnut~vecntena for the hm~tatlonof damage to structures from v~bratmnsfrom other sources, such as blast vlbrat~onsand ground motions Induced by explosmns and forced vlbrat~onsfrom rotatmg and Impact machmes are d~fficultto develop, although emp~r~cal and semi-empmcal approaches are ava~lable See Inst~tut~on of Structural Engmeers Structure-so11mteractlon (1977), Downck D J (1977), Blastmg practice (1972), Corbett B 0 (1961), R~chartF E (1960), Alpan I (1961), Barkan D D (1962)
3.3.4.5 Loading resulting from climatic changes The loadmg effects of chmat~cvanat~onsshould be cons~dered,mcludmg a) thermal expansion or contract~on,wh~chmay cause slgn~ficantchanges m strut loads m braced excavations and tle loads In anchored walls, b) expansion of the ground due to freezmg, wh~chmay mcrease the earth pressure on the structure, c) molsture content mcrease in shnnkable clays, wh~chmay cause sweUmg pressures, whde moisture content reductions may cause tenslon cracks whch may lead to a reduction in the stability of the so11 mass, d) abnormal tempemtures m the so11caused, for example by artific~alclunates, such as in boller houses or cold stores These may mcrease the loads to be carned by the retanmg wall or may lead to undesirable deformations in elther the retammg wall or the ground or both Mass gravlty walls, remforced concrete w& and remforced masonry waUs may be damaged by movement of the soil on whch they rest Movement may be caused by frost heave, for example of a chalk sod, or by changes In molsture content where the so11 is a shnnkable clay The foundatmns of such walls should be des~gnedm accordance w t h 3.2.2, but should be carned down to a sufficient depth to reduce the relat~ve movement of the foundatmn to a satisfactory value See also BS 8004 3.3.5 Water pressure 3.3.5.1 General The determmation of the water pressure on an earth retammg structure 1s mportant The assessment of the des~gnlevel of the water table necessitates tak~ngmto account natural vanatlons In the water table, the provision of effectwe dranage and the drainage charactenst~csof any fill and drainage layers provided The mfluence of ramstorms on the seepage pattern behmd the structure should be cons~dered,part~cularlyfor sandy s~ltsand s~ltsSee Terzaghi 1943, Terzagku, I C E Proceedmgs 1939, 12, 106-141, Terzagh and Peck 1967
BS 8002 : 1994
Section 3
3.3.5.2 Water table and seepage forces If the equihbnum level of the water table is well defined and measures are taken to prevent it changmg during heavy m n or flood, the design water pressures can be calculated from the position of the eqwhbnum water table, mak~ngdue allowance for possible seasonal vanatlons, othenv~sethe most adverse water pressure condit~onsthat can be ant~c~pated should be used m deslgn In clay solls the equ~hbnumwater table can be determmed only from piezometnc readings taken over an adequate length of time The water pressure due to the temporary f m g of cracks in clay soils should be allowed for in 'undmned' analyses m terms of total stress The water pressure to be used in effectwe stress so11 analyses should be calculated from the ground water regune in the vlcinity of the structure In weak rocks it will be necessary to measure the water pressures on discontinu~tysurfaces Account should be taken of seepage flow occurnng around the structure where a difference in water pressures is hkely to exlst on opposlte sldes The distnbution of pore water pressures wdl not then be hydrostatic and should be deternuned from a flow net, whch adequately represents the hydraul~cand permeab~htyconditions ln the vlcinlty of the structure, see figure 8 Where layers of markedly d~fferentpermeab~lityexlst the water levels relevant to each permeable stratum should be taken Into account Alternatwely the pore water pressure distnbution can be calculated based on the slmphfylng assumptlon that the hydraul~chead vanes hnearly down the back and up the front of the wall (Burland et al, 1981) Figure 9 shows such a d~stnbutionadjacent to a wall m a so11 of ISOtrOplc permeab~l~ty A h e a r dlss~pationof seepage pressure will gwe reasonably reliable results for retamng walls where the seepage flow upwards through the passtve zone in front of the wall is free
I
to diss~patelaterally as well as vertically This assumptlon should not be made for cofferdams where the w ~ d t hof the cofferdam 1s less than four tunes the d~ferenhalhydrostatic head The concentration of seepage flow from the opposlte s~desof the cofferdam ~nthe relat~velyrestncted passwe zone m the bottom of the cofferdam makes the assumption unsafe, for such cofferdams a flow net should be constructed On the basm of figure 9
3.3.5.3 Drainage The design should take account of the mfluence of the dramage cond~t~ons behmd and through the wall Allowance should be made for possible changes in the ground water levels due to temporary or permanent modification whlch may be made to the water cond~t~ons by the earth retairung structure itself, both permanently and dunng construction It may be necessary to prov~dea dramage layer m order to prevent softening and subsequent loss of strength of coheswe backfill materials, to prevent the Ingress of water mto the f~ssuresformed dunng hot, dry spells or to reduce the effect of frost actlon behind the retamng wall Where the ground water reame IS modified by drams and t h s modificat~onIS assumed In the d e s m to be permanent, the drains should be des~gned,mstalled and mamtamed so as to function in the mtended manner throughout the hfe of the structure Adequate drainage behmd retaimg walls IS Important to reduce the water pressure on the wall, wthout dramage, the water pressure can exceed the effective earth pressure Where there is a poss~b~hty of a hgh equ~hbnumground water table. full water wressure should be taken into acco&t up to the hghest level of the soffit of the dramage outlet
Figure 8. Flow net determination o f pore water pressure
.
BS 8002 : 1994
Section 3
NOTE Assume head difference ( h + r length ( 2 d + h - 2 - 37
-
J?
is d~ss~pated uniformly along flowpath of
Figure 9. Linear variation in hydraulic head On sites where there is a steep and unpermeable slope above the wall, a sudden heavy storm may produce a flow of water down the hdlside If the volume of water flowmg mto a dramage system exceeds that drammg out, then hydrostatic pressure w~llbulld up behmd the wall Where t h s IS likely to happen, full hydrostat~cpressure should be allowed f o r m the design of the wall When the surface of the backfill to the retaming wall IS horizontal and carnes no surcharge, the top layer of f a should have a low permeabhty and slope towards a surface gutter to prevent the saturation of the backfill by run off from ramwater For granular backf* of high permeability, no special dramage layer IS necessary, but some means of draming away any water, whch has percolated through the backfill, should be provided, particularly where the structure IS founded on an impemous matenal For cohesionless backfills of med~umto low permeability (2 x mls or less) and for coheswe soils, it IS usual to place a drainage layer behmd the wall to prevent the bu~ld-upof hydrostatic pressure (see figure 10) This may not be apphcable to basement walls The dmnage layer is usually vertical, it is generally unpracticable to provlde a dramage layer at an angle even when fill IS placed behnd the wall Vanous construction methods may be used for the dramage layer, a s follows a) a blanket of rubble or coarse aggregate, clean gravel, or crushed stone, or
b) hand-placed pervious blocks as dry walhng, or C) a graded fdter dram, where the backfang consists of fine-gmn matenal The graded filter should prevent fine-gram soil from entenng and clogmg the dmnage layer Migration of fine particles may result m undesirable settlement of the adjoining ground, for example behmd the abutments of a road or r d w a y bndge Guldance on the design of filters is gwen in 6.4.4.5 of BS 8004 1986, d) a geotext~lefilter, m combmat~onwith a permeable granular matenal, may be used as an alternative to a graded filter The geotextde should be durable, resfitant to damage dunng mstallat~onand correctly designed w ~ t hrespect to water flow and pore sue Care should be taken to ensure the contmulty of the geotextde filter at connections, e) a geotext~lecomposite (fin-dram), conslstlng of a geotextlle filter futed to one or both faces of a permeable core A geotextlle fin-dram may avold the need to Import granular matenals to the site and may s~mphfythe method of construction The fin-dmn should be durable, resistant to damage dunng mstallat~onand should be correctly designed to possess adequate long-term in-plane flow characteristics when subject to both normal and shear stresses from the adjacent soll or f a The present apphcatlon of fin-drams IS mamly in smaller structures with lower apphed stresses
Section 3
BS 8002 : 1994
Graded f ~ l t e rdram
Concrete or puddled clay
Figure 10. Graded filter drain 3.3.5.5 Waterfront condrtsons
The water entenng the dramage layer should dram Into a dramage system wh~chshould allow free exlt of the water e ~ t h e by r the provlslon of weepholes, or by porous land drams and plpes lad at the bottom of the dramage layer and led to sumps or sewers vla catchplts Weepholes should not be used where this may cause unacceptable d~sf~gurement of the front face of the wall Where weepholes are used, they should be at least 75 mm In dlameter and at a spacing of not more than 1 m horizontally and 1 m to 2 m vertxally Puddled clay or concrete should be placed lmmed~atelybelow the weepholes or plpes and m contact with the back of the wall, In order to prevent the water from reachmg the foundat~ons
Mant~mestructures are subject to the effects of waves, surge tides and flood condltlons Overtoppmg of the wall may occur, resultmg m ~nundat~on of the retamed sods, followed by a rap~ddrop of water level on exposed face The des~gmshould take account of the resultmg hydrauhc pressures and the poss~blebu~ldup of ground water levels behmd the structure The maxlmum out-of-balance hydrostatic head w h c h mav result from a t ~ d avanation. l should be used m thk d e s m , part~cularlywhere the wall can be overtopped See part~cularly4.7.3.6 and sectlon 5 of BS 6349 Part 1 1984
3.3.5.4 Water pressure in tension cracks
3.4 Resistance to movement
Where there 1s a low equil~br~um water table, tenslon cracks may form behmd the retmmg structure m a retaned clay soil and they may extend over the full retamed he~ghtof the clay sod The cracks can become filled with water so that the wall may have to be deslgned to wthstand full water pressure from the surface to the base of the crack The pressure on the wall should be taken to be hydrostatic down to that level where the total soil pressure exceeds the posslble water pressure Where a tension crack may form ad~acentto the wall, the deslgn should be checked as follows a) All clays The end-of-construction condlt~on, wlth soil parameter c,, w t h the tenslon crack fully or part~allyfilled wlth water b) Hard clays o r weak rocks The final equihbnum condlt~on( c ' , p l ) ,with the tension crack fully or partially filled w t h water, to a level h~gherthan the equ~ltbr~um water level
3.4.1 General
Res~stanceto the dlsturbmg forces on earth retammg structures may be provlded by the mobd~zedpasswe sod pressures of an embedded wall, or by a combmat~onof base resistance and passwe so11 pressure on a gravlty or free-standmg structure In a d d ~ t ~ oresistance n may be provided by struts and wahngs m trench excavatlons, by ground anchorages and by the stabihty of the bulldmg m basement construction 3.4.2 Passive earth resistance 3.4.2.1 General
Passwe earth resistance e assumed to mcrease h e a r l y wlth mcreasing depth However vanatlons from a h e a r relat~onshpmay anse as a consequence, for example, of wall flexure or where the wall has an extenswe depth of penetration below dredge or excavat~onlevel in front of the wall
In evaluatmg the reslstance to lateral movement for deslgn purposes, the des~gnsod strength, denved ~naccordance w ~ t h3.1.8, should be used In assessmg the passwe reslstance the des~gnshould mcorporate the obhgatory 'unplanned' excavatlon, see 2.2.2 3 4.2.2 Cohesionless soils The bas~cformula for passlve reslstance IS appl~cableIn the followmg sunple sltuatlon - un~formcohes~onlesssod, - no water pressure, - mode of deformatlon such that earth reslstance increases hnearly with depth, - u~uformlyd~stnbutedsurcharge only In these restricted clrcumstances, the passwe pressure at depth z IS gwen by
The total passwe thrust normal to the wall between ground level and depth z IS then
3.4.2.3 Clay soils 3.4.2.3.1 General Clays, in the long term, behave as granular soils exh~b~t~ fnctlon ng and dllat~on If a secant 9 value 1s selected the procedures descnbed in 3.4.2.2 apply and the values of Kp m annex A are apphcable If tangent parameters (c' , 9') are to be used, then the passwe reslstance between ground level and depth z IS @ven by
where u * ~ ,= K~~~~+ 2 ~ J' K ~
(29)
and the values of the honzontal component of K are gven ~nannex A Equatmn 28 can be apphe8 generally for layered sods and lrrespectlve of the mode of deformat~on,provlded passwe reslstance is relevant When a clay sod is subjected to rap~d shearmg it may be assumed to behave m an undramed cond~t~on A total stress analys~smay then be carned out uslng des~gnvalues of the undmned shear strength c,
If there m statlc ground water beneath a water table at depth +, then for z > r,
where where U
=
r,(z - z ~ ) and c, IS the deslgn value of undramed wall adheslon, see 3.2.6.
then
T h s equatlon IS general, it 1s not h t e d to uruform sods nor hydrostat~cwater pressures nor to modes of deformat~onsuch that earth reslstance Increases lmearly w ~ t hdepth More than one surcharge can be accommodated, but each must be u~uformly d~stnbuted Values of the honzontal component of Kp are @ven m annex A A further approach IS presented by Sokolovsk~(1965) In the spec~alcase of a smooth vert~calwaU and honzontally retamed so11 surface (6 = 0, P = O), the passive earth reslstance coeffic~entK p IS gwen by the Rankme's formula
This formula gwes the same results as the graph for
, - 0,/3
Kp m annex A, w ~ t h
b
P
=
0
3.4.2.3.2 Normally and lzghtly overconsolzdated clay As w t h actlve pressures, the passwe reslstance of clay sods IS cons~deredseparately for normally and hghtly overconsol~datedclays and for overconsol~datedclays Under long-term stress, the posltlve pore water pressures Induced by the shear stresses wlll d~ss~pate, lead~ngto consohdat~onand an Increase In strength of the so11 This wlll be accompamed by wall movement The most onerous cond~t~ou for eqluhbnum wlll generally exst lmmed~atelyafter construction and the desgn should be based on a total stress analys~suslng des~gnvalues of the undmned shear strength c, In some clrcumstances the strength of the clay WIU reduce w ~ t htlme and therefore a check of the des~gnshould be made usmg effectwe shear strength parameters
BS 8002 : 1994
Where excesswe disturbance to the sod may occur dunng constructton or through the mfluence of local works (e g by dnvlng of high d~splacement plles) the des~gnshould be based on the undramed shear strength of the sod remoulded at its natural water content Rel~anceshould not be placed upon the passwe reslstance of sensltlve clays except m speclal cases, e g cofferdams
3.4.2.3.3 Ovwconsolzdated clay In an overconsohdated clay non-un~formstrain of the clay may exlst within the passlve mass resultmg m varying sod strength values Generally, it IS only practicable to make allowances for the complexity of these charactenstics by uslng an approxunate method of analysts Two cases are considered a) Case 1 T h ~ sapphes to structures where there is no change in ground level m front of the wall from that exlstlng for a constderable penod of tlme pnor to constructlon The equhbnum state of stress m the passwe so11 mass IS not agn~ficantlyd~sturbedAn effecttve stress analysls w d take account of the loss of strength created by the dlssipatlon of the negative pore water pressures mduced by shear stram If laboratory test data IS ava~lablethen an appropnate deslgn value for c' may be used but m selectmg a c' value for design purposes account should be taken of possible loss of strength associated w t h wall deformat~ons For short-term temporary works m an overconsohdated clay of known low mass permeablhty of the order of 1 0 ~ 8 m/s or less, a total stress analys~smay be used based on the results of undramed tests on the sod For the upper layers of so11the undramed strength of the clay should be assumed to reduce to zero at the surface The des~gnvalues of the wall fnctlon parameters or the uudramed wall adhesion should be determmed in accordance \nth 3.2.6.
Section 3
b) Case 2 T h ~ auohes s to structures where excavat~onlowerithe ground level m front of the wall below the exlstlng for a considerable penod of t m e pnor to constructlon The soil subject to a passwe pressure in thls case undergoes loss of strength due to the release m overburden pressure T h ~ reduction s IU stress IS in a vertical d~rection,whereas the hor~zontalstress present w ~ t the h undisturbed soil w ~ lbe l augmented by the pressure imposed by the wall As m case 1 an effectlve stress analys~sshould be carned out Effective shear strength parameters should be used and the recommendat~ons regardmg the use of c' are also apphcable
3.4.3 Weak rocks The pattern of d~scontinu~t~es, e g jomts, fissures, etc , which are present wthm the rock mass IS mportant in determ~nmgthe passlve resistance ava~lablem weak rocks as it IS when assessing the actlve pressures The comments and recommendations relatmg to actwe pressures in 3.3.3.7are relevant and apphcable also to passwe reslstance 3.4.4 Layered soils The so11 pressures, at the mterfaces in a layered sod profile, are calculated from the common overburden pressure by applymg the relevant passlve earth reslstance coeff~c~ents for each stratum, as descnbed in 3.3.3.8 for actlve pressures A change m the soil resistance thus occurs at each mterface Examples of the est~mat~on of passive earth reslstance diagrams for vanous strata combinat~onsare gwen in figure 7 3.4.5 Water pressures and seepage forces The recommendat~onsfor water pressure and seepage forces for active pressures also apply to the passwe resistance of clay soils As for actlve pressures, see 3.3.5,the most adverse goundwater condt~onsthat can reasonably be ant~clpated should be used In the estlmatlon of passlve reslstance The mfluence of upward seepage within the passive zone of so11is Important It can reduce the effectwe overburden pressure in the extreme almost to zero
BS 8002 : 1994
Section 4
Section 4. Design of specific earth retaining structures 4.1 Interrelation of section 3 and section 4 4.1.1 General The nrouortlons of the wall should be d e t e n n e d in a&oidance w ~ t hthe requirements for eqwhbrium and deformat~onin sectlon 3 The equhbr~umof the wall should be determmed for the vanous fa~luremodes in accordance w ~ t h section 3, together \nth any further cond~t~ons glven in sectlon 4 for md~v~dual types of walls The des~gnstrengths and serviceabfity 11mts of structural materials should be as recommended in the appropnate structural codes of practice such as BS 8110 Part 1 and Part 2, BS 5400 Part 3 and Part 4. BS 5950 Part 1 and BS 5628 Part 1. Part 2 and &rt 3 4.1.2 Design Structural bendmg moments, shear forces and prop or tie forces should be denved from the equll~bnum calculatlons usmg des~gnearth pressures and water pressures, see sectlon 3 The deslgn sltuatlons, used to check the mtegnty of the structure, at ult~mateh m t state and serv~ceab~hty lumt state, should be the same as those used for the overall equihbrium and deformat~oncalculatlons There may be circumstances where more unfavourable, I e larger actwe earth pressures or smaller passwe resstance, should be used for the deslgn of sections, components, or supports than are used in the des~gnof a wall geometry for overall equhbnum, some of these are referred to m 3.1.9.
4.2 Gravity walls 4.2.1 General Grav~tywalls should be desgned to perform adequately at both semceabfity and ultmate limit states In equlllbnum w ~ t hthe desgn loadmg and design sod strengths m accordance with sectlon 3 The proportions of the wall should be determmed In accordance w ~ t hsectlon 3 See 4.1.1 and 4.1.2 4.2.2 Foundations 4.2 2.1 Bearing capacity design The foundations should be des~gnedfor beanng capaclty at the ultmate l m ~state, t \nth allowance for the mchnat~onand eccentricity of the forces on the fouudat~on,in accordance \nth the standard procedure using N,, Nq and Ny beanng capaclty factors, see Tenagh~and Peck (1967, p 217) 4.2.2.2 Serviceability limit state Des~gnfor the semceabihty limit state may be based on calculations of d~splacementand rotatlon under load Alternat~vely,in sods which are at least f ~ mor - medmm-dense, expenence may md~catethat serviceability can be sat~sfactonly assured by the beanng capaclty calculation, for undramed shear strength calculat~onsa m o b h a t ~ o nfactor greater than 1 5 (see 3.2.4) wdl
be required, in the range 2 0 to 3 0 The pressure on the sod under the toe of the wall should be checked to ensure that it does not exceed the allowable beanng pressure Reference should be made to BS 8004 4.2.2.3 Base resistance to slrding Base reslstance to shdmg should be checked in a d d ~ t ~ oton beanng capac~tyBase reslstance can be expressed either in terms of total stress or effectwe stress r = u' tan 4, where q, and tan Sb are the desgn values of shear strength at the mterface, as descnbed In 3.2.6 and u' 1s the effectwe mean normal stress on the base Any uphft pressures due to seepage should be taken into account in evaluat~ngu' All sods should be evaluated for base shdmg uslng equatlon 33 Granular sods wll behave as fully dra~nedat all times and clay so~lsw ~ leventually l come mto draned equd~bnum,when excess pore pressures arislng dunng constructlon wdl have dissipated and long-term values associated w ~ t h steady seepage w ~ lhave l ansen Where a 1s known or suspected that clay or sllty clay sods may be slow to consohdate, an add~t~onal undmned strength analysls usmg equatlon 32 should be performed unless tests show that the rate of consohdat~onwdl be sufficient to guarantee that the sol1 remams fully dramed dunng constructlon Soft clays subject to extra load~ngwdl tend to consohdate as pore pressures dlss~pate,so theu undramed strength 1s Uely to be more cntlcal than then fully dmned strength St~ffclays subject to excavation will tend to swell and soften as negatwe pore pressures are reheved, so t h e ~ fully r dramed strength wdl be more cntlcal than t h e ~ r undrained strength Part~cularcare should be taken w t h walls bullt on high plastlc~tyclays to determ~newhether res~dualshp surfaces already exlst in the clay due to penglac~alsohfluction or to prevlous landslides Where pre-exlsting shear IS suspected, and a shp surface can be exposed, the res~dualstrength on the suspect surface should be determined in a saturated, dmned shear box test at an appropnate level of normal effective stress The resistance to shdmg of a weak rock IS dependent on the onentatlon of beddmg planes or other d~scontinuitiesm the rock Where cast in situ construction 1s used, the resistance wdl also depend on the bond attamed m fractured or fissured rocks At worst a rock, other than shale, wdl behave as a dense granular so11 w ~ t han angle of sheanng reslstance related to its fragment sue and m m e d composltlon (see table 4) Strongly bedded or fohated shaley rocks may behave as stlff or hard clays, if there is doubt as to onentatlon of the shale laminat~ons,these should be assumed to be the most adverse poss~ble
BS 8002 : 1994
General rules for des~gnfor resistance to slidmg on weak rocks cannot be gwen because of the vanation in matenals For mqor structures or d~fficultgeologcal cond~tions,spec~ahstadv~ce should be obtamed from geotechn~calensneers If the base resistance to shding IS madequate then this should be increased by either w d e n ~ n gthe base, mchning the foundation or prowding a shear key (see figure 11) Shear keys should be located under the rear half of the base If shear keys are prowded then equihbrium of the combmed retammg walllsoll mass above plane BCDE m fgure 11 should be checked taking Into account the extra active pressure on EF and the extra passive pressure on AB Equlhbnum should also be checked above the surface ACDE taking into account the inclined nature of AC and the extra actlve pressure on EF Gu~danceon construction problems wtth inclined foundatlous and shear keys is gven in Hambly (1979) 4.2 3 Mass concrete retaining walls 4.2.3.1 General Mass concrete walls are suitable for retamed heights up to 3 m They can be designed satisfactorily for greater heights, but as the he~ght Increases other types of wall become more economlc The cross-sectional shape of the wall can be affected by factors other than stabihty, such as the use of the space in front of the wall, considerations of appearance or by the method of construction
Section 4
Where an mclmed or battered wall is impract~cable, economy of matenal will result if either the front or the back of the wall IS stepped or inched, see figure 12b, c and d Walls with a nominally vert~cal face should be battered back at approximately 1 m 50 to avold the ~Uus~on of tilting forward In choosing an economical section for the wall, the overall cost of construction should be considered, since the use of simphfied formwork and construction methods may result m a greater sawng in tune and cost than the mere reduction of the volume of matenals m the cross sect~on 4.2.3.2 Q p e s of wall and applzcabilrty Typical profiles of mass concrete walls are shown in figure 12 The simple form, see figure 12a, IS sultable for small retmmg walls, up to about 1 5 m in retamed he~ghtThere is a further economy when the wall is inclined or battered back agamst the backing to such an extent that the resultant compressive stress is u~uformover any sectlon, including the base and foundat~onof the wall Care is necessary to ensure that the wall, in itself, 1s not unstable during construction, see figure 12e A heavy masonry facmg may be used as permanent shuttenng to produce an mtegral mass concrete and masonry wall as in figure 12f 4.2.3.3 Materials Concrete should be generally in accordance \nth BS 5328 Part 1 and Part 2 and w t h sectlon 6 of BS 81 10 Part 1 1985 Aggregates should normally conform to the requirements of BS 882 and BS 1047
bl Shear key
Figure 11. Foundations of gravity walls
c) Mass concrete w ~ t h battered back
e) Mass concrete w~th stepped back and ~nchnedface
d I Mass concrete w ~ t hstepped back
f Mass concrete wall w ~ t h battered masonry face
Figure 12. Basic forms of mass concrete walls
Section 4
BS 8002 : 1994
-
..
4.2.3.4 Design 4.2.3.4.1 Equzlzbrzum of the wall See 4.1.1 and 4.1.2 A wall w ~ t ha stepped back should be des~gnedw ~ t ha 'virtual back' taken as the vertlcal plane from the heel or rear extremity of the base to the surface of the earth baxklng The further following cond~t~ons should be checked In a d d ~ t ~ oton those referred to In 4.1.1 and 4.1.2 a) Structural deszgn Normally mass concrete walls should be desgned on a no-tension basis under the des~gnearth pressures However provided grade 15 concrete or stronger 1s used and constructlon jomts are prepared in accordance w ~ t hBS 8110 Part 1 and Part 2 or BS 5400 Part 1 and Part 4 to transfer tens~leor shear stresses then permiss~blestresses of 0 28 N/mm2 m tenslon and 0 55 N/mm2 m shear may be used b) Foundatzon The pressure on the soil under the toe of the wall should be checked to ensure that it does not exceed the allowable beanng pressure It may be necessary to determ~nethe probable settlement espec~allywhere the wall carrles external loads, see 4.2.2. 4.2.3.4.2 Movement joznts ( e m a m o n and contractzon) Vertlcal jomts should be prov~dedat intervals dependent upon the expected temperature range and the shape of the structure Expans~onjolnts should be llned \nth a r e d e n t jomtlng matenal about 10 mm to 20 mm thlck and sealed w ~ t ha sealmg compound Su~tablelocatlous for joints are changes in level of the foundat~onor the top of the wall Jomts should also be promded where the nature of the foundat~onchanges, e g from one type of so11to another Generally there should be jomts between the wing walls of bndges and bndge abutments For detals of expansion, contraction and movement jomts and t h e ~ spacing r reference should be made to BS 8007 Movement ~ o ~ nint sthe masonry facmg or claddmg should be positioned and detalled in accordance w ~ t hthe recommendatlons of BS 5628 Part 3 or BS 5390 for stone masonry
Improve the fm~shedappearance by breaking the monotony or reducmg the Impact of blem~shesand discolourat~on The treatment of the finished surface by bush-hammenng or other exposed aggregate treatment may also lmprove the finished appearance, but such treatment 1s expenslve 4.2 3.4.5 Masonry claddzng
Where appearance and weathermg quahtles are ~mportant,the wall may be bulk with masonry claddmg separated from the structural wall by cavlty construction, see f~gure13 T h ~ w~ll s allow a certam amount of different~almovement between concrete and the masonry although the latter w ~ l l requlre provision of separate movement Joints Thls form of constructlon should be adopted where the mcompatlbility of masonry used as a stmctural facmg to concrete would set up undesirable Internal stress, but it should not be used where there 1s a nsk of impact damage, for example, where the wall adjolns a h~ghway Wall tles should be cast Into the concrete retamng wall and subsequently bult Into the masonry cladding T ~ e should s be spaced at ~ntervalsnot greater than 900 mm centres honzontally and 450 mm centres vert~cally,alternate rows staggered, and should preferably be stamless steel dovetall slot type w t h 'fishtad' anchors to ensure correct courslng w ~ t hthe masonry BS 5628 Part 1 prov~desrecommendatlons on the use of wall tles 4.2.3.5 Construction 4.2.3.5.1 Formwork
The formwork should be sufficiently rlgd to prevent undue deflect~onof the face and the jomts should prevent loss of grout or mortar from the concrete durlng constructlon
Where a wall is subject to hydrostatlc pressures, hydrostatlc uphft should be taken Into account at constructlon jolnts Honzontal construction joints should be designed to be watertight, but if they cannot be watertight the design should cater for full hydrostatlc uplift
4 2.3.5.2 Constructzon joznts The number of constructlon jolnts should be kept small, cons~stentw t h reasonable precautions agamst shrinkage The spaclng of construction lo~ntsshould have due regard to the economlc slze of the concrete pour and any need for the dissipat~onof the heat of hydrat~on Reference should be made to CIRIA Report No 49 for masslve walls where large pours are used In a stepped profile wall any horizontal Jomts should coinc~dew ~ t hthe positlon of the steps A long~tudmalgroove may be formed to generate resistance to the sheanng force at the joint Vert~calconstruction jomts should be at approximately 10 m centres and should comc~de w t h contracbon/expans~onjoints
4.2.3.4.4 Surface fzntsh Attent~onshould be glven to the surface fm~shof the exposed face of the wall Exposed in wtu concrete requ~rescareful treatment Formwork with a moulded surface, used w ~ t hcare, may
4.2 3 5.3 Concretzng Concrete of low strength, such as grades C7 5 and C10 of BS 5328 Part 1, will normally be sat~sfactoryprov~dedit 1s properly compacted to attaln the necessary deslgn density The
4.2.3.4.3 Ilydrostatu uplzft znjoznts
*
BS 8002 : 1994
Section 4
-.
Damp prmf course d reau~red Cop~ngin reconstructed or natural stone or precast concrete to BS 5642 Part 2
Damp proof course
Concrete or puddle day ~fweep holes used
Concrete or mortar ftll~ng
Figure 13 Masonry clad mass concrete wall with cavity watertcement ratlo should be low, consistent with adequate workability to obtam the necessary compactzon The deslgn or selection of a concrete mlx should provide the necessary nunimum cement content requved for durab~lltyThe cement content should not be less than shown m table 6 2 and table 6 3 of BS 8110 Part 1 1985 If the concrete 1s exposed to attack by sulfates reference should be made to table 6 1 of BS 81 10 Part 1 1985 Where there is hkely to be severe attack from the constituents of ground water, it may be necessary to promde add~tionalprotection to the back of the waU In waterfront structures the wall should have adequate resistance to chlonde attack and may need to be promded w t h a waterproof protection
4.2.3.5.4 Masonry faczng and claddzng Where a structural masonry facing 1s to be mcorporated or a masonry cladding is to be bulk as part of the retairung wall, the mlnimum quahty of the masonry unit and the mortar des~gnation requ~redfor durabil~tyshould be m accordance wlth the recommendations of BS 5628 Part 3 and BS 5390 for stone masonry The cholce of masonry umt and mortar des~gnatlonto be used will depend upon a number of factors ~ncludmgexposure, whether protectwe detahng to the masonry such as coplng and damp-proof course systems are used and ~f effectwe dramage and waterproofmg to the retanmg face are incorporated Where clay brlcks of quality FW and MN are used it may be necessary to use mortar contaming sulfate-reslstlng cement (see also 4.2.4.2 1, 4.2.4.2.2and 4.2.4.2.3)
4.2.4 Unreinforced masonry retaining walls 4.2.4.1 General Cnreinforced masonry IS sultable for small retalning walls, espec~allywhere the fin~shedappearance m important, such masonry' requlres mlnimal construction plant A s~mplestem wall, 1s sultable for small retained heights, up to 1 5 m, for greater he~ghts,a stepped or buttressed wall as m figure 14 may be appropriate The des~gnand construct~onof unremforced masonry should conform to BS 5628 Part 1 and BS 5628 Part 3 The selection and d e t a ~ h gof stone masonry IS gwen in BS 5390 4.2.4.2.1 Brzcks und bnckwork Bnck masonry units should conform to the appropriate Bntish Standards as follows
Calc~ums~hcate(sand-hme and flmt-hme) bricks Clay bncks Dimensions of bncks of specla1 shapes and slzes Precast concrete masonry
BS 187 BS 3921 BS 4729 BS 6073 Part 1
units
Clay and calc~umsillcate modular bncks
BS 6649
Clay br~cksof quality FL, FN, ML, or MN to BS 3921 1985, calc~ums k a t e br~cksof class 3 or stronger to BS 187 1978 or concrete bncks of mlnlmum strength 15 ~ l m m to BS ~ 6073 Part 1 1981 and Part 2 1981 may be used prov~dedthat the following protectwe detads are ~ncorporated Into the wall a) a bnck or slate damp-proof course a t the base of the wall and above ground level conformmg to BS 743 Other damp-proof course materials may be used 8 they can be shown to be su~tablefor apphcat~on,and, b) effectwe waterproofing treatment on and dramage to the retamng (I e back) face of the wall, and, c) an effective coplng w ~ t hdrlps w h ~ c hthrows water clear of the exposed wall surface, coplngs should be frost resistant, and, d) a contmuous impervious damp-proof course below the copmg, (slates, t ~ l e sand damp-proof course bncks are unsu~table) Where protectwe detalhng is not used or where there 1s a nsk of saturated bnckwork being subjected to freezmg, frost-resistant clay bncks (FL and FN quaMy), or calc~umsihcate bncks of class 4 or stronger should be used Where moderately frost res~stantclay bncks (ML and MN quality) are proposed in such sltuatlons, the manufacturer's advice should be sought Concrete bncks should be of mlnlmum strength 30 N/mm2 and have adequate cement content to ensure durablhty
Figure 14. Stepped and buttressed retaining walls in unreinforced masonry
Concrete br~cksshould not be used where sufficiently aggresswe sulfate ground condrt~ons exlst unless they are protected or have spec~f~cally been manufactured for the purpose BS 5628 Part 3 gwes recommendat~onsfor the select~onand use of bnck masonry 4.2.4.2.2 Other masonry unzts When masonry uruts other than brlcks are used these should conform to the appropnate Bnt~sh Standards Stone masonry Precast concrete masonry units Reconstructed stone masonry unlts
BS 5390 BS 6073 Part 1 BS 6457
Protectrve detahng to wall~ngmay be requ~redand some types of masonry unlt may not be su~tablefor use in aggressive sulfate condlt~onsBS 5628 Part 3 and BS 5390 for stone masonry glve recommendat~onsfor the select~onand use of mater~als 4 2 4 2.3 Mortars Mortars to be used should be selected in accordance w ~ t hBS 5628 Part 3 and BS 5390 for stone masonry The cho~ceof mortar des~gnat~on should be made pnnc~pallyon the bass of structural requirements, where these apply, and upon durabil~tycons~derat~ons Where damp-proof course (dpc) bncks are used in bnckwork construction they should be l a d m mortar des~gnatlon(I) and it 1s advisable to use the same mortar for bnckwork below the level of the dpc bncks Where cappmgs to br~ckmasonry are used mortar des~gnation(I) should be used w ~ t h clay bncks, mortar designatron (u) w ~ t hcalclum sil~catebr~cksand mortar des~gnat~ons (I) or (11) w ~ t hconcrete bncks Where masonry unlts contam hgh levels of soluble salts and the walls are hkely to remaln wet for long penods of t m e , for example when wall exposure to wind-dnven ram 1s except~onallyhqh, the mortar should be selected w ~ t hcare Where clay bncks of e ~ t h e FN r or MN qual~ty(see BS 3921) are used m these exposure cond~t~ons the use of mortar contammg sulfate-res~stmgcement throughout the wall constructlon should be considered Sulfates in suffment quantltles to be damaglng may be present m the ground, ground water, aggregates and backfill, protectwe detalmg to masonry where necessary or the use of mortar contammg sulfate-res~stingcement should be considered 4.2.4.2.4 Wall t z e s Where wall tles are requ~redthese should be specified m accordance wlth BS 5628 Part 1
4 2 4 3 Design 4.2.4.3.1Equzlzbrzum of the wall See 4.2.1 and 4.2.2. 4.2.4.3.2 Stmctural deszgn The structural des~gnof the walls should be m accordance wlth BS 5628 Part 1 4.2.4.3.3 Movement joznts Movement jolnts should be mcorporated as recommended In BS 5628 Part 3 Where buttressed walls are used, a buttress should be prov~dedeach s ~ d of e a jomt 4.2.4.4 Construction 4.2.4.4.1 General The general constructlon and workmansh~pof unremforced mass masonry walls should be in accordance w ~ t hBS 5628 Part 3 and BS 5390 as appropnate 4.2.4.4.2 W a ~ r o o f z n g Unremforced masonry walls should be prov~ded w ~ t ha waterproofing membrane to the reta~nmg (I e back) face of the wall The type of membrane to be used w ~ l depend l upon the nature of the retamed matenal and whether there wll be a permanent head of ground water behmd the wall Waterproofing membranes should be adequately protected pnor to and dunng backf~lhng Where movement jolnts are mcorporated, water bars may be requ~redat these jomts if ground water cond~t~ons warrant Water bars should not mterfere w ~ t hthe free actlon of movement jolnts 4.2.5 Reinforced soil Retalmng structures constructed w ~ t h remforcement embedded m the retaned so11 have advantages m embankments because the resultmg vertlcal or steep faces, as compared w ~ t hbattered slopes, lead to a reduct~onm the extent of land requlred Reference should be made to BS 80063) for detaled ~nformat~on on the deslgn and constructlon of structures mcorporatmg re~nforced so11 4.2.6 Gabions 4.2.6.1 General Gab~onsare large cages or baskets usually of steel wlre or square welded mesh, rectangular m shape, filled w ~ t hstone and used to bu~ldreta~nmgwalls, revetments and antl-erosion works, see figures 15 and 16 They can also be made of wlckenvork, bamboo slats, nylon or polypropylene, but there are reported Instances of fire damage to gabron walk constructed from flammable matenals
BS 8002 : 1994
Figure 15. Hexagonal woven mesh gabion cage (typical)
Section 4
.
Section 4
& Sta~nlesssteel clips
The w~dthof the honzontal tread of the steps should not exceed the depth of the gab~on Walls may have plane outer faces, preferably bullt to a batter for appearance and to mcrease the resistance to overturnmg Sunllarly walls wlth stepped faces should be t~ltedtowards the backfill Counterforts or buttresses may be mcorporated in the constructlon In large walls where the cross sect~onIS greater than 4 m w ~ d e an , economy can be made by usmg a cellular form of constructlon The outer and mner gab~onfaces are t ~ e dby bulkheads of gab~ons and the cells between them f~lledw ~ t hstone The slze and shape of the cells should be proport~oned to ach~evemternal stabhty In nvers and in t ~ d awaters, l the permeab~l~ty of a gab~onwall 1s an advantage smce water in the backfill dunng falhng levels, can dram freely The nature of the backfill may necessitate the use of a filter behmd the wall, to prevent the leachmg of fines In cold c h a t e s gab~onsare able to resist the actlon of frost heave The l ~ f eof a gab~onwall 1s not necessarily l ~ m ~ t e d by the effective hfe of the cage or basket if the shape of the wall IS such that the stone f f i n g remams substant~allystable after fa~lureof the cage through corrosion or abras~onof the wlre mesh If soll cond~t~ons are su~tablefor a n@d structure, the wall may be made permanent by groutmg the gab~onwall w ~ t ha cement grout, but t h ~ schanges the nature of the wall
4.2.6 3 Materials 4.2.6.3.1 Hexagonal woven wzre mesh The nettmg w mechatucauy woven m a continuous sheet, to form a hexagonal mesh wh~chcan stretch or contract m two d~rect~ons in its own plane so that a rectangular wlre mesh box filled with quamed stone or nver shmgle can deform m any d~rect~on, see frgure 15
Galvan12edor plastic coated Iacng w e
Figure 16. Welded mesh gabion cage (typical)
4.2 6 2 Types of wall a n d applicability The permeab~l~ty and flexlb~htyof gab~onsmake them sn~tablewhere the retamed matenal 1s hkely to be saturated and where the beanng qual~tyof the soll 1s poor W~remesh gab~onsare of two forms baskets, which are used for walls, and mattresses w h ~ c hare used for revetments and the hnmg of nver and channel banks The bas~cshape of gab~onreta~nmgwalls IS trapezo~dal,but the outer and mner faces may be stra~ghtor stepped, the latter bemg more common
4.2.6.3.2 Welded wzre mesh Welded wire mesh 1s an oblong or square mesh manufactured from cold reduced steel wlre, produced m accordance ulth BS 1052 It should be electncally welded at every mtersection, @vmga mlnlmum average weld shear strength of 70 % of the mmunum ultlmate tens~lestrength of the wwe The welded mesh 1s cut Into panels w ~ t hflush edges to s u ~the t dlrnens~onsof the s~des,top, base and d~aphragms(where necessary) of the baskets, and jomed together wlth stamless steel chps or galvan~zedsprmg steel spht nngs See figure 16 The gab~onsare dehvered to the s ~ t eflat-packed The n@d~ty or flexlb~htyreqn~redby the desgner can be ach~evedby selectmg vanous slzes of wlre gauge, but in general baskets made from welded mesh are less flexlble than a comparable woven wire mesh basket
Section 4
BS 8002 : 1994
4.2.6.3.3 Other meshes Other meshes may be used such as cham hnk, expanded metal and plg nettmg They have one or more of the following disadvantages a) tendency to unravel if one wlre IS broken, b) no selvedged wire so that true rectangular shapes are d~fficultto form, c) low reslstance to corrosion Gabions made on site from rolls or sheets of mesh rarely have d~aphragmpanels Incorporated so that structures bu~ltfrom them are l~ableto progressive fallure particularly should the mesh on the outer surface be ruptured 4.2.6 3.4 Corroszon and damage of gabzons In the use of gab~onsthe followmg matters should be cons~dered a) Unprotected Uncoated wlre gab~onsare normally used only for temporary works, but if the wlre d~ameter1s 5 mm or more, the expected l ~ f of e unprotected steel may be suffic~entfor such gab~onsto be used for certain permanent works b) Galuanzzed wzre Hexagonal woven mesh gabions should be made from wlre galvanued to BS 443 For welded mesh gab~ons,the panels of mesh w h ~ c hform the cages should be hot dip galvan~zedto BS 729 after weldmg Galvanued gablons may be used where the expected hfe of the galvan~zedwlre IS suffment for the mtended hfe of the structure The sod and water w ~ t h whlch the structure IS in contact should be assessed to determme 1) sod res~stivlty, 2) redox potent~al, 3) d~ssolvesalts such as chlonde Ion content and the total sulfate content, 4) pH value, 5) molsture content of the so11 If the cond~tionsare aggresswe to the galvan~zed wlre coatmg, the use of polyvmyl chlonde (PVC) coated wlre should be cons~dered C) PVC coated wzre The PVC coating should conform to BS 4102 The rad~alth~cknessof the coatlng apphed to the galvan~zedwire core should be a mlmmum of 0 25 mm The PVC should he suffrc~entlybonded to the galvan~zed wlre core to prevent a capillary flow of water between the w r e and the PVC coatmg leading to corros~on d) Damuye by ubraszon Galvan~zedand PVC coated wlre may be damaged by abrasion, by movmg shingle in nver beds and on coastal foreshores in mountam riven, where the heavy waterborne mater~alusually travels along the
bed, PVC gab~onmesh has been satisfactory in the constructlon of nver walls w ~ t hvertlcal water faces but antl-scour aprons wlth hor~zontal surfaces should be avo~ded Galvan~zedmesh IS more eas~lyabraded in these sltuatlons On coastal foreshores, PVC coated gabions are unsat~sfactorywhere large shmgle, or heavy abras~vematerial, IS hkely to be thrown agamst, or, washed over the structure by wave action
4.2.6.3.5 Assembly ofgabzon unzts The edges of the end panels and of the d~aphragm panels, where prov~ded,are futed to the s ~ d e sby lacmg w ~ t h2 2 mm mmlmum bindlng wlre, galvaluzed or PVC coated, to match the gab~on mesh 4.2.6.3.6 Gabzon szzes Box gab~onsare normally ava~lablein half metre modules in lengths of 2 m to 6 m, 1 m to 2 m w ~ d e and in depths of 0 3 m, 0 5 m and 1 m Boxes should, where possible, be f ~ t t e dw ~ t h transverse vert~cald~aphragmpanels a t 1 m centres to prevent undue d~stortionand stone mlgratlon 4.2 6 3.7 Stone fzllzng Stone should conform to BS 5390 for hardness, crushmg strength and reslstance to weathenng Naturally occurnng rounded stone or quarr~ed stone are acceptable The lower limiting slze is controlled by the dlmens~onsof the mesh, although 5 % ' may be down to 50 mm to 80 mm To ensure efficient constructlon, the slze should be as small and as unlform as possible, for manne structures 175 mm 1s the usual mlnlmum The maxlmum recommended slze is 200 mm 4 2 6.4 Design 4.2.6.4.1 General Small gabion walls should be deslgned on the same pnnc~pleas a gravlty mass wall, no allowance bemg made for the strength or mass of the wlre mesh, see 4 2.1 and 4.2.2 Examples of gab~onwalls are shown In figure 17 The dens~tyof the stone fill should be taken as 60 % of the sohd matenal 4.2 6 4 2 Equzlthrzum of the wall The retamed soil wlll exert actwe pressure over the entu-e wall height, but w t h no hydrostat~c pressure The cross sectlon of a gab~onwall, as a mass gravlty structure, should be proport~onedso that the resultant force at any horizontal sectlon hes w i t h ~ nthe m~ddlethlrd of that sect~onThe thrust exerted by the backfill on a gab~onwall acts a t an angle to the perpendicular to the wall T h ~ sangle can be assumed to equal the des~gnvalue of m' due to the roughness of the gab~onsurface, w h ~ c hmay be assumed to be a so11to soil f n c t ~ o nsurface
Figure 17. Examples of g a b ~ o nretaining walls When the retained so111s supported by a heel to the wall the so11may be assumed to be a part of the wall and the design assumes a v~rtualvert~cal rear face When calculat~ngthe resistance agamst shding forward the angle of fnction should be taken as that of the foundat~onso11 In accordance w ~ t h3.2.6 and not as that between stone rubble and the soil The gab~onwall can be bu~lton a sloped foundat~on to Increase thls reslstance Checks should be made at selected levels above the base of a gab~onwall, to ascertam that the resistance to shdmg IS suffic~entto prevent shear failure through the wall, lgnonng the effect of the wire mesh
4.2.6.5 Construction 4.2.6.5.1 Posztzonzng cages Empty cages may be placed srngly or joined together in groups Woven wlre mesh gab~onsmay be stretched wlth a small wmch before they are mred to adjacent unlts that have already been filled Underwater gab~ons,by t h e ~ nature, r are pre-filled before they are placed by crane The cages should be t~ghtlyf~lledw ~ t hsome overfillmg to allow for subsequent settlement Honzontal Internal brac~ngwlres should be fitted between the outer and mner faces at 330 mm centres in both woven mesh and welded mesh gab~onswh~chare deeper than 500 mm When filled, the gab~on11ds should be properly closed w~thoutgaps, and w~reddown The vertical jolnts
between ~ndlv~dual umts should be staggered in adjacent courses, to @ve a better appearance and to prevent the format~onof weak vertical shear planes Curves and angles In the face of the structure may be formed by cutting and foldlng the wlre mesh to make specially shaped unlts
4.2.6.5.2 Marzne applzcatzons Where gab~onsare subjected to wave actlon, there should be a min~malamount of movement of the stone filhng lns~dethe baskets The fillmg should be tlghtly packed and the wlre mesh should be taut It IS good practice to open the baskets after a few t~deshave passed through the work and to add stone to make good any settlement that has occurred in the frllmg Any loose stone left over after construction should be removed and not left on the foreshore 4.2 7 Cribwork 4.2.7.1 General Cnb walls are another alternat~veto concrete and masonry mass gravlty walls They are built of ~ n d ~ v ~ dunlts u a l assembled to create a serles of box-hke structures contalnmg su~tablegranular free dralnmg fill, to form a gravlty retaimng wall system The units should be so spaced that the f ~ l l rnatenal is contamed w ~ t h ~ the n cnb, is not affected by chmatic changes and acts in conjunction w t h the cnbwork to support the retamed earth There are two bas~ctypes of cnb walls, t~mbercnbs and remforced precast concrete cnb walls, see figure 18
Section 4
BS 8002 : 1994
a] Front elevation
Double th~cknessof wall lnd~cated thus
urcharge
b l End view on X
Dsmensmns dlustratwe only
Figure 18. Sectlon and elevation of typical crib wall Economy of crib uluts is effected by open-faced walllng The headers are supported on top of the stretcher course The interspace can be planted with rock garden type vegetation which helps to blend the wall into the enwronment By varylng the des~gnof the units, wahngs with a closed face can be achieved Cnb walls are usually bullt to a batter wh~ch should not be steeper than 1 honzontal to 4 vertical Low walls w ~ t ha he~ghtless than their thickness may he vert~cal The thickness of the wall can be vaned by multiples of the module length of the standard header units, the wall reducing in thickness at upper levels as the retainmg force5 dimln~sh
4.2.7.2 Tgpes of wall and applicability 4.2.7.2.1 nmbw m b s These may be bulk usmg whole logs or sawn timbers If whole logs are used it 1s necessary to form plane faces at ihe pomts of contact to dlstnbute the load and ~rovldeanchorage between adjacent members ~ h e d u r a b ~ofl ~t ht e~tmber in relat~onto the ln~tlalcost and requ~redMe of the 3fr11ct uw should br ~.ons~dtm!d\CIIIISI d f ~ c ~ ~ n t (lulnbil~tymi). Iw achievvd I>) ublnr: rl~nbel-\\ h ~ c h I> ~ ~ ; u u ~ nKl ~l y S I ~ ~ I1 I0 Ia~~a,:li I \,y \\.ootl-tlc*~rroy~~~g bc cwosmd ur orgmuwls. timlx,ib h ~ ~ ~grntwlly l r l chc~~~it.allv lur-~rr~ml'c.cl h.1'or.t. hemz I ) u ~ l t111loI he cnb, see BS 5268 Part 5 Timber c n b structures are formed w ~ t hfront and rear stretcher units tied
sp~kesso as to tle together the stretcher courses See figure 19
*
y False
header
1200 mm stretcher
Ooubie th~ckness Batter wall 1 4
Dimensions dlustratwe only
Figure 19. Examples of timber cribwork
Section 4
BS 8002 : 1994
4.2.7.2.2 Concrete m b s Remforced precast concrete c r ~ bunlts can be brought on the slte fully cured ready for use The Infill 1s bulk In as each course is assembled so that the cnb wall 1s a fully operatmg structure, to the he~ghtbu~lt,throughout its constrnctlon The unlts are llrn~tedin sue if they are to be manually placed The use of mechan~calplant enables larger unlts to be used, speeds up the construction and the plac~ngof the fill matenal Cnbwork uses less concrete than a concrete gramty wall and IS qu~ckly constructed
TI@J
To ensure the assembled structure acts as a senes of box contamers, the face stretchers should be pos~tlvelyanchored by ~nterlock~ng headers for the full thickness of the structure and the headers should be allgned vert~callyto transm~tthe load dlrectly throughout the helght of the wall wthout mducmg bend~ngmoments m the supporting stretcher unlts Instead of a back row of stretchers stablhty may be ach~evedby uslng headers 'T'or 'Y' shaped on plan so as to contaln the mfill matenal Examples of remforced concrete cnbwork are shown in figures 20 and 21
LFace
stretcher
b l ALternatlve d e s ~ g nf o r unlts
a ) Rear vlew of assembly
u
Figure 20. Examples of r e!inforced concrete cribwork
Section 4
b i Su8tabIe for wallr up to 2 m wlih noiurcharge
a1 Su8iabie far walls up to 2rn w i h surcharge up i o 25' slope
lunenslons lllustrat~veonly
Figure 21. Further examples of reinforced concrete cribwork -
4.2.7.2.3 Appl.~cabzlzty Cnb walls may be used for permanent and temporary retalnlng walls to embankments, cuttlngs and bndge approaches When used to support an exlstlng slope it IS advisable to construct the wall to the maxunum batter (1 honzontal m 4 vertical) tnmmmg the emsting slope accordingly and buildlng the uruts to it Cnb walls should not be used for retalmg slopes wh~chare hable to shp The excavation for the foundations below the toe of the exlstlng slope may preclpltate the shp and it is ~mpracticalto extend the cnbwork below the level of potential shp planes Cnb walllng can cany surcharged slopes \nth normal angle of repose above the top of the wall Foundat~onsfor burld~ngsor other structures should not Impose loading onto a cnb wall or its foundation Cnb walling IS normally bullt in stralght Lengths, although speclal u n ~ t sare avadable to permlt curvature to a nununum radms of approxunately 25 m in both direct~ons Spec~al unlts are requlred for bonding two walls together at cornem in battered wpll construction
4.2.7.3 Materials 4.2.7.3.1 Tzmber The stresses for different grades of t~mbershould be as recommended In BS 5268 Part 2 4.2.7.3.2 Reznforced concrete Precast concrete unlts should be in accordance with BS 8110 Part 1 or BS 5400 Part 4 4.2.7.3.3 Fzllzng The fill should be durable, mert and free dralnmg A wlde range of matenals is su~tableand locally excavated matenal is normally used Coarse sand, gravel and rock rubble, should be used whenever obtainable, these materials reduce the nsk of d~stortionof the cnbs Measures for preventmg the loss of fill through the openlngs may be necessary Where ground conditions permlt, the welght of the wall structure may be Increased by uslng lean mlx infdl at the base so lun~tmgthe construction to a smgle module thck Where t h s IS done a land dram should be formed a t the rear of the wall and weep pipes brought through the mfill on the face to prevent the bu~ldup of hydrostatic pressure
51
Section 4
BS 8002 : 1994
4.2.7.4 Design
4.2.7.5 Constructcon
4.2.7.4.1 Equzlzbrzum of the wall
4.2.7.5.1 Foundattons
A cnb wall should be des~gnedas a gravlty mass wall, see 4 2 1and 4.2.2 The des~gncross sectlon of the wall should be taken as the area enclosed by the back and front faces of the cnb The weight of the wall should be taken as the weight of the matenal comprising the cnb, together wlth the welght of the filling contamed between the front and back faces The back of the cnb should be cons~deredas the back of the wall The effect of fnct~onon the rear of the wall WIU add to the stabhty and should be mcluded Whdst the effect of vehcular wheel loads can be allowed for in the stabhty calculations ~t IS preferable that roadways should be kept back from the wall a d~stanceof e ~ t h e 4r 5 m or the wall he~ght,whichever is the greater veh~clewheel loads may then be ignored
Where the allowable ground beanng pressure IS adequate the cnb wall may be erected w~thouta separate concrete foundat~ou It should be built off stretcher units set on a granular bed
4.2.7.4.2 Detazlzng of crzb walls
Header unlts should be des~gnedas beams over thelr unsupported length, to carry a load equal to the we~ghtof the superimposed fill with m-um consol~dation The stretchers should be designed to resist bend~ngcaused by the honzontal component of the earth pressure behmd, together with the pressures Induced by the compaction of the fill matenal The connection between the headers and stretchers should be deslgned to remt the reactions from the stretchers mainly by mechamcal interlock which is normally provlded by using recesses or dowels T h s mterlock~ngalso ass~stsassembly by avmg pos~t~ve locat~ondunng construct~on It IS usual to desgn the unlts for the maxlmum loading cond~t~on at the base of the wall and make all the units standard for use throughout the wall The umts should be detailed and manufactured to provlde plane beanng surfaces whlch are suffic~entlylarge to prevent crushmg f a h r e from the loadmg involved after due allowance has been made for reduction in beanng area due to manufactunng and erectlon tolerances, but it may be necessary to prov~dea mortar bed between such beanng surfaces 4.2.7.4.3 Weepholes
Weepholes wdl be requlred if the lnfdl is not free drammg, for example if lean mix concrete lnfill has been used to mcrease stab~l~ty The lnfill zone immediately behind the wall should be bullt w t h free-dranung matenal 4.2.7 4.4 Plantzng Rock garden types of vegetation may be planted after construction A sultable amount of topsoil 1s exchanged w ~ t hthe f ~ land l rammed home so as to key well in and provlde sufficient root anchorage
4.2.7.5.2 Posztwnzng of unzts The first row of stretchers should be pos~tionedto h e and level on the prepared foundat~onand held in place by the lnterlockmg header unlts, the batter should be checked On slopmg ground the foundation should be stepped to follow the slope, the steps bemg spaced to suit the u n ~ module t lengths 4.2.7.5.3 Cmnpactzon and fzllzng The cnb should be filled to the top of each course of stretchers as the erection of the wall proceeds The fill should be compacted to prevent the development of voids and to avold disturbmg the algnment of the crib
4.3 Reinforced concrete and reinforced masonry walls on spread foundations 4.3.1 Rexnforced concrete walls (other than
basement walls) 4.3.1.1 General
Re~nforcedconcrete and remforced masonry retamrung walls on spread foundat~onsare gravlty structures m wluch the stabhty agamst overtum~ng IS provlded by the we@ of the wall together, generally, \nth the welght of the retamed matenal where t h s rests on the base slab The vanous structural elements of the wall are designed to reslst bending 4.3.1.2 Types of wall and applicabilzty
The followng are the main types of wall a) Cantzlever or stem wall (Twalls) A vert~calor inclined slab rnonohthc m t h a slab base (see figure 22) b) Counterfort wall A vertlcal or mclined slab supported by counterforts monohtluc w t h the back of the wall slab and base slab (see figure 23) c) Buttressed wall A vertlcal or mclmed slab supported by buttresses monohthic w t h the front of the wall slab and the base slab (see figure 23) d) Reverse cantzlever wall (L-shaped walls) A vertical or inched slab monol~thicwith a slab base that projects m front of the wall slab e) Precast retazntng wall Retaming wall units, designed as cantilevers are avalable as precast concrete umts, standard slzes are avdable up to 4mlugh
BS 8002 : 1994
Section 4
b l,Reinfoned concrete L -wall
a ) +forced T -wall
concrete '
(may be prov~ded with key)
L -wall with k
Figure 22. Basic forms of reinforced concrete cantilever o r stem wall For he~ghtsup to about 8 m a cantilever wall w generally econonuc, for greater helghts a counterfort wall is more appropnate, othennse the th~cknessof the stem of the cant~leverwall becomes excesswe Buttressed re~nforcedconcrete reta~ningwalls are seldom used The illusion of the retamrung wall t~ltmgforward should be avoided w t h all types of walls, by battenng back the exposed face at approx~mately1 ~n50
4.3.1.3 Materials Matenals for re~nforcedconcrete work should be in accordance with BS 8110 Part 1, BS 5328 Part 1 and Part 2 or BS 5400 Part 4
4.3.1.4 Design 4.3.1.4.1 Equz1zLn"zum of the wall See 4.2.1 and 4.2.2 4.3. 1.4.2 structural de,gn See 3.2.7 Gu~danceon structural deslgn is also Bven in DD ENV 1992-1-1 and Recommendatzons forperrnzsszble stress deszgn of reznforced concrete structures Inst,, Struct Eng (1991) Where re~nforcedconcrete walls relv on other structures for support the construc6on sequence of the structures and the wall and possible changes in use which may affect the supports should be taken lnto account in the des~gn
Section 4
BS 8002 : 1994
a I Relnforced concrete wall
b l Relnforced concrete wall
with counterforts
w ~ t hbuttresses
Counterforts
Base slab
I c I Sectional elevation of counterforted wall
Figure 23. Basic forms of reinforced concrete counterfort and buttressed walls
d) Detazlzng Attention should be gven to the detailmg of appropr~atednps at the top of the retamng wall, weepholes and other polnts where water may accumulate so that the uns~ghtly blemishes on the exposed concrete surface due to ramwater and waterborne detntus, may be avo~ded
4.3.1.4.8 Masonry claddzng Remforced concrete reta~ningwalls may be bulk with masonry claddmg where appearance and weathenng qualities are design factors Where masonry 1s used as a claddmg reference should be made to flgure 13 and 4.2.3.4.2, 4.2.3.4.5, 4.2.3.5.4 and 4 2.4.2 4.3.1.5 Construction 4.3.1.5.1 Constructzon joznts Construction joints should be kept to a minlmum Generally to fachtate constructlon there should be ajoint between the base, or splay and the wall stem, \nth addlt~onalhorizontal jolnts m the wall stem to s u ~ the t hfts of the formwork Vert~cal jomts should be pos~tionedat points of mlnimum shear and at approximately 10 m centres Reinforcement should pass through the jolnts Reference may be made to BS 8007 for detiuls of types of jolnts 4.3 1.5.2 Fbmnwork The design and constructlon of formwork should provide the requlred surface finish and should be appropnate to the method of plac~ngand compactmg The formwork should be sufficiently n g ~ dto prevent deflect~onof the face and t~mber suffic~entlytight to prevent the loss of grout or mortar from the concrete at all stages 4.3.2 Basement walls, excavat~on,support and retention systems 4 3.2.1 Types of wall a n d applicability The construction of basement walls wdl necessitate the constructlon of an ~mtialtemporary support to the earth Face, except where a free standmg earth face is poss~ble It 1s common for basement walls to be close to exsting bu~ldings,or roads The excavatlon techn~quesand t h e ~ effect r on neighbounng structures are important in the design and construct~onof basement walls Figures 24 to 29 mclnswe lnd~catethe mam range of walls and the temporary and permanent support r systems which may be used with t h e ~ advantages and disadvantages Reference should be made to The Deszgn and Constructzon of Deep Basements, Instn Struct Eng (1975) from whch the figures have been taken
4.3 2 2 Design 4.3.2.2.1 Equzlabnum of the wall See 4.1.1 and 4.1.2 The des~gnshould incorporate the restctlons from temporary supporting members wh~chmay be mcluded m the plane of the wall and whose poslt~onmay be vaned dunng constructlon The design should also mclude the reactions from permanent members on the wall such as columns, beams and floor slabs wluch may in a d d ~ t ~ oton vertrcal loads produce moments due to eccentricity of loading or end f m t y Where the basement wall 1s restramed from movement by one or more lateral floor slabs, each slab should be checked for adequate resmance to buckhng Where the basement wall 1s monol~th~c w ~ t ha raft foundatlon there 1s not normally a nsk of overstressmg the sod under the raft, but on soft or loose sods t h ~ sshould be checked Considerat~onshould be gven as to the extent to which 3.2.2.2 IS apphcable to basement wails Where the wall is promded w ~ t ha separate foundat~on,the sod stresses beneath the foundatlon should be checked 4.3.2.2.2 Excavatzon methods and support systems These should be considered at the des~gnstage and in part~cularthe followmg a) The struttmg promded, whether temporary or permanent, should ensure that at all stages of constructlon the stresses in the wall are w~thm the des~gnlunitations The location of permanent floor slab strutting may be different from that of temporary struttmg The design should be checked for both the temporary and permanent cond~t~ons b) For shallow depth basements and to fachtate constructlon it may be advantageous to desgn the retamng wall as a cantdever dunng the construct~onstage and as a propped wall for the permanent constructlon when the ground floor slab or beams have been completed to promde the prop actlon to the top of the wall at ground level c) Ground anchors may be used w t h advantage to fachtate constructlon to tle back the walls of basements instead of using struts, see 4.6.3 Where the basement IS suffic~entlyremoved from slte boundanes and from adjommg buddmgs, a slopmg, free standmg earth face excavatlon may be used The basement retmmg wails are then constructed w t h the excavated area and the floor slab(s) constructed before any backfihg (see figure 30)
BS 8002 : 1994
Section 4
11
" ll
~ e ~ n f o kconcrete d or g r a v ~ t y wall
NOTE 1 Advantages Suitable for large excavatlons m plan rather than m depth Evades ground water problems if sheet plllng can effect seal m underlnng stratum PuOTE 2 Dzsad~antages Slow and radically constrams programme and access Wall has to be self-supportmg to withstand sol1 pressures when dump~ngrrmoved NOTE 3 Source INSTITLITION OF STRUCTURAL ENGINEERS 1975 Deszgn and constructzon of deep basements
Figure 24. Temporary support against central dumping
NOTE 1 Advantages Sultable for excavatlons relatwely large m extent rather than depth Evades ground water problem e sheet p~llngcan effect seal m underlying stratem NCfl'E 2 Dzsadvantages Slow and r e s t r a m construction progmmme WaU has to be self-supportmg agalnst sol1 pressures when basement area ISexcavated NOTE 3 Smcxce INSTITUTION OF STRUCTURAL ENGINEERS 1975 Deszgn and construdum ofbasements
Figure 25. Temporary support by fully braced trench
Section 4
BS 8002 : 1994
3I ,I
Long flylng shores at requred centres Basement construct~on
I
---Vert~cal supports I to shores if requlred
A-5
---
II H
Struts help to reduce span of wal~ngs
bl Plan NOTE 1 Advantages Vanant of figure 43, but suitable for narrower excavatlons NOTE 2 Dzsadvantages Impedes constructmn Incorporation of monltonng jacks more drfflcult than for method shown in flgure 42 NOTE Source INSTITUTIOK OF STRUCTURAL ENGINEERS 1975 Deszgn and constructton of deep basements
Figure 26. Long flying shores across excavations
NOTE I Advantages Sultable for very deep excavatlons - li.ad~t~onalWlth mcorpomtlon of jacks for pre-loadmg can be used where movements have to be restricted to mlnlmum NOTE 2 Dzsadt,antages Slow and very costly partlculary as wldth of excavation increases Constrams constructLon programme greatly because of access difficulties NOTE 3 Source INSTITUTION OF STRbCIZIRAL ENGINEERS 1975 Deszgn and construetzon of deep basements
Figure 27. Fully braced temporary support
BS 8002 : 1994
Section 4
Plates for supporting floor slabs
I Cylinder belled wt to form f ~ n a l foundation base
-1 -2
-3
-.
-4 L
I n - s ~ t uc o d n cast in cylmder shaft
r
Flrst basement floor level wal~ngslab
NOTE 1 Advantages Good for d e e p excavations Affords speedler constructmn On Superstructure NOTE 2 h a d v a n t a g e s Excavatlan and removal of spoil form enclosed area relatively difficult NOTE 3 Source INSTITUTION OF STRUCTURAL ENGINEERS 1975 Dengn and construct~onof deep basements
Figure 28. Concurrent upward and downward construction
Section 4
BS 8002 : 1994
..
Walmg beam at f l r s t
-
Sheet pde wall
base
Wal~ng slabs Inserted as excavat~on proceeds downwards
-
5
6
Lift of r e t a ~ n ~ nwall g b u ~ l tw ~ t hwallng slabJ
G Latt~cecolumn
Fnal column loads t r a r h i t t e d onto raft
L Foundat~onr a f t at s ~ x t h basement floor level
NOTE I Adwantayes Good method for deep excavations Temporary strutting ehmmated Temporary beams ehmmated NOTE 2 Dxadvantages Excavatmn under slabs and removal of spoll relatively dlfflcult NOTE 3 Source WSTITUTIOK OF STRUCTURAL ENGINEERS 1975 Deszgn. and constructzn of deep basemenJs
Figure 29. Floors cast on ground with excavation continuing below
NOTE 1 Advantages For large excavatlons a is fast, cheap and gwes completely accessible site NOTE 2 Dzsadvantages Only pratlcable m relatwely stable sod and requires a large open fleld slte Dewatenng necessary if pemeablhty and water table hlgh NWE 3 Source INS'IlTUTION OF STRUCTURAL ENGINEERS 1975 Deszgn and constmtzon of deep basements
Figure 30. Open cut 4.3.2.2.3 Effect on nezghboumng stmcctures The followmg matters should be cons~deredat the des~gnstage a) Where excavatlons for basements are close to extstmg buddmgs, then some movement, part~cularlysettlement, of the adjouung ground may occur due to mward yleldmg of the excavation face and support system, both temporary and permanent Ground water lower~ngdunng construct~onor ground heave of the basement excavatlon may also cause movement of the adjomlng ground b) The foundations to walls of adjolnlng bulldings may need to be underpmned where they are at a shallow depth and close to the basement excavatlon and wtlun a h e drawn at a slope of 1 honzontal to 2 vertical from the base of the basement excavation c) The excavation and support systems should be des~gnedto ensure that the settlement or lateral y~eldof the surroundmg ground surface 1s wtlun acceptable hmits particularly where the excavatlon adjoins a publlc hghway where dramage, electncity and gas semces are located Surcharge loads, see 3.3.4, should be carefully cons~dered 4.3.2.3 Construction Excavat~onsshould be carned out m accordance w t h BS 6031 4.3.3 Reinforced and prestressed masonry retaining walls 4.3.3.1 General Re~nforcedmasonry IS su~tablefor retalmg walls over 1 5 m h~gh,while prestressed masonry is usually economical for retalrung walls over 4 m
ktgh Both methods of construction prov~dewalk w ~ t hhigh appearance quaht~esand good weathenng capabhty Remforcement provldes flexural tensile capaclty to the wall sectlon and t h s allows add~tionallateral loads to be carned compared to the equ~valent unremforced masonry wall A number of structural arrangements are ava~lable Prestressed masonry IS a technique where precompression 1s mduced In the masonry cross sectlon thereby @wng effectwe flexural tens~lecapacity and enhanced resistance to lateral loadmg Prestressmg IS usually by post-tenwonlng Prestressmg 1s usually carned out m conjunction w ~ t hgeometrical masonry cross sections such as diaphragm walhng The structural des~gnof remforced and prestressed masonry should conform to BS 5628 Part 2 and for general construction and workmansh~pwlth BS 5628 Part 3 or BS 5390 for stone masonry 4.3.3.2 Reinforced a n d prestressed masonry wall types Typtcal examples are a) remforced grouted-cawty (see figure 31), b) remforced Quetta bond (see figure 32), C)re~nforcedpocket-type (see figure 33), d) remforced hollow blockwork (see figure 34), e) post-tensioned d~aphragmwalhng and other geometnc sectlons (see figure 35)
Section 4
BS 8002 : 1994
Figure 31. Reinforced masonry: grouted-cavity construction
Figure 33. Reinforced masonry: pocket-type construction
Figure 32. Reinforced masonry: Quetta bond construction
Figure 34. Reinforced hollow blockwork construction
-
Figure 35. Post-tensioned masonry diaphragm wall construction
4.3.3.3 Materials 4.3 3 3.1 Masonry unzts The select~onand spec~ficat~on of masonry unlts should generally be ~n accordance w ~ t h4.2.4.2 Addd~onalrecommendat~onson masonry unlt surtabhty are provlded by BS 5628 Part 2
damp-proof course at the base of the wall and above ground level confonnmg to BS 743 should be consldered for use wlth bnckwork constructlon Other damp-proof course matenals may be used if they can be shown to be su~table 4.3.3.3.4 Concrete znfzll and grout These should be m accordance \nth BS 5628 Part 2 In some apphcatlons mortar can be used as mfill around reinforcement in remforced masonry dependmg upon the type of steel used and the exposure sltuatlon
4.3.3 3.2 Mortars The recommendat~onsfor mortars should generally n , 5628 Part 2 conform to 4.2.4.2.3 In a d d ~ t ~ oBS recommends that only mortar des~gnat~ons (I) and (11) should be consldered for use in remforced and prestressed masonry Mortar des~gnat~on (111) may be used m walls mcorporatmg bed jomt remforcement to enhance lateral load remtance
4.3.3.3.5 Reznforczng and pestresnng steel These should be selected and spec~fiedin accordance w ~ t hBS 5628 Part 2
4.3.3.3.3 Dampproof courses Care should be taken ~nthe speclficat~onof damp-proof courses for remforced and prestressed masonry so that the structural mtegnty of the walhng IS mantamed The use of a bnck
4 3.3.3.6 Wall taes Where wall tles are requ~red,for example m remforced grouted-cavlty wall constructlon, these should be specified and provlded in accordance w ~ t hBS 5628 Part 2
Section 4
BS 8002 : 1994
4.3.3.4 Design
4.4 Embedded walls
4.3.3.4.1 Equzlzbrzum of the wall See 4.1.1 and 4.1.2
4 4.1 General
4.3.3.4.2 Structural deszgn
The structural des~gnof walls should be in accordance w ~ t hBS 5628 Part 2 4.3 3 4.3 MovemoU joznts
Movement jomts should be mcorporated in reinforced and prestressed masonry earth retauung structures In accordance w ~ t hthe gu~dancegwen in BS 5628 Part 3 The locat~onof movement Joints m relatmn to remforcement and prestressing wlres or ban should be gwen careful cons~derat~on m the wall des~gnand layout 4.3.3.5 Construction 4.3.3 5.1 General
The construction of, and workmanslup for, masonry used m re~nforcedand prestressed masonry retaining walls should be in accordance wlth BS 5628 Part 3 or BS 5390 as appropnate Add~t~onal gu~dancespec~ficto re~nforcedand prestressed masonry 1s gwen m BS 5628 Part 2 Protective detadmg to wabng may be requ~red dependmg upon the type of masonry units selected for the wall constructlon and should be prov~dedm accordance w ~ t hthe recommendat~onsof 4.2.4.2.1 and 4.2.4.2.2
Embedded or sheet walls are bullt of contiguous or mterlockmg ind~v~dual p~lesor d~aphragm waU-panels to form a continuous structure capable of retauung so11 and to some extent, water P~les may be of t~mber,concrete or steel and may have lapped, V-shaped, tongued and grooved or mterlocking jolnts between adjacent plles D~aphragmwall-panels are formed of re~nforced concrete Embedded retamng walls may be cantdever, anchored or propped structures, see figure 36 Cant~leverwalls denve t h e ~ eqn~hbnum r from the lower, embedded depth of the wall, anchored or propped walls denve thelr equihbnnm partly from the lower embedded depth of the wall and partly from an anchorage or prop system wh~chsupports the upper part of the wall Spec~alcons~derat~ons, m respect of d~splacement and movement, apply to walls wlth multi-level supports 4.4.2 Types of wall and applicabil~ty
Cant~leverretammg walls are su~tablefor only moderate helght It 1s usual to hit the maxunum he~ghtof such sheet p~lecant~leverwalls to 5 m, but even t h s may be excessive where soft or loose sods occur in front of the wall Stdfer cant~lever walls, of concrete or steel mcludmg d~aphragm walls and heavy compowte walls, may be 4.3.3.5.2 Concrete znfzllzng and grmtzng satisfactory to he~ghtsof 12 m provldmg the Where cavltles or vo~ds,contalnlng remforcement, ground 1s strong enough to gve adequate support are to be filled w ~ t hconcrete ~nfillor grout, the The deflections at the head of a cantllever wall are remforcement should be properly located and sigmficant It IS preferable not to use cant~lever mfibng should be carned out in accordance w t h walls when semces or fonndat~onsare located the recommendat~onsin BS 5628 Part 2 There wholly or partly w t h m the actwe zone smce are two methods of mfilhg usmg e ~ t h e low-hft r or horizontal and vert~calmovement in the retamed high-l~fttechn~quesW ~ t hsome types of re~nforced mater~almay cause damage masonry wall constructlon (e g grouted-cav~tyand Quetta bond constructlon) add~t~onal care 1s needed Anchored or propped walls may have one or more to mantam cavltles and vo~dsclear of debns and to levels of anchor or prop in the upper part of the wall They can be desgned to have fixed or free pos~t~on remforcement correctly m order that earth support at the bottom, they denve theu effnent mfilhng can be achleved If mechanical stab~l~ty mamly from the anchorages or props They compaction of mnfdhng 1s carned out t h ~ should s are common m cofferdams with several levels of avo~dd~srupt~on of e ~ t h e the r masonry or the supportmg frames, see 4.5 remforcement For anchored or propped walls m the free earth 4.3.3.5 3 Reznfmcement and wall tzes cond~t~on, the penetration of the plles should be Mam and secondary remforcement, bed Jolnt des~gnedso that the passlve pressure in front of remforcement and wall t ~ e where s used, should be the p~leswdl resm the forward movement of the correctly located and remforcement should be toes of the p~les,but w~llnot prevent rotatlon The w~red-mwhere necessary Where d ~ s s ~ m ~steels lar plles are supported by tles at the top of the wall are used in the same constructlon they should be and the so11 at the base of the wall, in a manner posit~onedso as not to be m d~rectcontact slrndar to a vert~calbeam w ~ t hsrnple supports In tk~e fixed earth cond~t~on further penetration of the 4.3.3.5.4 Wateqn"oofzng p~les1s requ~redto ensure not only that the passlve The recommendat~onsIU 4.2.4.4.2 for unre~nforced pressures at the front of the wall reslst forward masonry walls apply to relnforced and prestressed movement but that the rotahon of the toe IS masonry retamlng walls restramed by the development of passlve pressures
sorls the calculations are based upon the undramed shear strength of the clay so11wth & = 1 - (4c,,/lyH) for soft to med~umclay The diagram for st~ff fissured clays is tentatwe, the lower pressures are relevant only when movement can be kept to a mlnimwn and construction t m e e short The Terzaghi and Peck (revised) (1967) method should not be used to calculate the assoc~atedp~lebendmg moments The moments can be denved sat~sfactonlyas shown in figure 38 For walls w ~ t h two levels of anchorages, a deslgn method based on assummg an equwalent anchor (B J Jack, 1971) has generally produced conservatlve des~gns
near the toe at the rear of the wall The provlslon of a simple support at the top due to the anchor or ties and a fixed support due to the so11 at the base of the wall is s~rmlarto a vert~calpropped cantdever 4.4.3 Des~gn 4.4.3.1 General The des~gnprocedure recommended 1s based on the calculat~onof the des~gnearth pressures as descnbed in 3.1.9 R-ad~t~oual methods of deslgn for embedded walls have been w~delyused, but these methods all have recogn~zedshortcommgs These methods are outhned in annex B and comments are Included on the apphcabhty of each method
4 4.3.3 Bendsng moment reduction The simpllfy~ngassumption made m des~gn concernmg the h e a r Increase \nth depth of actlve pressure and pasave resistance, takes no account 4.4.3.2 Equslibrium of the wall of the interaction between the soil and the See 4.1.1 and 4.1.2 structure Numerical studles (Potts and The general procedure IS first to determme the Four~e,1985), and model tests (Rowe, 1952), have depth of penetrat~onto ensure overall equ~l~bnum shown that this can have a s~gn~ficant ~ntluenceon of the wall and the so11 and secondly to determme the d~stnbutionof earth pressures in servlce and the structural des~gnof the wall to reslst the on the result~ngbendmg moments and prop forces Imposed loadmgs The d~stnbut~on of earth pressures is affected by Where the retammg wall 1s supported by a the deflected shape of the wall whlch 1s a function multl-stage support system, the loads on the of the flex~bilityof the wall relatwe to the sol1 supported system of struts or anchors may be This red~stnbut~on results In an mcrease of denved from the trapezo~dddlstnbutlon of actwe d~sturbmgforce at the wallng and at the toe of the pressure shown in figure 37 T h ~ smethod should wall and reduct~onof d~sturbmgpressure m the be used withm the lnmts gwen in Terzagh~and centre of the span For a relatively flex~ble Peck (1967, p 396 et seq) In add~tlonto the loads structure, such as an anchored sheet plle wall m due to earth pressures in granular matenals water dense sand, the effect of wall deformat~onw ~ l l pressure and pressures due to surcharge loads enhance the pressure actlng above the anchor should be added In granular matenals K, is posltlon w ~ t hreduced pressure belund the wall at determmed from the graphs tn annex A in clay lower levels, where the greatest deflectrons occur
SKmmZa \ \
Deadrnan
\
\ \ \
\
, \-, \
Alternative ground anchor
a1 Cantilever wail
bl Anchored wall
Figure 36. Types of embedded retaming wall
\\
1 ,
? "
cl h p p e d wall
Apparent earth pres
bI Sand
Softlmed~um day
S t ~ f fclay
Figure 37. Active pressure diagrams relating to maximum strut loads m braced earth retaining structures NOTE S m m Sozl Meckanzcs zn Engznemng Praetux (second edltlon) Terzaghi 62 1967 Repnnted by pemlsslon of John Wiley & Sons Inc
BS 8002 : 1994
Section 4
-
-
, Free w fixed earih support l a 1 Exra\ai>on for f i a r n e l l l
( e l Constiuctm beqns Frame 141 ~ n i i a l l e d
Ib l Frame Ill ~nsialled Excavat%onfor frame121
iil FiameslllandI21 m t a l l e d
i f l C o n ~ t r u c i mm n t ~ n u e ~ Frame (31' inrialled Frame 131 removed
l g l C o n r t r u r i m ronimoer Frame 121' m l a l l e d Frame 121 removed
Exravai>onfar frame131
I d 1 Frameill1 121 and 131 installed Flnal e x m a t t o n
Ih i Construti>on ronilnuer Frame ill' m i a i l e d Frame Ill removed
Figure 38 Illustration of method of calculation of bending moments and frame loads by successive stage analysis in cofferdams The r e d ~ t n b u t ~ oofn pressure results m a reduct~on of bendmg moment in the p ~ l eIn front of the wall the passwe resistance may equal or exceed the theoretical passwe values close to ground level and may decrease w ~ t hdepth towards the toe of the wall The nett result IS a reduct~onIn maxlmum bendmg moment compared w t h des~gnbased on a hnear mcrease In l~mltmgpressure w ~ t hdepth T h ~ s IS however, accompamed by an mcrease m the anchor loads A des~gnapproach by Rowe for sheet p ~ l ewalls IS descnbed by Barden m Part JII of CIRIA techn~cal note 54 (1974) Rowe's method resulted from an extenslve senes of model tests, (Rowe, 1952 and 1957) I t enables account to be taken of the flextb~htyof the wall and the st~ffnessof the proppmg system Compansons between measured and desgn values of maxlmum bendmg moment and prop force for a temporary anchored sheet pde wall m granular sod are gwen by Symons et a1 (1987)
should be allowed in the des~gn No red~stnbut~on of cant~leverwalls, for sltuatlons where e ~ t h e the r structure or the retamed so11 wll be suh~ectto v~brat~on or large Impact forces, for p ~ l e backfilled i after dnvmg or for pressure due to water In these circumstances the pressures actmg on the wall wlll be h~gherthan the actwe earth pressures Where there are dlffenng so11 strata, moment reduct~on should be apphed w ~ t hcautlon, smce so11 archmg IS less ldiely to be estabhshed through strata of varylng strengths Moment reduct~onmay be apphed where the wall IS free to deflect, w ~ t ha ngld wall no moment reduct~onshould be made No moment reduct~onshould be made where excesswe y~eld~ng of the anchorage system or movement of the p ~ l etoes may occur The reduct~onm bend~ngmoment IS accompamed by an mcrease In the forces m the anchorage system, see 4.5.2.2
4.4.4 Steel sheet piling 4.4.4.1 General A comparat~velysmall d~splacementof sod IS caused dunng dnvmg and suitable sectlons can be drwen into almost any so11 except strong rocks Reference should be made to grade 5275P for the properties of mlld steel and grade 5355P for hlgh y~eldsteel to BS EN 10025 19904) Where steel sheet p~llng1s manufactured to other standards, care should be taken that the des~gnstresses to be used are compatible w ~ t hthat part~cularquahty of steel 4.4.4.2 Destgn The structural design of the steel sheet p h g should be in accordance wrth BS 449 Part 2 The allowable stresses gwen therem may be increased by 12 % for temporary works of short durat~on The desgn of sheet plllng is linked drectly with dnving considerat~ons(see 4.4.4.4.1) Where, because of the s o h to be penetrated, there IS hkely to be hard or difficult drivmg then the plles should be des~gnedon the bass of a free earth support T h ~ sw~llglve the combmed benefits of a heav~er sectlon to fac~htatednvmg and a shorter requred penetratlon Into the hard mater~al In determmmg the sectlon needed, the th~cknessof a pile may have to be increased to allow for corroslon The calculat~onsshould consider the bendmg stresses and corroslon at several levels to determme the sectlon of p~hngneeded See 4.4.4.4.3.5 4.4.4.3 Section modulus of steel sheet prling Sectlons of 'Z' type steel sheet pdmg, wh~chhave t h e ~ rmterlocks in the flanges, develop the full sectlon modulus of an und~vldedwall of pdmg under all condit~onsSect~onsof trough type steel sheet p111ng w ~ t hclose-fittmg mterlocks along the centre hne or neutral a m of the sheet~ngdevelop the strength of the combmed sectlon only when the p ~ l ~ nisgfully drwen Into the ground Loads on the supportmg system may be denved from the d~str~bution of actwe pressure as shown m figure 37 based upon the Terzagh~and Peck (rev~sed)(1967) method The shear forces in the Interlocks may be cons~deredas res~stedby fnctlon due to the pressure at the wahngs and the restmnt exercised by the ground In certam conditions it 1s advisable to connect together the inner and outer plles in each palr by weld~ng,pressmg or other means, to ensure that the interlock common to the p a r can develop the necessary shear resistance Such cond~t~ons anse when a) the plhng passes through soft clay or water, b) the pthug is prevented by rock from penetrating to the normal depth of cut-off, c) the p~hngis used as a cant~lever, 4 J Tnese
1990
d) the pllmg IS supported by props or struts but is cant~leveredto a substantial d~stanceabove the h~ghestwahng or below the lowest walmg If any of these condit~onsanse and the pars of piles are not connected together as descnbed, a reduced value of th? sectlon modulus of the combined sectlon should be used m accordance with the manufacturer's recommendat~ons 4.4 4.4 Constructron 4.4 4 4.1 Dmvzng sheet pzles The stresses wh~chd l be imposed on the pies dunng dnvmg should be cons~deredwhen the sheet plle wall IS bemg deslgned P ~ l esectlons requu-e suffic~entdnving strength, othenv~sethey may suffer damage dunng mstallat~on,or a may be ~mposslbleto dnve the p~lesto the requlred depth and expensive delays may result Sheet plles may be mstalled by impact, v~bratoryor hydraul~cdr~versand dnvmg stresses w~llvary, dependent on the type of dnve used Impact methods generate the htghest stresses dunng mstallation, but have the advantage of bemg sultable for all soll types V~bratoryand hydrauhc pressmg systems do not Impose h ~ g hpeak stresses m the p~lesdunng mstallat~on,but are not fully effectwe m certam types of sod Unless the soil cond~tionsare consistent, some dnv~ngwith mpact hammers may be needed Panel dnv~ngof a senes of sheet plies mterconnected to form a panel, IS advisable as it ensures the maxlmum control of hne and verticahty and reduces dr~vmgresistance to a minlmum Further gu~danceon dnvmg practlce is Bven m BS 8004 A hghter section of sheet pile may be acceptable for structural purposes for soils w ~ t hh ~ g hvalues of mternal fnction or cohes~onas they apply smaller actwe pressures to the wall than weaker solls However stronger s o b generate more resistance to dnwng and consequently, a p ~ l e w h c h 1s sutable for structural requirements may be mcapable of withstandmg the dnvmg forces The c n t e r ~ afor adequacy of the pile section are that the p~lehead should not be damaged by the hammer impact, the plle shaft should sustam the dnwng force w~thoutbuckhng and the plle toe should not be damaged by the soil resistance Dnvmg forces vary \nth the dnvmg method, but table 5 , wh~chis based on unadjusted blow count values N obtamed in the standard penetratlon test, may be used as a gude m the absence of more detailed knowledge This gmde is based on expenence with p~lesof Bnt~shmanufacture and of approximately 500 mm width, dnven wlth impact hammers and usmg the panel dnvlng method of mstallation Sectlons of greater w~dth,or those dnven by other methods, may requlre somewhat heavler sectlons than those
were previously referenced as grades 43A and 50A to BS 4360
1990 and grades Fe 430A and Fe 6lOA to BS EN 10026
BS 8002 : 1994
Section 4
'hble 5. Selection of pile size to suit driving conditions m granular sods using impact hammers D o m m n t SPT N Value
d
b n u n u m wall modulus cm3/m Grade 5276P m l d steel to BS EN 10025 1990 ~
~~~
450
Recommended maw~mum d n n n g length m Grade 5356P hlgh y ~ l stee' d to BS EN 10025 1990
1
81 to 140
14200
30 +
KOTE I N is the standard penetratlon test (SPT) blow count Dommant means the hgh avera
for the sods Where plles are to be dnven only to a toe hold in rock, the SPT value should be dwlded by a factor of 4 for that St1ratum only
NOTE 2 For SPT values exceeding 50, pde damage, declutchmg andfor refusal may occur Addmonal conslderat~onshould be glven to the presence of cobbles or boulders, whlch may glve nse to obstructed dnvmg, damage andlor declutchng
ind~catedin the table Bble 5 IS based on the fact that in granular soils the major part of the resistance to penetration results from point resistance at the toe of the plle Shaft fnctlon w ~ t h the surround~ngsod contnbutes relatively httle to the overall resistance to p~lepenetratlon The r e q m d sectlon is, therefore, related to the density of the sods bemg penetrated by the pile toe at all stages of the dnve, the length of embedded shaft hawng only a small mfluence For cohes~vesolls, the reslstance to pile penetratlon results pnmanly from shaft adhes~onw ~ t hthe clay sod, there is v~rtuallyno point reslstance to the toe of the pde The overall resistance IS, therefore, a funct~onof the undramed shear strength of the so& the penmeter dimens~onof the plle sectlon and the length of pde shaft embedded m the ground A p ~ l ednver w ~ t hsufficient power to overcome t h s resEtance wfl be necessary to advance the plle Damage to the pile toe IS far less hkely than when penetratmg granular sods lhble 6 IS a gu~dewhen no other informat~onor expenence IS avadable 4.4.4.4.2 Weldzng
Fabrication of spec~alsheet plles such as corners, junctions, closure and tapers may be camed out by shop or s ~ t welding e All fabncat~onsshould conform to the appropnate Bnt~shStandards Site condit~onsmay r e q w e add~tionalprocedures and quahty control to ensure comphance to these standards
Pre-heatmg of the plles pnor to welded fabricat~on should be cons~deredif they are subsequently to be dnven on site dunng low temperature cond~tions T h s precaution 7N1U avo~dposs~bleembnttlement effects in the area local to the weld 4.4.4.4.3 Corroszon and protectzon of steel pzlzng 4.4.4.4.3.1 General
In many circumstances, steel corroslon rates are low and steel pll~ngmay be used for permanent works m an unprotected condit~on The degree of corroslon and whether protectlon 1s requlred depend upon the working enwonment wh~chcan be vanable, even wthm a s~nglemstallatlon Underground corrosion of steel p~lesdnven into undlsturbed sods IS neguble, lrrespectwe of the so11type and charactenstics the ~nwgnificant corroslon attack IS attnbuted to the low oxygen levels present m undlsturbed s o h For the purpose of calculations, a maxlmum corroslon rate of 0 015 mmls~deper year may be used In recent-fill soils or mdustnal waste so~ls,where corroslon rates may be hlgher, protectwe systems should be cons~dered 4.4.4.4.3.2 Atmosphmc corrosLon Atmosphenc corroslon of steel m the UK averages
approx~mately0 035 mmlside per year and t h ~ s value may be used for most atmosphenc environments
Section 4
BS 8002 : 1994
..
/ 'lhble 6. Selection of vile size t o suit driving conditions in cohesive solls I Clay descrmtxon
Soft to firm Flrm F ~ r mto stlff Stlff Very stlff Hard (c, z 200 kN/mm2)
1
1
1
Mmmum wall modulus,
crn3/rn Grade 5275P mlld steel to BS EN 10025 1990
Grade 5355 P hlgh y ~ e l d steel to BS EN 10025 1990
450 600 to 700 700 to 1600 2000 to 2600 2600 to 3000 Not recommended
400 450 to 600 to 1300 to 2000 to 4200 to
600 1300 2000 2500 5000
Maxlmum length rn
1
NOTE The a b ~ h t yof plles to penetrate any type of ground depends upon attention bemg w e n to good pile dnvmg practlce Tables 5 and 6 assume such good practlce
4.4.4 4 3.3 Cwoszon zn fresh waters
Corros~onlosses in fresh water lmmerslon zones are generally lower than for sea water so the effectwe hfe of steel plles is normally proport~onatelylonger However, fresh waters are vanable and no general adv~cecan be gwen to quantlfy the lncrease In the length of hfe 4.4.4.4.3 4 C o r n o n zn marzne envzmnments Marme environments may include several exposure zones w ~ t hd~ffermgaggressivlty and d~ffenng corroslon performance a) Below the bed level Where piles are below the bed level httle corroslon occurs and the corroslon rate gwen for underground corrosion 1s apphcable, I e 0 015 mmls~deper year b) Seawater zmmerszon zone Corros~onof steel p111ng m lmmerslon cond~t~ons 1s normally low, wlth a mean corroslon rate of 0 035 mm/s~deper year c) Tzdal zones Manne growths, m t h ~ zone, s @ve slgn~ficantprotection to the piling, by sheltenng the steel from wave actlon between tdes and by hm~tmgthe oxygen supply to the steel surface The corroslon rate of steels in the t ~ d azone l 1s s~mllarto that of lrnmerslon zone corroslon, 1 e 0 035 rnm/s~deper year Protection should be prov~dedwhere necessary, to the steel surfaces to prevent the removal or damage of the manne growth d) Low water zone In t~dalwaters, the low water level and the splash zone are reaons of h~ghestthckness losses, where a mean corroslon rate of 0 075 mmls~deper year occurs Occas~onally,hlgher corroslon rates are encountered at the lower water level because of specific local cond~tlons e) Splash and atmmphmc zones In the splash zone, wh~chIS a more aggressive enwonment than the atmosphenc zone, corroslon rates are sim~larto the low water level, i e 0 075 mm/s~de per year In t h ~ szone th~ckstrat~fiedrust layers may develop and at th~cknessesgreater than
10 mm these tend to spall from the steel, espec~allyon curved parts of the p~lessuch as the shoulders and the clutches Rust has a much greater volume than the steel from which it is denved so that the steel corroslon losses are represented by some 10 % to 20 % of the rust thckness The boundary between the splash and atmosphenc zones 1s not well defined, however, corroslon rates d ~ m m e hrapidly w ~ t hd~stance above peak wave he~ghtand the mean atmosphenc corroslon rate of 0 035 mm/s~deper year can be used for t h ~ szone 4.4.4.4.3.5 Methods of zncreaszng effectzve lzfe The effectwe Me of unpalnted or o t h e m s e unprotected steel pihng, depends upon the combmed effects of nnposed stresses and corroslon Where measures for mcreasing the effectwe Me of a structure are necessary, the f o l l o m g should be cons~dered a) Use of a heavzer sectzon Effectwe hfe may be mcreased by the use of adbt~onalsteel thckness as a corroslon allowance Max~rnumcorrosion seldom occurs at the same posltlon as the m a m u m bending moment accordmgly, the use of a corrosion allowance IS a cost effectlve method of mcreaslng effectwe hfe b) Use of a hzgh yzeld steel An alternatwe to usmg rmld steel m a heavler sectlon IS to use a hgher y~eldsteel and retam the same sectlon Although both types of steel have similar corrosion rates, the use of grade Fe 510A steel to BS EN 10025 1990 at BS EN 10025 1990 Grade Fe 430A stresses wdl allow an add~t~onal 30 % loss of perm~ss~ble thickness to be sustamed wthout detnment T h ~ method s m effect, blulds in a corroslon allowance and gives an Increase of 30 % in effective hfe of a steel pdmg structure for an mcrease of about 9 % In steel costs c) Organzc coatzngs Steel pdes should normally be coated under shop condltlons F'amts should be apphed to the cleaned surface by arless
Section 4
BS 8002 : 1994
E
Chamfer - tongued jolntlng
4
Made -up sect~onfrom planks
L = t / 3 ~ 5 0 m m
I Tlmber well splked or bolted together I Figure 39. Typical sections of timber sheet piles
I
1
Groove
Figure 40. Detail of driving edge
-
r
Tongue
4.4 6 Reinforced and prestressed concrete sheet
piles 4.4.6.1 General
Where concrete sheet pdes can be installed to ach~evethe requ~redground penetration they merlt conslderat~on,jettlng or prebonng can be used m suitable ground to ass~stthe dnvmg Good durab~lrtyand appearance can be obtaned, particularly w ~ t hprestressed concrete and precast product~onmethods The general requlrements for matenals, deslgn details and the manufacture and dnwng of remforced and prestressed concrete p~lesgwen in 7.4.2 and 7.4.3 of BS 8004 1986 are equally apphcable to remforced and prestressed concrete sheet p~les 4.4 6.2 Remforced concrete sheet piles The followmg recommendatlons apply to remforced concrete sheet p~les a) General The des~gn,manufacture and handhng should be m accordance w ~ t h7.4.2 of BS 8004 1986 b) Concrete For the vanous condit~onsof dnvmg and exposure the concrete mlx and strengths should be m accordance w ~ t htable 12 of BS 8004 1986 c) Reznforcement The remforcement should conform to BS 8110 Part 1 or BS 5400 Part 4, Part 7 and Part 8 for reslstlng the apphed forces The lateral remforcement is important m reslstmg the dnvlng stresses The cover requlrements in BS 8110 Part 1 should be the mlrlimum requlrernents d) Dzmens7~onsThe dlmenslons depend on design, the th~cknessshould generally be m the range of 120 mm to 400 rnm The normal w~dth of a concrete p ~ l e1s 500 mm but the w d t h at the head 1s reduced to smt standard dnwng caps The toe of the plle is generally wedge shaped to asslst close dnving e) Joants Sheet piles are prov~dedw ~ t htongued and grooved jolnts e ~ t h e trapezo~dal, r tnangular or sem~c~rcular m shape These asast m correct alignment of the sheetmg dunng dnvlng and also mmmuze the seepage of sod through the wall after completion Watertightness may be ach~evedthrough spec~allydes~gnedmterlocks or through grouted jolnts f) Drzvzng The recommendatlons m 7.4.2.5 of BS 8004 1986 should be followed m dnvlng concrete sheet p~les 4 4.6.3 Prestressed concrete sheet piles The main advantages are a) lugh strength in bending and ab111tyto withstand tensde stresses dunng dnvmg and to take hard dnvmg, b) relat~velycrack-free In handhg, pitchmg and dnvmg,
c) econonucal des~gnfor gwen loads and moments, d) greater dnmbhty In different environments The requlrements for matenals, manufacture and dnvmg of prestressed concrete plles should be as set out m BS 8110 Part 1 or BS 5400 Part 4, Part 7 and Part 8 and 7.4.3 of BS 8004 1986 In the des~gntenslons up to a maxlmum of 50 % of the modulus of rupture (m tens~on)may be perm~ttedprov~dedthe ult~matestrength requlrernents are sat~sfied In t h ~ srespect the gu~dancefor class 2 structures m 4.3.4.3 of BS 81 10 Part 1 1986 1s appropnate 4.4 7 In situ concrete pile walls 4.4.7.1 General Bored plles are used when a so11replacement rather than a so11 d~splaeementmethod of p111ngis required to form a retanmg wall and when there 1s a requirement to mirumue wbration They may be unsmtable where the ground water level on the retamed s ~ d eof the wall e h~ghThe~rbest appl~cat~on 1s in cohes~veso~lsFor the constructlon of bored plles reference should be made to BS 8004 An advantage of a bored plle retamrung wall is that only one plle need be bored at a tlme Hence, when workmg close to an exlstlng foundatlon, only a short length of the foundatlon need be exposed to any nsk of ground loss or movement at a gwen tune It may also be easler to overcome ground obstruct~onsthan w t h sheet p~llngor d~aphmgm wall constructlon In a d d ~ t ~ obored n, p h g systems have a capaclty to penetrate moderately hard bedrock matenals more easlly than alternat~ve methods 4.4.7.2 Q p e s of wall and applicability The follow~ngrecommendatlons apply to two types of plle wall a) Close bored or contzguous pzle walk These walls are bulk wlth p~lesat centres w h ~ c hare equal to or s W t l y greater than the external diameter of the caslng or hrung Borehole casmg 1s requ~redto r e t m granular and s ~ m ~ lunstable ar so~lsdunng bonng near the ground surface or throughout its length, depending on the ground condit~onsT h ~ type s of wall is unsu~tablefor retauung water-beanng granular so~lswhich are l~ableto bleed through the gaps between the plles, unless speclal measures are taken to prov~dea seal between adjacent plles b) Secant pzle w a l k Secant plles are bored at centres less than the dlameter of the caslng Alternate piles are thus of full clrcular sectlon, whde lntervenlng p~lesare cut away m part dunng the construction process A fully contlnuous relatwely watert~ghtretmmg wall may be constructed prov~dedthe tolerances of posttlotung and vert~cahtyare sufflc~entto ehminate gaps between p~les 73
BS 8002 : 1994
Section 4
-.
bored p ~ l ewalls may be deslgned to mcorporate vertlcal drams between p~les,so as to produce a Concrete and re~nforcementshould conform to the su~table water collectmg dramage system in front requirements of BS 8004, BS 8110 Part 1 or BS 5400 Part 4, Part 7 and Part 8 The mm should of the flrushed wall These may be accommodated at the tune when fin~shesare applied be designed to provlde the necessary structural Surface treatment to bored p ~ l ewalls may cons~st strength and the flow reqwemeuts to ensure of adequate compaction and contmulty Speclal methods of placement, for example by tremle a) structural concrete, or, should be taken Into account See Shwinslu Z and b) gunite, or, Flemmg W G K (1974) Reinforcement cages c) precast concrete panels, or, should be bu~ltto withstand the hftmg and handhng stresses These may be relatlvely high d) bnck facing Weldmg of reinforcement, where used, should use Where structural concrete or gurute is used w ~ t h techniques wh~chmamtam the full strength of the close bored plle walls, the so11should be cut back steel between the p~lesand the concrete surfaces should be cleaned, so as to provide a reasonably good 4.4.7.4 Design Where props or anchorages are used to support the structural key before erectlng shuttenng and castmg the fm~shmgconcrete wall at vanous levels, walmg beams should be A wall fin~shwhich prevents subsequent damp provided along the face of the wall at these lateral penetration can be obtamed uslng structural support levels to u n ~ f ybehav~ourThe wahng concrete provlded the rmnlmum fin~shdepth 1s beams may be designed as honzontally spanrung adequate It IS,however, more ddficult to obtam a steel or concrete beams, where steel beams are waterproof wall w ~ t hgunlte Both types of apphed used, a satisfactory method of wedgmg or m f i h g concrete are normally remforced with steel mesh should be provlded to take up the gaps between lndlvidual piles and the walmg beam The gaps Where a precast concrete cladd~ngIS used, occur as a result of surface lrregular~tlesor adequate fwngs should be prov~ded,e ~ t h e from r devlat~onsfrom true vertical~tyand positlon of the cappmg beam or by castmg n ~ b son the piles to ~nd~vldual plles A continuous structural beam support the precast unlts Where rubs are cast on, should be cast along the heads of the piles to u n ~ f y su~tablebars should be Incorporated m t h the p ~ l e them behav~ourboth as an earth retalmng wall and at the requlred levels These bars are subsequently In order to dlstr~buteany vertlcal load wh~chmay exposed by breakmg out, they are then bent and also be apphed bonded Into the rubs n e s , such as ground anchors, are normally passed Where e ~ t h e precast r concrete or bnck fac~ngsare between piles m close bored plle wall construction used the soil between the p~lesshould be cut back and where necessary the gaps between the plles 4.4.7.5 Construction should be filled m t h gunlte or trowelled on cement mortar to prevent the sod from falling and 4 4.7.5.1 'Iblerances accumulatmg behmd the claddmg Independent The normal tolerances whlch can be expected in bnck facmg may need to be provlded w t h a the formation of close bored pile walls are separate foundat~onand behlnd-the-wall dramage approx~mately1 in 75 to 1 m 100 for vert~cahtyand 50 mm for lateral plan tolerances measured at nght 4.4.8 Diaphragm walls angles to the h e of the wall The lateral tolerance may be Improved by formmg a 4.4.8.1 Geneml gulde wall in the ground on elther s ~ d eof the p ~ l e Diaphragm walls are cast or placed in the gmund usmg a benton~teor polymer suspenslon, as part of posmons T h ~ 1s s not a usual practlce and may be the construction process Excavation IS carned out costly Timber or concrete sleepers may be used to m the suspenslon to a w d t h equal to the thickness Improve accuracy at relatlvely low cost of wall requ~red The excavation equlpment uses The requ~redvert~calltytolerance for secant plles 1s elther rope suspended grabs, grabs mounted on normally of the order of 1 200 and for posltlonal Kelly bars, or reverse-circulation excavating tolerances of the order of 25 mm Where walls equpment The benton~tesuspension 1s designed to have to be constructed in close proxmuty to other mamta~nthe stabihty of the slit trench dunng structures the clearances required are dependent d i w g and untll the diaphragm wall has been on the part~cularp~lingequlpment to be used concreted (Instn Civ Engrs , Diaphragm Wall Conference 1975) 4.4.7 5.2 Fznwh.es Walls are formed in panels of predetermmed Bored plle walls may requlre treatment m order to length The length is controlled by the type of provlde a su~tablefm~shedsurface All surface equlpment and the economics of construction, finishes should take account of the potentla1 vanatlons in the profile of the wall face Close adjacent surcharges, the types of so11 bemg 4.4.7.3 Materials
74
excavated, the slze and we~ghtof reinforcement cage that can be handled, the total quantlty of concrete that can be placed in one panel and the posltlons of anchorages or struts and walings Reinforcement IS not commonly linked from one panel Into another, but m some systems special jomts have been devised, whlch transfer forces becween adjacent panels The concrete cast In sltu 1s placed by tremle, it 1s essent~althat the concrete flows freely wthout segregatlon so as to surround completely the remforcement and displace the bentonite 4.4.8.2 Materials See 4.4.7.3 for recommendations 4.4.8 3 Constructcon 4.4.8.3.1 G e m 1
The followng recomrnendatlons apply in the constructlon of d~aphragmwalls a) Guzde walls Rernforced concrete guide walls are first bulk to defme the line and thickness of the wall and to ensure a posltive head agamst any ground water pressure The walls should be remforced contmuously, they are usually cast in the dry on and agamst f m ground or alternatively, if necessary, the outer face of the guide walls may be supported by a backfill of lean mlx concrete b) Bentonzte Benton~te,whch conslsts manly of m~neralsof the rnontmor~llonitegroup, should be sultable for clvll engneenng purposes There are two forms of bentomte, sodium benton~teand calcmm benton~te,each has markedly different properties Sod~umbentonlte 1s usually used, e ~ t h e that r naturally occurring or obtmed by the chemical treatment of calcium bentonite Dunng excavation, the bentonite suspenslon becomes contaminated wlth solids from the soil Tlus may lead to the bulld up of t h c k filtercake at a faster rate than IS des~rableon the walls of the excavation and may cause d~fficultym displacing the bentomte suspenslon by the tremed concrete Accordmgly the denslty, viscosity, shear strength and pH value of the bentonlte suspension should be checked at vanous stages in the work and any nnsat~sfactory levels of contammatlon should be corrected before proceedmg w t h the rest of the construction (Fleming W G K et a1 1975) c) Constructzonjoznts A normal jomt is formed by lnsertlng a round stop end pipe at the end of the excavatlon Tlus IS wthdrawn when the concrete has set, so f o n n g a seml-circular joint against w h c h the concrete of the next panel IS placed d) Watertzghtness Sound workmanshp should be used in concreting and m the construction of jomts so as to ach~evea reasonable watert~ghtnessMovement a t jomts may occur
dunng or subsequent to the removal of earth from one slde of the wall because of varying plan conflguratlons, with stiffer panel sections at corners or because of ddferent depths of excavation To avoid differential movement and the associated leakages uluform~tyof stiffness 1s desirable, but is unpract~calto ach~eve Accordmgly prowslon should be made for the seahng of jomts by groutmg or other techn~ques subsequent to constructlon (Shwmskl Z and Flemme. W G K . 1974) e) Use of pan& to suiport vwtzcally and horzzontally applzed loads Diaphragm walls may support additional vertical and horizontal loads If vert~calloads are to be carned by end beanng of the bases attention should be gven to ensunng a clean panel base to ensure end beanng contnbutes s~gmflcantlyto the total beanng capaclty Load transfer to the so11 by fnct~onbetween the wall and so11may also be used but for tlus to be fully m o b h e d panels should be concreted wlthout any long delay following the completion of excavatlon f) Deep carcular excavatzons Deep circular excavations may be retamed by a senes of panels m contact forming a clrcular plan retainmg wall Where the circumferential thrust 1s to be taken by the rlng of panels directly and not by w a h g beams, the tolerances of vert~calltyand the physical mtegnty are particularly Important
-
4.4.8 3.2 Prestressed cast zn satu walls
Cast-in-place diaphragm walls may be prestressed using post-tensiomng techmques, the cables should be looped so that both ends are access~bleat the top of the wall A su~tableremforcement cage should be made for the attachment of the cable ducts so as to prov~dethe cable profile best sulted to the desgn The fixmgs should be sufficiently stable and robust to w~thstandthe stresses due to handling and the forces due to rmng concrete dunng concretmg Reference should be made to BS 8110 Part 1 or BS 5400 Part 4 for the general recommendations of post-tens~onedprestressed concrete deslgn and constructlon 4.4.8.3.3 Precast concrete walls A precast concrete diaphragm wall 1s formed of precast umts placed in a benton~tefilled trench, excavated by normal methods Part of the bentonlte suspension 1s first displaced by a cementfbentomte grout tremted Into the base of the excavatlon Precast wall panels wlth su~table arranged jointlng and locatmg systems are then lowered to the required depth in the trench The quantity of cement bentonlte grout should be such that when all the precast panels, corresponding to the trench length excavated, have been futed, they are fully unmersed in the grout mucture
BS 8002 : 1994
Section 4
I
.
The cement benton~tegrout 1s not requ~redto develop h ~ g hstrengths and needs to be only margmally stronger in its final state than the ground m wh~chthe wall 1s embedded and upon w h ~ c hit rehes for support The stage of d~splacementof benton~tesuspenslon by grout may be avo~dedby canylng out all the trench excavatlon usmg a cement benton~tegrout suspenslon, mstead of convent~onalbenton~te Several propnetary types of precast walllng system are ava~lable,they dlffer as regards the jolnts between panels and the way m wh~chthe wall 1s flxed in posltlon in the ground by the cement grout Some are re~nforcedconvent~onally,wMe others are of prestressed concrete Precast d~aphragmwall panels should conform to the recommendat~onsof BS 8110 Part 1 except ln so far as the desgn 1s mod~fiedin t h ~ sdocument Smce one of the purposes of usmg precast wallmg 1s to prov~dea satisfactory fin~shedappearance for the wall subsequent to excavatlon, care and accuracy are Important throughout fabncat~onand ~nstallat~on
1
Figure 41. Horizontal sheeting (lagging)
Figure 42. Vertical sheeting (lagging)
4.4.9 Sold~erlkingpiles 4.4.9.1 General These conslst of vertical members b d t at sultable centres w ~ t ha system of ground support spann~ng between them The p~lesare first mstalled along the penmeter of the proposed excavatlon Sheetmg, supportmg the ground, 1s placed in posltlon as excavation proceeds The sheetmg spans e ~ t h e honzontallv r between the sold~erlk~ng mles or vert~callybetween horizontal walmgs, see figures 41 and 42 Sheet plles ~nterlockingw ~ t h H-sect~onp~lesare also commonly used Sold~erllungplles may be used to support deep, narrow, shallow or w ~ d eexcavations m vanous matenals ~nclud~ng clays and sands Excavat~onm water-beanng ground may reqwre specml attention, t h ~ method s is unsu~tablefor the exclus~onof water and if so11 1s washed out from behmd the sheetmg unacceptable settlement may be caused to adjacent structures or semces
.
Section 4
Support to the piles may be by wahngs and struts, ralang shores, tie rods and anchorages or ground anchorages The plles should be designed to span vertically between the supports prov~ded The des~gnof the p~lesshould accommodate any resultmg vertical forces from mchned anchors or supports 4.4.9.2 Materials The followmg matenals are recommended for p~les and sheetings a) Pzles The p~lesmay be of re~nforcedconcrete or steel sections used elther smgly or in pam Where steel sectlons are used m pairs adequate batten plates or spacers should be prov~dedto ensure the composite action of both p~lesSee figure 43 b) Sheetzng (laggzny) Steel and concrete may be used for sheetmg, but the most common matenal 1s timber T h ~ IS s easlly handled and cut on slte to s u ~varlatlons t In site cond~tlonsStructural softwoods to BS 5268 Part 2 are su~table Steel trenchmg may be used as sheetmg, it 1s relatwely light and can be tnmmed to the correct length d necessary Concrete may be used, elther as prestressed precast planks or in situ concrete Precast planks are heavy to handle, d~fficult to cut to length and exh~bitbnttle fa~lurew~thout vmble or audible warnmg
BS 8002 : 1994
anchorages so as to accommodate the sltuatlon where an indw~dualanchorage fads to carry its full desgn load Adequate penetrat~onof the piles below formation level should be prov~dedto permit the mobhzation of suffment passwe resistance for both normal conditions and also to allow for the nsk of over excavatlon as well as to prevent mstab~htyIn the bottom of the excavatlon Where t~mbersheetmg m used a 50 % reduct~on may be assumed m the des~gnof the sheetmg members m the loadmg from the sod, due to the arch~ngactlon of the soil The rehevlng effect of the archmg IS much reduced \nth use of st~ffer sheetmg materials, for example concrete, due to the lower relative flexlbhty of the concrete sheetmg
4.4.9.4 Construction As excavatlon proceeds, exposlng the sold~erlkmg plles, the spaces between adjacent piles are closed w ~ t hhonzontal sheetmg In clay soils it 1s common practlce to uncover the face to a depth of 1 m between piles before the sheetmg is placed, but m loose waterlogged ground the sheet~ngshould be placed as soon as possible to prevent a cave-m Vanous methods of locatmg the sheetmg (lamug) are shown m figure 44 Gaps of 50 mm may be left between the laggmg to facilitate drainage of ground water, so reducmg the load on the soldler p~lesand supports In loose 4.4.9.3 Deszgn ground the 50 mm Raws should be wacked with straw to prevent a iosk of ground between the The earth pressure to be resisted depends on the louvres, wh~lstst111 allowmg water to percolate stiffness of the support system The design should Volds belund the sheeting should be packed as cater for all stages of excavatlon and support construction proceeds to prevent movement of the ~nstallat~on as well as for the final excavated supported ground towards the excavatlon The condition Where anchorages are provided to support the p~les sheeting should be firmly wedged or fixed to the sold~erplles as construct~onproceeds then the p~lesand the wahngs should be able to red~stnbute40 % of the load to adjacent
Figure 43. Composite steel soldier piles
t ' Sold~erplies
Sheeting
I Lagg~nq1
Bolt (w~thnut and washer1 welded to soLdler pile
Figure 44 Various methods of locating the sheeting (lagging)
..
Section 4
BS 8002 : 1994
4.5.1.3 Earth-filled double-wall and cellular
4.5 Strutted excavations and cofferdams 4.5.1 General 4.5.1.1 Single skin coflerdarns and strutted excavations These may be formed with piles supported either by a framework w~thinthe cofferdam, or by external anchorages around the penmeter Cantilever p ~ l ecofferdams may also be formed and are des~gnedm the same way as cant~lever retammng walls When the cofferdam is very large m area, but of relatwely shallow depth, cons~derat~on should be glven to incorporatmg external anchorages or to usmg rakmg struts from the foundations of the permanent structure, in order to acheve economy 4.5.1.2 Cofferdams for river crossings This type of cofferdam is used when a pipehne is to be la~dacross a rlver and a is ~mpracticableto close the watenvay See figure 45
cofferdams Earth-filled cofferdams are self-supporting gravlty structures, either parallel-slded double-wall cofferdams or cellular cofferdams The stab~htyof both types is dependent on the properties of the filhng and of the sol1 at foundat~onlevel, as well as on the arrangement and type of the steel sheet piling Typical uses are as dams to seal off temporanly dock entrances so that work below water level can be carned out in the dry and in the construction of permanent walls for land reclamation, quays, wharves and dolphins Double-waU cofferdams consist of two parallel hnes of steel phng connected together by a system of steel wahn@ and tle rods and sometimes struts at one or more levels The space between the lines of phng is filled wlth coarse cohes~onlessmatenal such as sand, gravel or broken rock
Flrst bulkhead
----Plan
Junct~on p ~ l e s i
Long~tud~nal sect~on
Figure 45. Cofferdamfor river crossing
LTlrnber packs a t Waling levels
Cross section A-A
BS 8002 : 1994
Section 4
.
4.5.1.4 Design 4.5.1.4.1 Pzlzng
In strutted excavations, the des~gnof the p111ng should be checked, not only m the final excavated cond~tionw ~ t hall frames in posltlon, but also dunng every stage of both excavatlon and construction of the supportmg framng, as well as the subsequent removal of the frames In a d d ~ t ~ oton the design surcharge loadmg gven m 3.3.4 the effect of lsolated heavy plant or equipment mposmg large pomt or lme loads should be taken into account The plles may be assumed to be s~mplysupported at the frames and at the excavatlon level (m both temporary and final cond~tlons)they will have e ~ t h e r'futed' or 'free' earth support in the embedded length depending upon the type of sod and the depth of cut-off of the p~les The most su~tablemethod of calculating bending moments m walls and the load in the vanous supporting frames, whether anchored or propped is by successwe analys~sof each stage of construction as shown m figure 38 Tlus ulll determine the bendmg moments and loadmgs occurnng dunng the temporary condit~ons,whch develop as the excavatlon 1s camed out, pnor to the ~nstallat~on of each frame These moments and loads are normally larger than those calculated for the structure m the completed condtion The most economlc sectlon of sheet pllmg WLU be the hghtest sectlon capable of be~ngdnven to the depth determmed m the desgn calculations and wh~chcan reslst the des~gnbendmg moments calculated m the cofferdam des~gnA heavier sectlon and/or a h~gherquality of steel may be required where long p~lesare to be dnven to deep penetrations or where hard dnvlng 1s to be encountered, also see 4.4.4.4.1 4.5.1.4.2 Equzlzbmum of the strutted ezavatzon or cofferdam See 4 1.1and 4.1.2 In strutted excavations, where so11 IS supported, the soil pressure d~agramin the fmal stage of excavation relatmg to the strut loads 1s parabohc but may be approxmated to a trapezoidal shape as shown m figure 37 These d~agramsdo not mclude the effect of water pressure, they provlde an envelope of strut loadmg wluch wll cover the maxlmum loads m the struts of the frammg system The d~agramsshould not be used to calculate the bendmg moment m the p111ng dur~ngboth of the temporary stages of construction, I e excavation and removal This method may also be applied to retanmg walls wlth mult~plelevels of tie-backs, but only when the wall 1s formed by excavation or dredgmg in front of the wall It should not be used for a backfilled type wall Altemat~velythe pressure and strut loads may be obtained by using a stage by stage method of analysis (James and Jack, 1975)
4.5.1.4.3 Coferdam zn water
See figure 46 Where a cofferdam IS to be bulk m stdl water the spaclng of the wahngs can be taken from standard handbooks If frames are mstalled 'm the dry' w ~ t hthe water in the cofferdam lowered m stages, then the tables in standard handbooks may not apply and a stage by stage analysis should be carned out 4.5.1.4.4 Cofferdam wzth unbalanced loadzng
(dock wall and r z v m d e constructzon) Thls type of cofferdam 1s subjected to a heavler loadmg on the landward s ~ d ethan on the seaward s ~ d edue , to the soil pressure, surcharges and add~t~onal head of water The water pressure 1s generally hgher on the landward s ~ d ePrecautions, dependent on slte cond~t~ons, should be taken to overcome the unbalanced loadmgs Su~table alternative methods are a) removal of sod on the landward side, so as to reduce the sod pressures, b) provlslon of a restncted area adjacent to the cofferdam, with~nwh~chplant and veh~clesare prolub~tedso as to reduce the surcharge, C)placing spo~lon the water s ~ d eof the cofferdam to mcrease the passive reslstance, d) provlslon of an anchorage system on the landward side, e) using ralung struts mside the cofferdam 4.5.1.4.5 Double-wall cofferdam The mner h e of phng should be desgned as an anchored retaming wall, whde the outer h e of pdmg acts as the anchorage The mdth of the dam should be not less than 0 8 of the retamed he~ght of water and/or sol1 The inner wall is usually prov~dedw ~ t hweep holes near the bottom to reduce water pressure and to prevent a decrease in the total shear strength of fill matenal Complete dramage of the fill may be impossible and so allowance should be made for a hydrostatic head mthm the fill of at least 0 3 of the retamed height The penetration of the p h g mto the soil below excavation or dredged level should be sufficient to develop the necessary passlve resistance and prevent honzontal shding and to control the effects of seepage T ~ I type S of cofferdam is uneconomical if there is rock at excavatlon level unless the rock 1s such that steel p ~ h can g be drlven Into it to an adequate penetration If the rock is hard it may be possible, at low water, to excavate a trench into which the phng may be concreted to o b t m the necessary seal and honzontal reslstance for the cofferdam Where rock may be encountered grade 5355P or higher to BS EN 10025 1990 steel sheet plles
-
I
External top wal;ng and t ~ erod Water Level
v -. ..
,
t
Y
,-
v Water level
. . . .
1
+
A
r
c % . " ?
b
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L 0,
,
c
;
-
.
m
m
-
.
_ N
-
4
N
r
>
O
-0
m
0,
c
Wabngs
t
x
Struts
Sea or river be
1
Figure 46. Cofferdam in water
should be used These p~lesw~thstandharder dnvmg, and also punch Into the rock \nth less nsk of buckhng, than plles of grade 5275P steel to BS EN 10025 1990 4.5.1.4.6 Arrangement of supporttng frames
Framing w t h m a cofferdam should be arranged to permit carrymg out constructlon work withm the cofferdam in a satisfactory manner The spacmg of members hornontally and vertically should be such as to permit the use of plant dunng excavatlon and the constructlon of a permanent structure wltlun the cofferdam Vertlcal spacing of the frames should be des~gnedto ensure that the p~lesand the frames are not overstresscd m any temporary c o n d ~ t ~ odunng n cofferdam excavatlon 4.5.1.5 Construction 4.5.1.5.1 iMznzmum depth of cut-ofS zn coheszonless sozls When a cofferdam 1s constructed m cohes~onless sod, and the water level outs~dethe cofferdam 1s appreciably h~gherthan the excavatlon level, the length of the pdes should be sufficient to prevent the so11 m the bottom of the cofferdam becommg unstable due to the upward flow of water from under the toes of the pdes The upward flow of water into the bottom of the cofferdam may mduce so11 instablhty w t h m the cofferdam and also a reduct~onof passlve resistance The sections of
p~hngand the bottom frames should be checked In manne cofferdams t h ~ nsk s of mstab~htymay be reduced by placmg a less permeable blanket around the outs~deof the cofferdam, nght up to the sheet p~hng,so as to ~ncreasethe lengths of the seepage paths m a manner s~milarto the increased path length w h c h results if the cut-off IS mcreased When water 1s pumped from a sump at excavatlon level, the sump should be s~tuatedas far as poss~ble from the walls of the cofferdam Any flow of water Into the cofferdam may carry fmes from the surroundmg so11 and cause subs~denceor p~pmg Local mstabhty may occur m condit~onsof pipmg as the passwe r e s ~ t a n c eis destroyed The flow of water beneath the toes of the p~les may be reduced where it is poss~bleto lower the ground water level outside the cofferdam A system of well-pomts or filter wells at plle toe-level can, as an altemat~ve,control the flow of water mto the cofferdam Where the flow of water mto the cofferdam 1s hkely to be excesswe, excavatlon and pos~tiotungof frames should be carned out whdst the cofferdam remams flooded A plug of concrete should then be placed by treme a t excavatlon level and the cofferdam pumped dry T ~ concrete E plug should be e ~ t h e of r suffment welght to reslst the uphft forces or should incorporate plpes to reheve the water pressure
4.5 1 5 2 Pmentzon of heave
4.5.2 Struts, ties, walings and anchorages
In soft cohesive solls there 1s a nsk of the flow or heavmg of the bottom of a deep cofferdam Where a cofferdam is founded in cohesive soil whlch 1s underlam by water under arteslan pressure, rehef wells may be needed to reheve the arteslan pressure
4.5.2.1 General
4.5.1.5 3 Czrcular cofferdam
The p~lesshould, where possible, be p~tchedand the whole c~rclecompleted before dnvmg 1s commenced The plles should be driven m stages as the hammer works its way several tlmes around the circumference A d~aphragmwall can be used to form a clrcular cofferdam Earth pressures should be calculated as for straght-sded cofferdams and plles should be supported by c~rcularnng beams, mstead of wahngs and struts This leaves the central area of the excavat~onclear of obstruct~on Due to the devlatlons in practlce from a true circle the rmg beams are subjected to eccentric loadmg and a check should be made for buckhng in the nng w ~ t hthe radial wahng load q m kN/m) determmed from
where Young's modulus of wahng matenal E in N/mm2, Moment of inertla about 'xx'axls m em4, I R Rad~usof cofferdam m metres
-
4 5 1 5.4 Earth-ftlled cofferdam
Clays or s~ltsshould not be used as fillmg matenal and any soft soils of these types which may be enclosed w~tlunthe sheet pdmg should be removed before placmg filling Under tidal cond~tionsthe water level outside the cofferdam may be above or below that withm the so11 ins~dethe cofferdam and the cofferdam should be des~gnedfor these vanable load~ngcondlt~ons Weepholes, w ~ t hgraded filters if necessary, should be provlded near the bottom of the exposed portlon of the p~leson the Inner s ~ d eDrainage of the fill~ngshould be undertaken to reduce the pressure on the inner hne of phng and to prevent a decrease in the total shear strength of the fillmg If complete dramage of the f~lhngis imposs~ble,an allowance should be made for any water pressure acting on the piling
wlth other types
4.5.2.1.1 Walzngs
The cho~ceof timber or steel for cofferdam bracing IS controlled by the external loadmgs upon the p~les under the most severe condltlons as well as the mternal d~mens~ons of the cofferdam Good qual~ty timber such as Douglas f ~ or r p~tehpme may be used for lighter loadmgs Heavier loadmgs requlre the use of steel members, I e universal beams su~tablyremforced by web st~ffenerswhere necessary to safeguard aganst web bucklmg and tors~onalrotation, particularly where wahngs are suspended on hangers Recommended maxlmum stresses to be employed m the design of cofferdam walmgs should be based on the stresses @ven m BS 449 Part 2 Remforced concrete wahngs may be used in permanent cofferdams and m temporary works where the cofferdam w ~ lbe l open for a long penod of tlme They are espec~allyuseful when constructmg a curved wall or c~rcular cofferdam Remforced concrete prowdes h~gh st~ffnessand ehmmates the nsk of web crushmg and bucklmg which may occur when steel wahngs are poorly des~gnedor badly assembled Posmve support methods should be used to locate the frames, usmg hangers or brackets Where diagonal struts are used, or where wahngs act as struts to other walings thus lmposmg an axlal load, the members should be des~gnedto withstand the combmed stresses due to the axlal and bendlng loads 4.5.2.1.2 Struts
As w ~ t hwahngs, good quahty timber may be used for hghter loadmgs but steel struts should be used for heavier loadlngs Where loadmgs are 'severe' steel box piles or tubular steel piles may be used as struts The design should make allowance for the self we~ghtstresses and for the consequent eccentricity due to deflection as well as for accidental construet~onloadmgs 4.5.2.2 Design 4.5.2.2.1 General
The loads to be earned by the struts, tles, wahngs and anchorages should generally be determmed in accordance w ~ t hsect~on3 and 4.1.1 and 4.1.2 However, if stresses m the sheet p~hnghave been modifled by bendmg moment reduction, the calculated load m the struts or tles should be ~ncreasedby 25 % Walmgs and anchors should be des~gnedto carry these strut or tle rod loads
The deslgn should also accommodate the poss~ble fadure of an ~ n d ~ v ~ dstrut u a l tle rod or anchor The wall and wahngs should be capable of red~stnbutmgthe load from the falled tle rod or anchor An Increase may be made m the allowable stresses up to yleld values for steel and 0 8 x ultunate values for concrete and tunber 4.5.2.2.2 Walzngs The honzontal react~onfrom an anchored sheet wall 1s generally camed to the struts, tles or anchorages by wahngs, where wal~ngsare om~tted the attendant nsks should be carefully cons~dered The loads on the walmgs are obtamed by cons~denngthe same cond~tlonsas those used to obtmn the bendmg moments in the p~les (see 4.4.3.2 and 4.4.3 3) The loadlng from the sheetmg members onto the w a h g should be d~stnbutedevenly by means, for example, of tmber wedges The wahngs should be des~gnedas continuous beams malong due allowance for the end spans The total system IS statically mdetermmate (or hyperstat~c),an exact analys~swould cons~derthe elastmty of the tle rods, the n s d ~ t yof the wahng and the stresses Induced dunng boltmg operations In practlce it 1s usual to adopt a s~mpl~fied approach uslng an assumed bendmg moment wL2/10 where w 1s the un~formlyd~stnbuted loadmg and L IS the span length Where walmgs are placed behmd the p ~ l ethey should be connected to the sheet p~hngby bolts w ~ t h adequate beanng plates and washer plates, the steelwork should be of generous proportions to allow for corrosion and the stresses Introduced when ahgnmg the p~lmgand to avo~dthe nsk of web buckhg, stiffeners should be prov~dedas requlred Where the anchorages are Inched the vert~calload component d be camed by walmgs To ensure an of the vert~calloads the adequate d~stnbut~on clutches should be welded at the heads of the pdes When d~agonaltle rods are used to support end returns of the sheet p~hng,the des~gnshould cater for the honzontal components m the plane of both the wall and the return 4.5.2.2.3 Steel walzngs Steel wahngs should be des~gnedin accordance w ~ t hBS 449 Part 2 They commonly consBt of two spaced structural steel channels placed horizontally w ~ t hthew webs back to back The channels are spaced a suff~c~ent &stance apart to allow the tle rods to pass between the web and be mstalled \nth ease, tak~ngaccount of any ~nchnat~on of the tie rods Bolted or welded channel spacers are used to mamtam the requ~red spacmg when the channels are connected When the tle rods are honzontal, channel spacers at appromately 2 0 m to 2 5 m centres are generally
adequate, but when w h g s are used m conjunction w ~ t h mchned tle rods, it IS advisable to reduce t h ~ spacmg s To ensure the contmuity of steel walmgs, the jolnts should be located opposlte the troughs of the p h g adjacent to the tle rods at approxlmately one fifth of the span The jomts In upper and lower channels should be staggered and placed one on edher s ~ d e of the tle rod If the jolnts cannot be spaced at the des~redpomts, the walmgs should be desgned to reslst both the bendmg moment and the shear at the polnts where they occur Web st~ffenersmay be necessary at anchor head pomts to reslst web buckhng due to the reactlon of the tle rods All permanent steelwork should be gwen protectwe treatment m accordance w ~ t hBS 5493 The nunlmum sue of bolts for wallng spl~ce connections should be M20 and the rnlnlmum size of bolts for attachmg w a h g s to sheet p~lesshould be M30 Bolts for walmg sphce connectlons should conform to BS 4190 Bolts for attachmg wahngs to sheet plles should conform to BS 4190 or be des~gnedas short tle rods (see 4.5.2.2.6) 4.5.2.2.4 Tzmber wulzngs Tunber walmgs should be deslgned m accordance w ~ t hBS 5268 Part 2 Steel spreaders of suffic~ent area and stiffness should be provlded at the tle rods to avo~dover-stressmg the wallngs The wallngs should be connected to the sheet p~lmgby bolts w ~ t hadequate washers or beanng plates The wahgjomts In tied walls should be located at approxlmately one f ~ f t hof the span and should be sufficlently strong to ensure contmurty and to take shear forces from anchor head loads In permanent works the steel bolts and plates used m wallngs should be galvan~zedm accordance wlth BS 729 or sherad~zedto BS 4921 All tunbenvork should be s v e n protectwe treatment in accordance w ~ t hBS 5589 4.5.2.2.5 In s?tu and precast concrete walzngs Concrete walings should be des~gnedin accordance w ~ t hBS 8110 Part 1 The mlnlmum charactenstic compressive strength of the concrete at 28 days should be 25 N/mm2 Where walmgs are m contact \nth sod, sea water or fresh water aggresswe to concrete then the mnumum strength at 28 days should be 30 N/mm2 A concrete capping beam 1s usually constructed on top of a permanent sheet p~lewall Where t h ~ s capplng beam IS used as a wallng it should be sufficlently strong to transmlt vert~caland honzontal loads from anchor tles Into sheet p111ng unless other provmons such as welding at clutches are made The w a h g should be deslgned as a beam, supported on elast~csupports prov~dedby sheet pil~ngboth m honzontal and vertlcal d~rect~ons Sufficient remforcement should be prov~dedIn the capplng beam to cope w ~ t hshear forces arlsmg from anchor head loads and also wlth shrinkage and temperature stresses
Section 4
BS 8002 : 1994
.-
4.5.2.2.6 The rods
4.5.3.2 Materials
Where tie rods are employed they may be critlcal to the stab~htyof sheet piled walls The des~gn should prowde for the Increase in stresses whch may anse from corroslon and also through the bendmg of tie rods due to the ground around them settlmg For protection agamst corrosion buned permanent tle rods should be gwen a su~table sheathmg, wrapplng or coatmg, suffic~entlyflexlble to accommodate the extenslon of the tle rods under load Provls.on should be made m addrtion for corroslon at a rate of not less than 0 05 mmlyear and if the tles are placed m aggressive ground or sea water, a greater wastage for corrosion should be provlded Whllst cathod~c protection reduces corroslon some corrosion should be assumed in the deslgn Where cathodic protection 1s used to protect the tle rods all of the steel components of the wall should be s~milarly protected The tie rods should be provided w t h washers and beanng plates to gwe adequate beanng on the walmgs and on the concrete anchor blocks, d used me rods havmg lengths over 12 m should be jolned by couplers or turnbuckles The rod d~ameterat the root of the threads of nuts and turnbuckles should be adequate to take the des~gntle load Upset ends or rolled threads may be used to prowde for the threads Tie rods should be deslgned m accordance w t h BS 449 Part 2 uslng grade Fe 430 or grade Fe 510 steel to BS EN 10025 1990 or smdar hot rolled non-alloy steel grades If settlement of the sod below tle rods IS bkely to occur, then the ends of the tle rods should be designed to allow for the movement Alternatively the tle rods may be enclosed m flexlble plastic ducts
4.5.3.2.1 Pzle sectzon The shape of the Interlock is deslgned to take the hlgh clrcumferentlal tens~leforces and at the same tlme permit suffic~entangular dewation between adjacent plles to enable cells of a practical diameter to be formed Smce straght web p~leslack bendmg strength In the flat posltlon, care should be taken In handhng and stonng the p~lesThe mlnlmum ultlmate Interlock strength 1s gwen in the manufacturer's hterature
4.5.3 Cellular cofferdams 4.5.3.1 General
Cellular cofferdams are self supporting structures, constructed uslng stra~ghtweb steel sheet plles dnven to form cells of vanous shapes (see figure 47) and filled w t h sand, gravel or broken rock They can be founded on rock, sand or stlff clay and ut~luedas e ~ t h e temporary r or permanent structures to r e t m cons~derablehelghts of sol1 andlor water The stabihty of a cofferdam depends upon the tens~lestrength of the sheet p h g (especially the clutches), the properties of the fillmg, the shape and slze of the cells and the foundat~onmatenals The outward pressure of the f d h g produces U h c~rcumferent~al tensile forces m the p h g , w h c h the stra~ghtweb piles are deslgned to reslst, unhke trough shaped plles sectlons whch are unsuitable
4 5.3.2.2 Fzll
The fill matenal should have a hlgh crushing strength combmed w ~ t ha h~ghangle of sheanng resistance to prowde the necessary shear strength with~nthe fill together w ~ t ha shdmg resistance at the base The fffl should be free drairung and be relatively mcompress~ble,I e coarse granular solls Well graded sand is preferable but where un~form sand is to be used, fine matenals should be avoided Well graded crushed rock and muctures of sand and gravel are Ideal Backfill placed behind the cellular cells should be coarse sand, gravel or clean rubble The fill should have a permeabhty greater than approximately m/s 4.5.3.2.3 Cell shape As shown in figure 47 vanous geometncal shapes are poss~ble,each of whch have advantages and
disadvantages 4.5 3.3 Tgpes of wall and applicatron NOlT The followmg h t s descnbe the advantages and dmdvantages of various types of wall
4.5.3.3.1 Czrcular dzaphragm cells
a) Advantages 1) Each cell IS a self-supportmg unit 2) Each cell can be fffled Independently of adjacent cells 3) Clrcular diaphragm cells can be constructed In rough and f l o m g water (max~mumveloclty about 1 3 mls) b) Dzsadvantages 1) Res~stanceto large water pressures IS restncted by mterlock tension 2) Interconnect~ngarcs Increase stresses and deformations of the m a n cell 3) P~tchmg,closmg and dnvmg of cells requlres great care to avoid developmg excesswely hgh forces m the mterlocks
Note Dimens~onsA,B,R,D and L depend upon dimensions of stra~ghtweb sect~onused
C~rcularcells
Cross
Cross walL Diaphragm cells
?igure 47. Types of cellular cofferdams
4.5.3.3.2 Dzaphragm cells a) Advaniages 1) Large water pressures can be ressted by increasing d~aphragmw d t h 2) Interlock tenslons are umform and smaller than those of a cn-cular cofferdam wlth den tical radius and he~ght b) Dzsadvantages 1) Cells are not mdependently stable It is therefore adv~sableto include a full c~rcular cell at mtervals 2) D~fferencem fill and water level between adjacent cells should be controlled sufficiently to avo~dd~splacementof the diaphragm 3) Several templates required dunng construction
4.5.3.3 3 Cloverleaf cells a) Advantages 1) Cells are mdependently stable 2) Larger size cells can be bulk by tlus method b) Dzsadvantages 1) More sheet piles are required than clrcular or diaphragm cofferdams
2) FU and water levels m adlacent compartments of one cell should be reasonably uniform
4.5.3.4 Descgn The des~gnmethods used are essent~allyempmcal and d ~ f f ein r some respects from each other The following methods and references should be considered The Terzaghi (1945) method and subsequent modifications to it by TVA (1957), U S Corps of Engmeers (1958) and Dept of the Navy (1971) together w ~ t hCummings (1960) and Bnnch Hansen's methods (1953), Ovesen N K (1962), Lacrout Esng and Luscher (1976) In all methods, the safety of the cofferdam IS evaluated aganst fallure by the followng modes a) burstmg of the cells due to fa~lureof the sheet pile Interlock in tension, Expenence has shown that 'at rest' pressures can occur b) sl~dmgon the base, c) excessive leaning or tdting of cells due to shear fa~lureof the fill, d) fadure of the foundation matenal The forces on the cofferdam dunng its vanous stages of construct~onshould be deternuned
Section 4
BS 8002 : 1994
In~t~ally, dunng the fillmg of the cells, espec~allyif hydrauhc filling 1s used, the level of the so11and water may be at least as h ~ g has the lowest part of the top of the cell skin The water level will be lower outslde than Inside and this is particularly slgn~ficantunder t ~ d a cond~t~ons l at low water Further lnformat~onon the design of cellular sheet pile structures IS gwen m BS 6349 Part 2 4.5.3.5 Construction 4.5.3.5.1 Fbundatzons
Rock prov~desa suitable base for cellular cofferdams and penetrat~onof the plles 1s not essent~alfor stab~lltyIf there are gaps between the toe of the plles and the rock then measures should be taken to restnct seepage of water and to prevent the loss of fill W~thsand and gravel foundations, the pillng should penetrate sufficiently to prevent seepage affectmg stab~lityA berm should be provlded on sands, ms~dethe cofferdam and woss~blyoutside, to prevent scour
cells are f~lled 4.5.3.5.2 Draznage
In order to provlde stabihty of a cellular cofferdam the elevation of the phreatlc h e should be mamtamed as low as practicable and dmnage holes should be provided as low as possible m the h e of plles of the cofferdam, together wlth fdters as necessary 4 5.3.5.3 Dewaterzng
When the unloaded s ~ d eof the cofferdam 1s dewatered the rate of lowenng the water level should not exceed the dramage capacity of the rells ..-.
4.5.3 5 4 w o r m a n c e
A cellular cofferdam IS a flexlble structure Honzontal movements of the plles and vertlcal movements of the f~lhngshould be expected Durmg flll~ngof the cells barrelhng of up to 150 mm is not unusual and additional deformation due to the pull from the adjacent cells is to be expected Honzontal deflect~onsat the top of hlgh cofferdams can be s~gnificantand movements up to 500 mm are not unusual F ~ h g espec~ally , on the unloaded s ~ d eof a cofferdam, can settle and total settlement may be 150 mm or more
4.6 Anchorages 4.6.1 General
An anchorage for an earth retammg structure 1s a system mstalled in the retained ground mass to prov~dea tensde form of support to the structure Anchorage systems permlt a clear excavation and they may be used as an alternatwe to struts
When considering anchorages for retanmg walls, the su~tabfityof the proposed installat~on techtuque to ground cond~t~ons, the posslble effect on adjoming bu~ld~ngs and the nghts of adjoimg owners under whose bu~ldmgor land the anchorages are to be Inserted should be determmed Anchorages for retanmg walls are of three general types, ground anchorages mcludmg rock and soil anchorages, tenslon plles and deadman anchorages (see figure 48) They may be used solely dunng constructlon, may form part of the permanent structure or may be des~gnedto perform a dual functlon In order to produce a sat~sfactorydeslgn, anchorage loads should be evaluated together, where poss~bleand practical, w ~ t han assessment of consequent deformations The consequence of fa~lureof any mdividual anchorage should be evaluated, together w ~ t han exammatlon of the overall mass stabfitv and/or effects of aouws of
4.6.2 Equil~brium See 4.1.1 and 4.1 2 4.6.3 Ground
4.6.3.1 General The des~gnand constructlon of ground anchorage systems and the material and components emnloved together w t h the necessaw corrosion prdtee"t1on are dealt wlth In BS 8081 to wh~ch reference should be made It w recommended that spec~ahstwork of t h nature should be undertaken only by persons w t h the necessary geotechn~cal knowledge and expenence 4.6.3.2 Design
Cons~derat~on should be Bven to the following a) It IS usual for anchorages to be dnlled m c h e d below the horizontal m order to reach more competent ground Account should be taken of the effects on so11 loadmgs, both belund and in front of the wall, due to the component forces anslng from the inclmat~onof the anchorage b) The overall safety factors should be determmed by the Me, nature and purpose of the anchorage c) Vanous propnetary computer programs are avadable to assist in the analysls of multi-anchored walls 4.6.4 Tension piles 4.6.4.1 General
Tension p~les,whether raked or vert~cal,are commonly used under rehevmg platforms to retanmg walls and to form raked tenslon pile anchorages
..
BS 8002 : 1994
Section 4
Ground level
Ground level
Balanced sheet p ~ l eanchorage
f Deadman anchors
Ground level
Mass concrete anchorage
" T
Sheet pdes
+
Ground anchw
Figure 48. Types of anchorage d) The ult~mateskm fnct~onof p~lesm 4.6.4.2 Design cohes~onlesssoils may be greatly reduced if plle Axla1 uphft forces on tenslon pies are res~stedby mstallat~onor subsequent relatwe movement skm fnct~onon the plle shaft and any reststance between the p~lesand sod causes degradat~onof offered by enlargement at the base or on the shaft the so11 part~cles T h ~ degradat~on s may occur of a p ~ l e(e g by under-reammng) Gu~danceon the part~cularly ulth pdes dr~ven mto calcareous evaluation of skm fnct~onIS gwen m 7.5 of sods, or where pdes embedded In these solls are BS 8004 1986, and Tomhnson M J (1987) subjected to cycl~cuphft loading However the most rehable gude to the capaclty of a tenslon p ~ l eIS glven by an uphft load test If t h ~ ~ e) The welght of the p~leacts to reduce the total 1s ne~therdesirable nor pract~calthen the foUowmg uphft load appl~edto the p ~ l ehead matters should be taken Into account when f) Because of the reduct~onIn skm fnction from assessmg the ultmate uphft capaclty from the peak to a lower res~dualvalue, tenslon calculat~onsbased on the properties of the so11 or fallure m a p ~ l emay be sudden and catastropluc rock m wh~chthe p ~ l eIS embedded Accordmgly, the des~gnshould be based on a a) Under condlt~onsof cycl~cloadmg or creep conservatlve assessment of the representatwe resldual strength, takmg account of the caused by sustamed loadmg the uphft skm fnct~onresistance of p~lesm coheswe so~lsand reduct~onbelow remoulded cntxal state strength weak rocks can fall from a peak to a res~dual of plastlc clays (see 2.2.3) and the smoothness of value, part~cularlyfor long plles the p~lesurface (see 2.2.8) The assessment of the deslgn value of s h n fnct~on(see 3 2.6) b) Irrevemble uphft movement of tens~onplles m should be based upon an upper h m ~ of t the coheswe so& subject to cychc loadmg 1s unl~kely representatwe res~dualstrength A conservatlve to occur untd the peak cychc shear stress is 80 % assessment should be made of the largest tensde of the ultunate state capaclty load whlch may be apphed by the p~le C) The skm fnct~onresmtance to cychc uphft (see 3.1.7) loadmg of p~lesin cohes~onlesssolls may be 30 % to 40 % lower than that @ven by statlc sustamed loadmg, see Peuch A A (1982)
Section 4
BS 8002 : 1994
.
The des~gnof tens~onplles should also take account of any resistance afforded by an expansion of the base or shaft (e g under-reammg) Full length reinforcement will be requwed to ensure that the mtegnty of the pile 1s mamtamed under tensde loadmg 4.6.5 Deadman anchorages 4.6.5.1 General These usually conslst of sheet plle or concrete anchor walls, the> may be contmuous or a serles of separate unlts Sheet p ~ l eanchorages are of the balanced or cant~levertype Wlulst a concrete anchorage does not requlre the use of wahngs for the distnbut~onof the load from the tle rod, it 1s necessary to excavate to the full depth of the anchorage T h ~ smay cause dlfficult~eswhen the water table 1s near the ground surface The anchors are connected by tendons to the earth retammg structure to form a complete anchored wall system The proport~onsand depth of embedment of the deadman are generally governed by the workmg load of the anchor and by the effectwe passwe res~stanceavadable m frout of the anchorage 4.6.5.2 Design 4.6.5.2.1 General See 4.1.1 and 4 1 2 Further general guidance on the desgn of deadman anchorages 1s gven in BS 6349 Part 2 There are three basic desgn requlrements to be satsfled by the anchorage a) The anchorage should not yleld by movmg forward a sgn~ficantdlstance The reslstance to movement IS related d~rectlyto the effective
Anchored wali
4.6.5.2.2 Overall equzlzbnum
Where the passwe sol1 zone in frout of the anchor wall does not mterfere w ~ t hthe actlve so11 zone behind the m a n retamng wall, see figure 49, the desgn of each can be undertaken mdependently, see 4.1.1 and 4.1.2 The average tension in the tie rods should be determined as sufficient to ensure equilibrium of the main retaming wall, under the required des~gn sltuatlon The specification of the des~gnsltuatlons (see 3.2.2) should mclude the h~ghestground water level, the lowest external water level and the maxlmum live load wh~chcan reasonably be combined The maxlmum hve load wll mclude, where appropnate moonng forces, repeat crane loadmgs and surcharge loads on the retamed surface behmd the main retanmg wall
Non interference of active zone t o anchored wall and passive zone to Deadman
/"-
Active zone to anchored wall
I
passwe reslstance of the sod and the fnct~on between the anchorage and the so11 b) The deadman should not undergo excesslve settlement or rotatlon m relatlon to the tendons Thls requirement 1s seldom sgn~ficantm undisturbed granular soils but where the anchorage is located m uncompacted fill, or weak so&, it may be necessary to provlde a foundat~onto the deadman or to use an alternatwe anchorage system A certam amount of movement may be accommodated by the provlslon of pln-jolnted tendons and tendon ducts c) The deadman unlt should be des~gnedto resist the bendmg moments and shear forces resultmg from the tle bar forces and earth pressures actlng upon it
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Figure 49. Non-interference of zones for anehored wall
.
Section 4
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In order to prevent progresswe fadure of a wall followmg the uncouphng or rupture of a smgle tle rod, the specificat~onof the deslgn sltuatlon should include the assumpt~onthat any t ~ rod e may be defectwe and that the tension m the defectwe rod is shared between its nearest ne~ghbours Where the active and passwe zones of the m a n and anchor walls necessarily mterfere, the anchor wall should be assumed not to develop full passlve reslstance due to lateral stress rehef followmg actlve movement of the mam wall The anchor wall should be deepened to develop the required reslstance Cons~derat~on should be gven to des~grungthe combmatlon of both walls, as a single unit s ~ m ~ lto a r a mass wall, m whch the effective resistance of the lower falure plane, see figure 50, will then be invoked to preserve the overall equ~hbnumof the block The mternal equll~bnum of the smgle unit should also be determmed and in part~cularthe moment equabnum of the walls about thelr tle rod connections Two levels of tles may be necessary 4.6.5.2.3 Deadman deszgn The reslstance to forward movement of the anchor wall IS the Mference between the passwe reslstance of the so11 m front of the anchor wall, lgnonng any surcharge or l ~ v eload on t h ~ ground, s and the actwe force on the back of the anchor wall ~ncludlngany surcharge or hve load on t h ~ ground s The depth and length of the anchor wall should be suffment to reslst the total anchor force uslng the mobhzed sod strengths (see 3.2.3 to 3.2.5) The factor of safety for anchorage system and ~ndiv~dual members should not be less than 2 In accordance with BS 8081
The existence or proposed construction of buned semces and excavations w ~ lreduce l passlve reslstance and a consewatwe level should be assumed for deslgn A group of ~ndiv~dual deadman anchorages may be cons~deredas a contmuous anchorage where the spaclng between mdlvldual anchorages does not exceed the depth to the top of the anchor Corrosion protection should be prov~deddependmg on the locatlon of the anchorage (see BS 8081) At the end of the assumed Me of the system the calculated factor of safety should be not less than 75 % of that ongulally assumed or 1 75 whichever is the greater
4.7 Waterfront structures 4.7.1 General The deta~leddeslgn of waterfront structures IS dealt w ~ t hm BS 6349 Parts 1 to 7 to wluch reference should be made 4.7.2 Concrete and reinforcement The durablhty of re~nforcedconcrete m mantime cond~t~ons depends on the quahty and ~mpermeabllityof the concrete In preventmg steel remforcement from corrodmg Cover to remforcement m mantune structures should be preferably 75 mm but not less than 50 mm T h ~ s should be the mlnlmum d~stancefrom the surface of any steel remforcement h k s , tendons, or sheath to the surface of the concrete Sulfate Ions, present m seawater at relatwely h g h concentrations, react wlth tncalclum alummate In hardened Portland cement The rate of attack,
A c h e zone t o main wall
Passwe zone to rear wall
/ Figure 50. Double wall construction where zones interfere
BS 8002 : 1994
wh~ch1s greatest In warm or polluted waters, can be reduced by lmuting the proportion of tr~calcmm alummate in the cement and in UK waters a m a m u m of 10 % is recommended The tncalcmm alurnmate content should not be less than 4 % in order to avoid attack of steel reinforcement by chlondes 4.7.3 Design 4.7.3.1 Equilibrium of the walls See 4.1.1 and 4.1.2 4.7.3.2 Design level and overdredging Where ~t is necessary to mmtaln, at all tunes, a minunum depth of water at the face of a waterfront structure, it is normal to ident~fya level above which all material may be removed This level is arnved at by cons~denngthe loaded draught of the vessels using the berth, the underkeel clearance required and the t~dalrange This level with a further obhgatory allowance of 0 5 m IS the design level Overdredgmg, that is, the removal of matenal below the des~gnlevel, may be mewtable as a dredger will not be able to produce a w e n level without tolerance Overdredgmg IS sometimes used In order to achieve extended mtervals between successwe dredang campagns 4.7.3.3 Hydraulic fill and backfilling generally Where fill is deposited on the landward slde of waterfront structures to form a level quay surface or a geneml area of reclamahon, the fill may be placed before or after building the waterfront structure, dependmg on the type of structure and the method of construction Nl may be placed hydrauhcally, by pumpmg from dredgers or by depos~tmgdry matenal from the shore, using earth moving equ~pment Care should be exercised to ensure that dunng fiUmg, stabihty is mamtained at all stages of construction Dry backfthng is generally placed by conventional tippmg and the fill may be cohes~veor noncoheswe I t should be as free as practicable from organic matter and should be selected to ensure its su~tab~lity for its purpose as a filled area Where hydrauhc fill is used, provision should be made for the dmnage of water from the fill The structure should be designed to support the standing hydraulic head and the resulting lateral pressure which will occur dunng hydrauhc filhng The hydraulic fill level may be m excess of the fmal fill level The resultmg hgher lateral pressure should be allowed for m the deslgn In addnon there may occur excesswe deflect~onsin the structure, for t h s excess stress cond~tionreduced factors of safety may be used Hydraulic f ~ lshould l be granular and should be well graded so that it consohdates well and provides a dense and homogeneous fill behind the waterfront structure Cohes~vematenals ansing from a
Section 4
dredgmg operation are generally unsn~table The consohdation period for such matend requlres a tlrne measured in years Sand and gravel mlxtures are normally suitable for use as hydraulic fdl Matenals with a sgnificant content of coarse grained matenal may cause pumping problems if there is a need to pump over long distances, due to the greater slurry veloc~ty and hence greater energy Input required If the hydraulic fdl matenal contams a sigmficant proportion of fines, problems may anse due to the natural tendency of the fines to segregate Furthermore, in such matenal excess water may take some time to drain out There should be, at all tlmes durmg the filling process, adequate anchorage capaclty This may requlre early f ~ n immed~ately g in front of and behmd the anchorage Undue settlement of filled matenal behind a waterfront structure 1s undes~rable,particularly in future load bearing areas close to the waterfront structure T h s may requu-e the prior removal of soft and organic matenals from the existmg bed levels Backfill placed in the dry should be deposited in honzontal layers of th~cknesscompatible w ~ t hthe nature of the matenal and the type of compactlon equipment used to acheve the design dens~ty Attent~onshould be gwen to the compactlon of the matenal ~mmediatelybehmd the structure, addit~ondloading may be imposed by the compaction process This should be evaluated with respect to the capaclty of the structure to wthstand such loading, see 3.3.3.6 Backfill matenal which 1s t~ppedthrough water should be granular and, as with hydraulic fill, due account should be taken of the vanations m fill density wh~chcan occur 4.7.3.4 Scour and its effects A change in one of the parameters wh~chdefine the sed~menttransport pattern may dlsturb the dynamic equilibrium of the system In the context of waterfront structures scour may be caused by change m velocity or drection of the current This, In turn, may be due to the construction of new works, the act~onof sh~ps'propellers, the removal or deposition of bed matenal or other action causlng change in a steady current, turbulence or edd~esSllts, sands and gravels are suscept~bleto scour in d~minishingorder of sensitlvlty, wlule stlff cohesive soils are resistant and even comparatively soft clays may remain stable m conditions where a granular matenal nught be eroded Scour may be reduced, or even prevented, if waterfront structures are desgned so that the emsting current or tidal regune is d~stnrbedas little as poss~ble An ideal solution may be ~mpossible,as patterns for ebb and flow may not be syrnmetncal and operat~onalrequirements may impose constramts In such cond~t~ons, the best compronuse should be sought
.
.
Section 4
Methods of measuring currents and sedlment transport are gwen in sectlon 2 of BS 6349 Part 1 1984 Where the constructlon Itself causes a change in current velocity for example at bndge plers, embankments, tralnmg works or reclamation works, an assessment should be made of the degree to wh~cht h s may cause scour in the exlstlng sea or nver bed lf necessary the des~gnof the works should provlde for scour protectlon or for deepening of the bed wh~chmay result Scour protectlon w ~ t h ~ the n t ~ d azone l is commonly achleved by some form of armounng and reference should be made to sectlons 2 and 7 of BS 6349 Part 1 1984, and sectlon 8 of BS 6349 Part 5 1991 For protectlon m a submerged locat~onit 1s usual to provlde a natural or art~ficialrubble apron Where scour problems occur at an exlstlng structure they may be removed by su~tabletraning works, often qulte s~mplem form It e advlsable that hydrauhc model stud~esbe undertaken to d e t e n n e the most suitable form and to ensure that no detnmental effects are mduced elsewhere It is advlsable to carry out routme hydrograph~c surveys at the face of waterfront structures if the bed 1s l~ableto eroslon to ensure that the bed IS not lowered to a polnt where the stab~l~ty of the structure e jeopardized The frequency of the surveys should be based on expenence of the rate of bed eroslon 4.7.3.5 Tide and river levels The des~gnof waterfront structure in t ~ d awaters l should normally be based on m e s t and lowest astronom~calt ~ d e sThese are respectwely the hlghest and lowest levels that can be pred~cted under average meteorolo@zal cond~tionsand under any combmat~onof astronomcal condrt~onsand are glven in Admiralty t ~ d etables Water levels may be rased above the predicted hlgh t ~ d elevels by storm surges, by Intense meteorological depress~onscausmg se~ches,by onshore wmds and, m the case of estuaries by Increased nver flow ansmg from storm run-off It 1s advisable to estabhsh from data, if available, the probable frequency of the extreme condrt~ons, though m Bnt~shwaters it 1s common practlce to subtract predcted t ~ d elevels from recorded t ~ d e levels at slack water to gwe posltwe or negatlve storm surges More d e t d e d mformat~onon these matters is gwen m sectlons 2 and 4 of BS 6349 Part 1 1984 and section 2 of BS 6349 Part 2 1988 For waterfront structures on nnpounded systems, des~gnshould be based on a sltuatlon where accidental draw-down mrght occur whlch would lower the water level to mean low water spnngs The maxlmum level to whrch water may normally nse should be taken as mean h~ghwater spnngs unless speclal clrcumstances ~ndlcatethat a h~gher level may be possrble Reference should be made to sectlon 2 of BS 6349 Part 2 1988
BS 8002 : 1994
4.7.3.6 Tidal lag and ground water The he~ghtof ground water behmd a waterfront wall depends on a) the heght of the water on the outer face of the wall, b) the Inflow Into the ground of water from landward and from the outer s ~ d eof the wall, c) the permeab~l~ty of the ground behmd, through and under the wall, d) the dramage, d any, prov~dedto cater for the ground water If there are semi-permeable layers m the ground a lugh water table may occur and there may be several water tables at d~fferentlevels At t m e s the water table in the ground at a d~stanceback from the waterfront is h~gherthan that ~mmed~ately behnd the waterfront wall T h ~ s may affect the load~ngon the wall and may reduce the capaclty of an anchorage system by 1) provldlng buoyancy to the anchorage, 2) reduclng the resistance of the ground in front of the anchorage, 3) creatlng water pressure behnd the anchorage Emergenc~esmay anse affectmg the backfill, for example by I) dramage outlets bemg frozen or othenv~se blocked, 11) burst~ngof a water man, IU)storm water from waves or tidal surges overtoppmg the wall In such instances h~gherground water levels than those ment~onedabove need to be mvestgated and cons~deredm the deslgn 4.7.3.7 Wave pressures Where wave actlon may have a s~gnificanteffect on the structure careful assessment should be made of the hydrodynam~cforces on the wall submerged below water level and the helght, length and angle of approach of waves should be taken Into account In the design when assessing the total hydraulr pressure Draw-down m the wave trough IS usually more Important than pressure from the wave crest When the retanmg wall IS relat~velyimpermeable, the t ~ d alag, l that IS the d~fferent~al level between ground water level on the actwe s ~ d eand low t ~ d e level on the passlve s ~ d eshould , be Increased to at least one half the wave he~ghtto represent a wave trough where a standmg wave can occur Also, when an impermeable retalnmg wall retams permeable solls, the effect should be cons~deredof wave action gradually bu~ld~ng up water levels In the retamed soils Effects of waves on waterfront structures are cons~deredIn d e t d m sectlons 4 and 5 of BS 6349 Part 1 1984 to whch reference should be made Where appropnate, the hydrodynam~cforces for submerged walls anslng from selsmlc actrvlty, should be taken Into cons~derat~on
4.7.4 Construction 4 7.4.1 General
Waterfront structures should be constructed in accordance w ~ t hthe requirements of BS 8110 Part 1, mod~fledas necessary for the mantlme environment by the recommendat~onsof section 7 of BS 6349 Part 1 1984 4.7.4.2 Posszble effects on coastal or river regimes Structures which project into flowmg water or wh~chchange the nature of the flow boundanes may induce reglme changes, particularly w ~ t hsoft or rnob~lebed matenal and where silt transport 1s h g h A firm bed material, such as gravel, rock or s t ~ f clay, f 1s much less l~kelyto be affected by the construction of a waterfront structure espec~allyif the current veloc~tydoes not exceed 1 m/s and there IS little material in suspension Where a new structure 1s constructed along a sgmflcant dlstance of the waterfront it may affect the existlng flow pattern of the rlver or the tidal stream if resistance to flow is reduced, for example by the replacement of an exlstmg natural bank or sloping foreshore \nth comparatively smooth or more vertlcal structure A structure of t h s kind WIN,In these circumstances, tend to attract current flow, possibly to a sufficient extent to mduce scour or erosion in its Immediate vlc~rutyand create a flow pattern wh~chcould not have been Inferred from observat~onof the prevlous und~sturbed resme condition
Large sohd structures projectmg into a strong current create changes in the flow resme and may induce local scour and accretlon, part~cularlyif mcorrectly aligned wlth the dlrectlon of flow Abrupt shoulders or return ends, may also create suffic~entlydis1,urbed con&tions to affect the new structure and possibly the navlgat~onof craft m the vlcmlty Open p~ledstructures are less likely to induce changes and, as a rough w d e , an obstruction by phng in the reglon of 15 % of a cross sectlon normal to the flow should not generally create alterations to flow cond~t~ons Coastal bcaches whlch are subject to llttoral dnft are hkely to be sensltlve to the effects of a waterfront structure if a new structure sqmficantly mpedes httoral dnft or 1s large enough to change the effects of wave action, accretlon and erosion are probable and these are hkely to cause local changes m the beach alignment A reversal of the effects described above may also be brought about by the removal of an emstmg waterfront structure Speclallst adv~ce should be sought where a structure by reason of its size or character 1s likely to mduce regme changes In certam circumstances it may be poss~bleto introduce des~gnfeatures, for example, rubble aprons, to mlnunlze the effects of regme changes on the structure
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BS 8002 : 1994
Annexes Annex A (normative) Graphs for K, and K,
Annex A
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Annex A
BS 8002 : 1994
Hor~zontal surface
60
8 =0
50 40 30
20
10
15
20
25
30
35
40
Des~gn values of 0'
Figure A.2 Passive resistance (horuontal component)
- Horizontal ground surface behind wall: Values of K p
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Annex A
Annex A
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s
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-BS 8002 : 1994
Annex A
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Annex A
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100 90 80 70 60 50 40 30
i
20
t
-+-
s" * C w c 0
a
E
100 90 + c 80 0 70
' 0
60 50
40 30
20
10 10
15
20
25
30
35
40
L5
Des~gn values of @'
d/@'. 0
Figure A.6 Passive resistance (horizontal component)
- Sloping ground surface behind wall: Values of Kp
-Annex A
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..
20
25
30
35
Des~gn values of @'
*/@'=o 66
Figure A.7 Passive resistance (horizontal component)
- Sloping ground surface behind wall: Values of KD
Annex A
10
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15
20
25
30
35
40
45
Deslgn values of @ '
6/08 = 1
w
Figure A.8 Passive resistance - Sloping ground surface behind wall: Values of Kp (horizontal component)
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Annex B
Annex B (informative) Traditional design methods for embedded walls B. 1 General Vanous different methods of des~gnare descnbed m t h ~ sannex (see figure B 1) They have trad~t~onally been used part~cularlyfor the d e s ~ n of small to med~umsized embedded or sheet plle retamng walls They mvolve the determmation of an overall factor of safety Fp The methods are Subject to varlous shortcom~ngsw h c h are o u t h e d below For these methods the des~gnof a propped or cantilever wall can be broadly considered in two parts, first the determmation of the depth of penetratlon required to ensure overall stabhty of the sod and the structure and secondly the structural des~gnof the wall stem to resist the ~mposedloadmgs The depth of penetration should be determined from a stab~htyassessment based on hmamg equ~hbnummethods of analys~sm wh~ch condlt~onsof fa~lureare postulated and a factor of safety applied to ensure that such a failure does not occur T h s can be d o ~ in e several ways a) by usmg a multiplymg factor to Increase the depth of penetratlon from that required for hm~tmgequ~libnum, b) by factonng sod strength, or c) by vanous methods of factonng the nett or gross forces unposed on the structure The size of the factor of safety used in design IS dependent on wh~chmethod of des~gn1s used The factor of safety should be suffic~entlylarge to cater for uncertamt~esin the parameter values and to sat~sfyserv~ceabil~ty requrements by preventing unacceptable deformat~onsunder workmg condit~ons,smce thls 1s generally a more adverse loadmg than lumtmg equ~hbnum
Compansons between a number of methods in current use for the d e s w of small to medlum sued retalrung walls are gwen in the report on a ~ a m e t r l study c (Potts and Burland, 1983). carned but at the 1nstlgat;on of the comm~tteeresions~ble for t h s code of practlce The methods descnbed below have been descnbed in some detail in CIRIA -- report 104 (1984) Where pract~calexpenence wlth some of the methods IS lumted or confined to part~cular apphcat~ons,the des~gnshould be checked against a d~fferentmethod to ensure compatzb~htyFor each of these methods the representatwe strength values should be used and not the des~gnvalues descnbed in 3.1.8 B.2 Gross pressure method T h ~ method s has been used for many years It cons~stsof factonng the gross passwe pressure diagram The method IS mconslstent for cohesive sods w ~ t hlow values of 9' when the factor of safety on the stability of the wall, F, may exceed the ratio KpIK,, for example wrth nn~formclays under undramed condit~ons,when 9' = 0 and KpIK, = 1 In these cond~t~ons, below a certam depth of penetratlon, wh~chis dependent on the wall geometry, loading and sod parameter values, the calculated factor of safety Fp decreases w t h lncreaslng depth of penetratlon, because In effect, the bulk we~ghtof the sod on the passwe side of the wall is factored (CIRIA report 104, 1984) For the same reason, except where a larger factor of safety Fp IS desirable m order to prevent excesslve movement of the wall, the value of Fp = 2 IS consematwe when Kp/K, IS not very large (I e 9' less than approximately 30°) T h s apphes m part~cularto st~ffclay soils
1
a1 Gross pressure method
b l Net total pressure d h o d
C)
Net ava~lable~ass~ve
Figure B 1 Different methods of assessing t h e ratio of restoring moments t o overturning moments (from CIRIA Report 104 (Padfield and M m , 1984)) reproduced by permsion of the D~rectorGeneral of CIRIA
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.a
Annex B L
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.
In practice, there 1s no advantage m exceedmg the depth of penetratlon beyond wh~chthese anomahes occur m the value of the factor of safety Also ~t IS now common to use lower values of Fp for low values of p' e g CIRIA report 104 (1984) recommends a value of Fp = 2 0 for p' > 30°, Fp 1 5 to 2 0 for p' ranglng from 20° to 30° and Fp 1 5 for p' < 20° Desp~tethese mcousistenaes, the method continues to be used with success and IS popular because of its s ~ m p l ~ c ~ t y
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B.3 Net available passive resistance method A descr~pt~on of t h ~ method s 1s @ven ~nthe paper by Burland, Potts and Walsh (1981) The method has part~allyovercome the anomaly ~nthe gross pressure method m regard to the factor of safety wh~chreduces w t h lncreasmg depth of penetratlon In t h ~ smethod the factor of safety 1s appl~edto the moment of the net avdable passwe reslstance T h ~ s1s the d~fferencebetween the gross passwe pressure and those components of the actwe pressure, w h ~ c hresult from the we@ of the sod below the dredge hne In effect, the dead we~ghtof the so11below the dredge hne, on both s~desof the wall, 1s factored T h ~ smethod requlres different factors of safety F, to be used through the range of values for p' B.4 Strength factor method The method apphes a factor of safety to reduce the strength parameter values of the so11 and IS analogous to that used for the calculat~onof slope stab~htyThe effect of factonng strength parameter values IS to Increase K,, reduce Kp and mod~fythe d~stnbut~on of earth pressures relatwe to that obtamed usmg the gross pressure method T h ~ s dtstorts the pred~ctedvalues for moments in the wall stem, accordmgly the method should be used only to determme the depth of penetratlon The requ~reddepth of penetratlon IS determmed at h t m g moment equ~hbnumusmg the sod forces calculated from the reduced strength parameter values In cohes~onlesssod the angle of sheanng reslstance at hmtmg equ~hbnump', is taken as tan- (tan pflF,) S~mllarly,for a total stress analysls c,, c,lF, where cum IS the value of the undramed shear strength at hmltmg equhbnum When the sod possesses both cohes~onand fnct~on different values of F, may be used for each parameter However a sunphfied approach, more wdely used, is to reduce both strength parameters by a smgle factor of safety such that for an effectwe stress tan p', = tan @/F, and c', = c'lF, Usmg thLs approach reduced angles of wall fnct~on6 , and adhes~onc,, should be determmed by mamtalmg a constant value of the ratlo 6,/@, and cWm/c', equal to the assumed values of Sip' and c,/c' The results obtamed w t h the method are sensltlve to the value of F, chosen CIRIA report 104 (1984), recommends values of F,
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wh~chare appl~cablefor stiff clay and are not appropnate for other types of so11 such as soft clay or sand In applymg the factor of safety to the so11 strength the method has the merlt of factonng the parameters wh~chfrequently represent the greatest uncertamty m dessn although care should be taken m selectmg the values of F,
B.5 Nett pressure method A method based on applymg a factor of safety to the total nett passwe forces, has been used w ~ t h success ~nthe des~gnof steel sheet p ~ l ewalls, m predom~nantlygranular matenals, see B r ~ t ~ sSteel h P h g Handbook (1988) The nett passwe forces are based on the horizontal pressure d~stnbut~on dmgram, wh~ch1s denved by subtractmg the actlve earth and water pressures from the passlve earth and water pressures The result 1s equ~valentto applymg a lower factor of safety to the gross pressure (Potts and Burland 1983, I F Symons of the 1983) The method rehes on red~stnbut~on pressure on the walls because of them flexb~hty The des~gnof the anchorages should take Into account the effects of such red~stnbut~on of of the pressures on the wall Red~stnbut~on pressures can have only a margmal effect on a cantilever retanmg wall The safety of a wall des~gnedby the nett pressure method depends mamly on the cho~ceof conservatwe values of so11 parameters In the absence of adoptmg conservatwe values, the deslgn produced by t h ~ s method may result m a retalrung wall w ~ t han madequate level of safety
B.6 End fixity method A des~gnmethod based on the assumption of futed earth support cond~t~ons was developed ongmally for flex~bleplle walls embedded in sand, see Terzagh~(1954) Where the embedment below the lower ground level IS m cohes~veso& the fixed earth support cond~t~on should be assumed only for structures of a temporary nature, because of the long-term deformat~oncharactenst~csof such so~ls However, when the wall 1s des~gned~n terms of effectwe stress and steady state cond~tionshave been reached, that 1s the pore pressures are steady, then the fmed earth support cond~t~ons may be used When a wall 1s very st~ff,e g remforced concrete pdes, the free earth support cond~t~on should be used, smce the st~ffness of the wall may prevent the rotatlon of the toe of the wall suffmently, such that the passwe pressure requ~red on the rear face for fixed earth condit~onsis not allowed to develop In order that end f m t y may develop at the toe of the p~le,the embedment should be greater than for the free-earth cond~t~on Prov~d~ng that the wall sectlon and the props are adequate, there e no falure mechanism whch 1s relevant to an overall stab~htycheck 'Rad~t~onally, for a retammg wall
BS 8002 : 1994
embedded In sand, a factor of safety of 1 1s used (Terzagh~,1943), dependmg on the confidence that can be placed on the sol1 parameter values and other vanous factors w h c h may affect the des~gn Although satisfactory desgns have resulted from the use of t h ~ method, s the assumptions made confhct wlth the necessity for large displacements of the wall for the full moblhatlon of the passwe pressures The advantage of the method IS the reduct~onin plle bendmg moment m consequence of the behamour of the p ~ l eas a propped cantllever
Annex C (informative) Bibliography C.1 Publications referred t o in text ALPAN 1 1961 Machme foundat~onsand soll resonance GVotechnzgue, 11 BARKAN D D 1962 Dynamzcs of bases and foundatzons McGraw-H111, New York BIRT J C 1974 Large concrete pours - Survey of current practzce CIRIA Report 49 BOLTON M D 1986 The strength and d~latancyof sands Geotechnzque, 36 (1) 65-78 BOLTON M D 1991 Geotechnacal stress amlyszs for brzdge abuhnent deszgn TRL Contractor Report 270, Transport Research Laboratory, Crowthorne BRINCH HANSEN J 1953 Earth pressure calculatzons The Danish Techmcal Press, Copenhagen BROMS B 1971 Lateral pressure due to compactlon of cohes~onlessso~lsIn Proc 4th Intnl Conf on Sozl Mech Found Engg , Budapest, 373 - 384 BURLAND J B , POTTS D M and WALSH N M 1981 The overall stabhty of free and propped embedded cantllever retanmg walls Ground Engznemng, 14 (5), 28 - 38 CARDER R D , MURRAY R T and KRAWCZYK J V 1980 Earth pressure agaznst a n expmmental retaznzng wall backfilled wzth szlty clay Transport and Road Research Laboratory Report LR 946, Cmwthorne CARDER D R , POCOCK R G and MURRAY R T 1977 Expemmental retaznzng wall faczlzty lateral stress measurements wzth sand backfzll Transport and Road Research Laboratory Report LR 766, Crowthorne CIRIA5) The standard penetrataon test (SPT) Methods and use Report ERlCP17, London CIRlA 1974 Large concrete pours - A survey of current practzce Report 49, London.
Annex C
CIRIA 1987 Pressuremeter testzng - Methods and znteqretatzon Ground engmeenng report, CIRIA Book 3, CIRLAIButterworth - Hememann, London CIRIA 1974 A comparzson of quay wall deszgn methods - Techn~calNote 54 London CLAYTON C R 1 1990 The mechanzcal propertzes of the chalk Proc Internat~onalSympos~um, B r a t o n , 213-232, Thomas Telford Ltd, London CLAYTON C R I , SIMONS N E and MATHEWS M C 1992 Szte znvestzgatzon 2nd ed Blackwell Saent~fic,Oxford CLAYTON C R I , SYMONS 1 F and HIEDRA COB0 J C 1991 The pressure of clay backfill against retamlng structures, Canadzan Geotech Journal, 28, (Apnl), 282 - 297 CORBETT B 0 1961 Settlement of turbo-alternator blocks D~scuss~on Proc 5th Intril Conf on Sozl Mechanzcs and Foundatzon Engznemng, Pans, (3), 220 - 221 CUMMINGS E M 1960 Cellular cofferdams and docks naris of the Ammcan Soc of Czv Engrs , 125 DEPARTMENT OF THE NAVY 1971 Deszgn manual Sozl mechanzcs, foundatzons and earth structures, NAVFAC DM-7, Bureau of Yards and Docks, Wash~ngtonDC DEPARTMENT OF TRANSPORT 1991 Speczfzcatzon for hzghway works HMSO, London DEPARTMENT OF TRANSPORT Departmental standard BD 30187 Backfilled retaznzng walls and bmdge abutmexis HMSO, London DOWRICK D J 1977 Earthqualce reszstant deszgn a manual for engzneers and archztects Wlley, London FARRAR D M 1971 A laboratory study of the use of wet fzll zn embankments Transport and Road Research Laboratory Report 406, Crowthorne FLEMING W G K , FUCHSBERGER M , KIPPS 0 and SLIWINSKI Z 1975 D~aphragmwall spec~ficationProc Conf on Dzaphragm Walls and Anchorages Instn Civ Engrs London HAMBLY E C 1979 Brzdgefoundatzons and substructures Department of the Environment Bu~ld~ng Research Establishment, HMSO, London HYDROGRAPHER OF THE NAVY Annual pubhcat~onAdmzralty tzde tables Hydmgrapher of the Navy, Taunton, Somerset TA1 2DN INGOLD T S 1979 The effects of compactlon on retaimng walls rnotechnzque, 29 (3), 265 - 283 INSTN OF CIV ENGRS 1975 D~aphragmwalls & anchorages Proc Conf organzsed by Instn Czv Engrs , Stpt 1974, London INSTN OF CIV ENGRS 1979 Proc Conf on Clay Fzlls London
--
..
..
Annex C
WSTN OF STRUCT ENGRS 1975 Deszgn and construclzon of deep basements London INSTN OF STRUCT ENGRS 1977 Structure-sozl znteractzon A state of the a r t report London INSTN OF STRUCT ENGRS 1991 Recmmendatzons for permzsszbk stress deszgn of reznforced concrete structures London ISHIHARA J 1993 L~qu~faction and flow fallure dunng earthquakes 33rd Rankme Lecture, Gdotechnzpe Instn CIV Engrs JACK B J 1971 Wall desgn Ground Engzueerzng, London, 4 (5) JAMES E L and JACK B J 1975 A design study of diaphragm walls Proc Conf on Lhaphragm Walls and Anchorages Instn Civ Engrs 41 - 49 KERISEL J and ABSI E 1990 Actzve and pasave earth pressure tables 3rd ed A A Balkema, Rotterdam KREY H 1936 E r d d m k , Erwzderstand and Pragahzgkezt des Baugrundes W Ernst u Sohn, Berlin LACROIX, ESRIG & LUSCHER 1976 ASCE spenahty conference on lateral stress and earth retaning structure LAMBE T W and WHITMAN R V 1969 Sozl mechanzcs John W~ley,New York LAMBE T W and WHITMAN R V 1979 Sozl mechanzcs, SI verszon John Wlley & Sons, New York MEYERHOF G G 1953 The beanng capaclty of foundations under eccentnc and inclined loads In Proc 3rd la Conf Sozl Mech Found Engng 1, 440 - 445 NOBELS' EXPLOSIVES COMPANY LIMITED 1972 Blastzng practzce Stevenston, Scotland OVESEN N K 1962 Cellular coflerdams, calculatzons methods and model tests Damsh Geotechnical Institute Bulletm No 14 PADFIELD C J and MAIR R J 1984 Deszgn of retaznzng walls embedded zn stgf clay CIRIA Report 104 CIRIA, London PAPPIN J W , SIMPSON B , ELTON P J and RAISON C 1986 Numencal analysls of flexlble retanung walls Symposzum on computer applzcatzons zn geotechnzcal engznemng The Mldland Geotechmcal Soclety PARSONS A W 1976 The rapzd measurement of the mozsture condztzon of earthwork materzal Transport and Road Research Laboratory Report 750, Crowthome PARSONS A W and BODEN J B 1979 The mozsture condztzon test and zts potentzal applzcatzons zn earthworks TRRL Report SR 522
BS 8002 : 1994
PARSONS A W and TOOMBS A F 1987 The preczszon of the mozsture condztzon test TRRL Report RR 90 PEUCH A A 1982 bas^ data for the design of tension piles in sllty soils Proc 3rd Int Conf on the behavzour of offshore structures Cambridge (USA) 1, 141 - 157 PO?TS D M and BURLAND J B 1983 A parametme study of the stabzlzty of embedded earth retaznzng st.ructures TRRL Supplementary Report 813 Transport and Road Research Laboratory, Crowthorne RICHART F E 1960 Foundation Vibrations Proc Ammcan Soc of Czv Engrs ROWE P W 1952 Anchored sheet plle walls Proc Part l Instu Czv Engrs London ROWE P W 1957 Sheet plle walls at failure Proc Part I , Instn Czv Engrs London SEED H B and WHITMAN R V 1970 Design of earth retannng structures for dynamlc loads, ASCE Spec Conf Lateral stresses zn the ground and the deszgn of earth retaznzng structures ASCE, 103 - 147 SEED H B , IDRISS I M and ORANGO I 1983 Evaluation of hquefactmn potentla1 using field performance data Proceedzngs Amwzcan Soczety of Czvzl Engzneers, 109, No 3, 458 - 482 SLIWINSKI Z and FLEMING W G K 1974 Pract~calconsiderations affecting the construction of diaphragm walls Proc Conf of Dzaphragm Walls and Anchorages, 1-10Instn Czv Engrs SOKOLOVSKI V V 1960 Statzcs of sozl medza Butterworth Scientif~c Pubhcations, London SYMONS I F and MURRAY R T 1988 Convent~onal retaning walls pilot and full-scale stud~esProc Instn Czv Engrs Part 1, 84, June, 519 - 538 SYMONS I F , CLAYTON C R I and DARLEY P 1989 Earth pressure agaznst a n ezpmmental retaznzng wall backfilled wzth szlty clay Transport and Road Research Laboratory Report RR 192, Crowthorne SYMONS I F , LIlTLE J A , MCNULLY T A , CARDER D R and WILLIAMS S G 0 1987 Behavzour of a temporary anchored sheet pzle wall on the AI(M) a t Hatfield TRRL Research Report 99, Transport and Road Research Laboratory, Crowthorne TERZAGHI K 1939 Soil Mechan~cs- A new chapter in engmeenng sclence Proc lnstn Czv Engrs London, 12, 106 - 141 TERZAGHI K 1943 Theoretzal sozl mechanzcs John W~leyand Sons Inc , New York TERZAGHI K 1945 Stabihty and stiffness of cellular cofferdams Trans of the Amemcan Soc of Czv Engrs 110, 1083 - 1202
BS 8002 : 1994
Annex C
..
TERZAGHI K and PECK R B 1967 Sozl mechanzcs zn engznemng practzce 2nd ed John Wlley and Sons Inc , New York Chapman & Hall Limlted, London THORBURN S 1963 Tentatwe correction chart for the Standard Penetrat~onTest in non-coheswe soils Czvzl Engznemng and Publac Works Revzew 58, No 683, 752 - 753 TOMLINSON M J 1987 Foundatton deszgn and constructzon 5th ed US ARMY CORPS OF ENGLNEERS 1958 Deszgn of pzle structures and foundutzom manual C.2 Additional recommended publications (not referred to m text) BRITISH STEEL 1988 General Steels Pzlzng Handbook 6th ed Bntlsh Steel plc, Scunthorpe FARRAR D M and DARLEY P 1975 The operatzon of earthmmng plant on wet fill Transport and Road Research Laboratory Report 688, Crowthome FEDERATION OF PILING SPECIALISTS 1991 Speczfzcatzon for steel sheet pzlzny, Federation of Phng Specialists, 39 Upper Elmers End Road, Beckenham, Kent BR3 3QY
INGOLD T S 1977 Performance of a retalmg wall w ~ t hdeep foundat~onsGround Engzneerzng PARSONS A W and DARLEY P 1982 The effect of sozl cond.ztzons on the operatzon of earthmovzng plant Transport and Road Research Laboratory Report 1034, Crowthorne PO?TS D M and FOURIE A B 1985 A numencal study of the effects of wall deformation on earth pressures Int J n l for Nummcal and Analytzcal Methods zn Geomechanzcs 10, 383 - 405 ROWE P W 1950 The dzstrzbutzon of lateral earth pressure on a stzff wall due to surcharge 45, No 531, Civil Engmeenng and Pubhc Works Review ROWE P W and PEAKER K 1965 Passive earth pressure measurements G6otechnzque 15 (1) 57 - 78 SYMONS I F 1983 Assessmg the stability of a propped in s ~ t uretamlng wall in overconsol~dated clay Proc Instn Czv Engrs Part 2, London, 75, 617 - 633 TERZAGHI K 1954 Anchored bulkheads Trans of the Amemcan Soc of Czv Engrs 119, 1243 - 1324 TOMLINSON M J 1977 Pzle deszgn and constructzon practace 3rd ed Viewpomt Pubhcatlons
.
K,, values of Annex A Kp, Values of Annex A S, values of 228 q' for rock Table 4 o',,t 2 2 3, Table 2, Table 3 Actwe earth pressure 1 3 1, 3 1 9 3 3 3 Actwe pressure cohesionless sod 3 3 3 2 clay smls 3 3 3 3 Adhesion undratned wall 2 2 8 Anchorages 4 6 deadman 4 6 5 Anchored embedded walls 4 4 1 Angle of sheanng reslstance 2 2 3 Apphed loads 3 1 7 At-rest earth pressures 3 3 2 Back dramage 4 7 3 8 Base frtctlon 2 2 8 Base reslstance to shdmg 4 2 2 3 Basement walls 4 3 2 Beanng capaclty gravny walls 4 2 2 1 Bendmg moment reductmn 4 4 3 3 Benton~te4 4 8 3 1 Boreholes 2 1 2 Buttressed walls 4 3 1 2, 4 3 1 4 4 Calculations based on totalleffective stress 3 2 3 Cantdever concrete walls 4 3 1 4 3 Cantilever embedded walls 4 4 1 Cellular cofferdams 4 5 3 r-. h .a-l. l.i.
as f111 2 2 i strength of Table 4 Chemrcals m ground 2 1 3 Crcular cofferdam 4 5 1 5 3 Clay soils actwe pressure 3 3 3 3 passwe resistance 3 4 2 3 Chmate 2 1 5 Climate changes effect on loads 3 3 4 5 Close bored pde walls 4 4 7 2 Cofferdams 4 5 construction 4 5 1 5 frammg 4 5 1 4 in water 4 5 1 4 3 \nth unbalanced loads 4 5 1 4 4 Cohesionless soils 3 4 2 2 Compaction earth pressures 3 3 3 6 Compatlbdlty of defomatrons 3 1 5 Compos~tesoldler piles 4 4 9 1 Concentrated loads 3 3 4 3 Concrete crlbs 4 2 7 2 2 mfilllng to remforced masonry walls 43352 walines4521 1 . 4 5 2 2 5 Concrete constructmn mass walls 4 2 3 5 3 Consewatwe values of sol1 parameters 1 3 2 Construct~onjomnts m concrete walls 4 2 3 5 2 In ranforced concrete 4 3 1 5 Construction of cellular cofferdams 4 5 3 5 cofferdam 4 5 1 5 concrete pde walls 4 4 7 5 concrete walls 4 2 3 5
cnb walls 4 2 7 5 dnphragm walls 4 4 8 3 gabions 4 2 6 5 masonry walls 4 3 3 5 reinforced concrete walls 4 3 1 5 s o l d ~ r l k m gpdes 4 4 9 4 tlmber sheet walls 4 4 5 4 waterfront structures 4 7 4 Corroslon of concrete 4 3 1 4 7 of gabions 4 2 6 3 4 protectmn of deadman anchorage 4 6 5 2 3 of steel mles 4 4 4 4 3 ~ o u n t e r f o kwalls 4 3 1 2, 4 3 1 4 4 Cnbwork 4 2 7 Cntlcal state strength 2 2 3, 3 1 8, Table 2 Damage to gabmns 4 2 6 3 4 Damp proof course for masonry walls 4 3 3 3 3 Deadman anchorages 4 6 5 Deformatmns, compat~bil~ty of 3 1 5 Depth of slte mvestlgatmn 2 1 2 Deslgn base fnctmn 3 2 6 Deslgn earth pressures 3 1 9 Deslgn method 3 2 ~esGn of basement walls 4 3 2 2 cofferdams 4 5 1 4 2 concrete mle walls 4 4 3. 4 4 7 4 cnbs4 2 ? 4 deadman anchorages 4 6 5 2, 4 6 5 dlaphragn walls 4 4 3 embedded walls 4 4 3 gablons 4 2 6 4 ground anchorages 4 6 3 2 masonry walls 4 2 4 3 mass concrete wall 4 2 3 4 remforced masonry walls 4 3 3 4 soldler pies 4 4 3, 4 4 9 3 steel sheet plles 4 4 4 2 struts 4 5 2 1 2 strutted excavations 4 5 1 4 2 t m b e r plles 4 4 3, 4 4 5 3 waterfront structure 4 7 3 Desgn philosophy 3 1 Deslgil situatmn 1 3 3, 3 2 1, 3 2 2 Deslgn sod strength 1 3 4. 3 1 8 Deslgn surcharge load 1 3 5, 3 3 4 Deslgn to structural codes 3 2 i Design undralned wall adhesmn 3 2 6 - -----using effectwe stress 3 2 5 uslng total stress 3 2 4 Deslgn value of parameters 3 1 6 of soil parameters 1 3 6, 2 1, 3 1 8 Des~gmwall fnctlon 3 2 6 Detadmg of cnb walls 4 2 7 4 2 Diaphragm
masonry walls 4 3 3 2 walls4 4 8 Dlsturb~ngforce 1 3 8, 3 3 Double waU cofferdams 4 5 1 4 5 Dramage 3 3 5 3 behmd waterfront walls 4 7 3 8 Dramage layer 3 3 5 3 Dralned shear strength 2 2 3 Dnmng sheet plies 4 4 4 4 1 Dnvmg tunber p k s 4 4 5 4 Durablbty of concrete walls 4 3 1 4 7 Dynamlc apphcatmn of loads 3 3 4 4 Dynamlc loads 3 3 4 4
Earth-fllled cofferdam 4 5 1 5 4 Earth pressure coefflc~ent1 3 9, 3 1 9 Earth pressures At-rest 3 3 2 des~gn3 1 9 due to compactLon 3 3 3 6 on gravlty walls 3 2 7 hlgher than normal 3 1 9 Earthquake desxgn cntena 3 3 4 4 Ell walls 4 3 1 2, 4 3 1 4 3 Embedded walls 4 4 Eau~hbnum calculatlo; 3 2 1 , 3 3 1 of cnb walls 4 2 7 4 1 of deadman anchorages 4 6 5 2 2 of embedded walls 4 4 3 2 of remforced concrete walls 4 3 1 4 1, 421,422 of reinforced mamnry walls 4 3 3 4 1 of retamng wall 4 1 1 of waterfront structure 4 7 3 1 Excavatmn unplanned 3 2 2 2 R112 2 7 hydraulic 4 7 3 3 Flexlble sheet plle walls 4 4 3 3 Flood t ~ d e s2 1 4 Flownet3352 Flow of water into cofferdam 4 5 1 5 Forrnwork to concrete walls 4 2 3 5 1 Foundations of gavlty walls 4 2 2 Framing for cofferdam 4 5 1 4 6 Freshwater carrosmn of steel pdes 4 4 4 4 3 3 Frost heave 3 3 4 5 Fully active earth pressure 1 3 11 Fully passwe earth resistance 1 3 12 Gab~ons4 2 6 desmn of 4 2 6 4 equz~briumof 4 2 6 4 2 hfeof4262 Galvanized mre gabmns 4 2 6 3 4 Geological lnvestigatmn 2 1 2 Geometry of structure 3 2 2 Geotechnlcal data 2 1 Geoteade filter 3 3 5 3 Graded filter dram 3 3 5 3 Granite as fill 2 2 7 Granular soils 3 4 2 2 Grawty walls 4 2 Ground anchorages 4 6 3 Ground water 2 1 3, 3 2 2 Grouted cavlty masonry walls 4 3 2 2 Gmde walls for dmphragm walls 4 4 8 3 Heave in cofferdams 4 5 1 5 2 Hollow block walls 4 3 3 2 Hydrauhc fill 4 7 3 3 Hydrostat~cpressure on mass walls42343 Hydrostatic uplift 2 1 3 mjolnts4 2 3 4 3 In sltu concrete plle wall 4 4 7 Klng pdes 4 4 9 'Lwalls4312,43143 Lagmg4 4 9 2 Layered soils 3 3 8 , 3 4 4 Lghtly overconsolldated clay 3 3 3 4 h m l t state 1 3 13, 3 1 2 h n e loads 3 3 4 3 Lqufactmn of sands, slits 3 3 4 4
Loads apphed 3 1 7 cllmatlc changes 3 3 4 5 concentrated 3 3 4 3 dynarnlc 3 3 4 4 surcharge 3 1 7, 3 3 4 uniformly dlstnbuted 3 3 4 2 onwall3 2 2 on waterfmnt structures 4 7 3 10, 47311,47312 Llfe of eablons 4 2 6 2 Life of L t p i e s 4 4 4 4 3 5 Llmlt state 3 2 2 Mason1y claddmg to mass concrete walls 42345,42354 Masonry u'alls ranforced 4 3 unremforced 4 2 4 Mass concrete walls 4 2 3 Mass permeablllty of clay 3 3 3 5 Matermls for cellular cofferdams 4 5 3 2 concrete piles 4 4 7 3 cnb walls 4 2 7 3 remforced concrete walls 4 3 1 3 ranforced masonry walls 4 3 3 3 umber plles 4 4 5 2 1 Mlmlnum surcharge 3 2 2 2 Wlnlmum unplanned excavdoon 3 2 2 2 Moblllzatmn factor 1 3 14, 3 1 8 Mortar for masonry walls 4 2 4 2 3 remforced masonry walls 4 2 4 2 3 43332 Movement jolnts in masonry walls 4 2 4 3 3 m mass walls 4 2 3 4 2 m remforced walls 4 3 1 4 6 Normally consohdated clay 3 3 3 4 Overconsol~datedclay 3 3 3 4, 3 4 2 3 3 h n l a l load fdcton 3 2 7 Paswve earth resistance 1 3 15. 3 4 2 2 cohesmnless so1153 4 2 2 granular smls 3 4 2 3 hghtly overcmsohddted clay 3 4 2 3 2 normally consohdated clay 3 4 2 3 2 overconsohddted clay 3 4 2 3 3 weak rocks 3 4 3 Peak soil strength 3 1 9 Phdosophy of dewgn 3 1 Plies tensam 4 6 4 Pocket-type masonry walls 4 3 3 2 Porewater pressure 3 3 5 2 Precast concrete diaphragm walls 4 4 8 3 ? walls411 4 5 Pre3sure due to wares 4 7 3 7 Pre3trezsed ctrncrete sheet prles 4 4 h. 4 4 6 3 d~aphragmwalk 4 4 8 3 2 masonry walk 4 7 ?
Propped embedded walls 4 4 1 Protection of concrete 4 3 1 4 7 PVC coated wme gablons 4 2 6 3 4 Quetta bond masonry walls 4 3 3 2 Rankme's formula 3 3 3 2, 3 4 2 2 Rapld sheanng 1 3 16 Remforced concrete qheet p k s 4 4 6, 4 4 6 2 walls 4 3 Remforced masonry walls 4 3 . 4 3 3 Ranforced soil 4 2 5
--Res~stanceto movement 3 4 Resistance to shdmg gavlty walls 4 2 2 3 Reverse cant~leverwalls 4 3 1 2, 4 3 1 4 3 Rwer crossings by cofferdams 4 5 1 2 Rwer levels 4 7 3 5 Rock2 2 6 Rotatmn of wall 3 1 9 Scope of code 1 1 Scour 4 i 3 4 Secant q' 2 2 3 Secant pile walls 4 4 7 2 Seepage forces 3 3 5 2, 3 4 5 Servrceablllt> lmlt gavlty walls 4 2 2 2 Servlceablllty hmlt state 1 3 18, 3 1 2 Sheet plle walls 4 4 1 Sheet piles mseawater4444 3 4 llfeof444435 SlltS 2 2 5 Site date 2 1 Slte lnvestlgatmn 2 1 2 So11 werght of Table I Sod denslty, 2 2 2, Table 1 So11oararnetem conservative values 1 3 2 desigm value 1 3 6 etaluatlon 2 2 2 representatwe 2 2 2, 3 1 8 Sod properties 2 2 Sod strength 3 1 8 des.gn 3 1 8 peak 3 1 9 Soldm plies 4 4 9 SPT for rock 2 2 6 SPT value 2 2 4, Rgure 2, Table 3 Standprpes 2 1 3 Steel sheet p h g 4 4 4 Steel struts 4 5 2 1 2 Steelwallngs452 1 1 , 4 5 2 2 3 Stone for gabmns 4 2 6 3 7 Strength of so11 changes m 3 2 3 Structural codes 3 2 T
Structural deslgn 3 2 7 of relnforced walls 4 3 1 4 2 Structure selection of 1 5 1 Struts 4 5 2 1 2 Strutted excavatmns 4 5 Surcharge 3 1 7 mmmum 3 2 2 2 Surcharge loads 3 3 4 Surface flnlsh concrete walls 4 2 3 4 4 Symbols 1 4 Teewalls4312.43143 Tensmn cracks 3 3 3 5, 3 3 5 4 Tenslon pries 4 6 4 TIdelag4 7 3 6 TIde levels 4 7 3 5 TIerods452 2 6 TIes4 5 2 n m b e r cnbs 4 2 7 2 1 Tlmber sheet plles 4 4 5 Tmber struts 4 5 2 I 2 T~mberwahngs 4 5 2 1 1, 4 5 2 2 4 Total stress analysis 3 3 3 3 Trees 2 1 6 Types of gab~ons4 2 6 2 of mass concrete walls 4 2 3 2 Ultlmate hmlt state 1 3 19, 3 1 2 Undrained shear strength 2 2 3 U m f o n t y coefficient Table 3 Uniformly dlstnbuted loads 3 3 4 2 Unplanned excavatmn 1 3 20, 3 2 2 1, 3222 Unre~nforcedmasom walls 4 2 4 Uphft to waterfront structures 4 7 3 9 Vertlcal sheetlng 4 4 9 2 Wahngs 4 5 2 1 1 Wall adhesmn 2 2 8 Wall displacement 3 2 5 WaV fnction 2 2 8, 3 3 3 2 Walls on spread foundatmns 4 3 Water levels 2 1 3 pressure 3 3 5, 3 4 5 pressure r e m e 3 2 2 3 table 3 3 5 2 Waterfront condltlons 3 3 5 5 Waterfront structures 4 7 Waterproofing masonry walls 4 2 4 4 2 Wave a m o n 4 7 3 7 Wave pressure 4 7 3 7 waves2 1 4 Weak rocks 3 3 3 7, 3 4 3 Weepholes 3 3 6 3 m cofferdams 4 5 1 5 4 Welght of soil Xhle I Welded wlre mesh gabmns 4 2 6 3 2 Weldmg sheet plies 4 4 4 4 2 Woven wre mesh gabmns 4 2 6 3 1
blank
109
List of references (see 1.2) Normative references BSI publications BRITISH STANDARDS IKSTITUTIOPI, London
BS 187
1978
BS 449 BS 449 Part 2 BS 729 1971
BS 1377 BS 1377 BS 1377 BS 1377 BS 1377
Part Part Part Part
1969
2 3 5 6
1990 1990 1990 1990
BS 1377 Part 7 1990 BS 1377 Part 8 1990 BS 1377 Part 9 1990 BS 3921 1985 . RS 4102 1990 BS 4360 1990 BS 4729 1990
BS 5268 Part 3 1985 BS 5268 Part 5 1989 BS 5328 BS 5328 Part BS 5328 Part BS 5390 1976 BS 5400 BS 5400 Part BS 5400 Part BS 5400 Part BS 5400 Part BS 5400
1 2
1991 1991
1 2 4 7
1988 1978 1990 1978
Part 8
1978
Speczfzcatzon for calczum szlzcate (sandlzme andflzntlzme) brzcks Spenfzcatzon for testzng nnc coatzngs on steel wzre and for qualzty requzrements Spenfzcatzon for the use of structural steel zn buzldzng Metmc unzts Speczfzcatzon for hot dzp galvanzzed coatzngs o n zron and steel artzcles Speczfzcatzon for materzals for damp-proof courses Speczfzcatzon for aggregates from natural sources for concrete Spenfzcatzon for azr-cooled blastfurance slag aggregate for use zn c o n s t m t z o n Speczfzcatzon for m z l d steel wzre for general engzneerzng puToses Methods of test for sozls for n v z l engzneerzng purposes Classzfzcatzon tests Chemzcal and electro-chemzcal tests Cmpresszbzlzty, pemneabzlzty and durabzlzty tests Consolzdatzon and pemneabzlzty tests zn hydraulzc cells and wath pore pressure measurement Shear strength tests (total stress) Shear strength tests (effectzve stress) In-sztu tests Speczfzcatzon for clay brzcks Speczfzcatzon for steel wzre and wzre products for fences Speczfzcatzon for weldable structural steels Speczfzcatzon for dzmenszons of brzcks of spenal shapes and szzes Speczfzcatzon for sheradzzed coatzngs on zron or steel Structural use of tzmber Code of practzce for pemnzsszble stress deszgn, rnaterzals and workmanshzp Code of practzce for trussed rafter roofs Code of practzce for the preservatzve treatment of structural tzmber Concrete Guzde to speczfyzng concrete Methods for speczfyzng concrete m m e s Code of practzce for stone masonry Steel, concrete and composzte bmdges General statement Speczfzcatzon for loads Code of practzcefor deszgn of concrete brzdges Speczficatzon for materzals and workmanshzp, concrete, reznforcement and prestresnng tendons Rtcommendatzons for materzals and workmanshzp, concrete, reznforctmt-nt and prestresszng tendons Code of practzce for protectzve coatzng of zron and steel structures agaznst corronon Code of practzte for preservatzon of tzmber
BS 5628 BS 5628 BS 5628 BS 5628 BS 5642 BS 5642
Code of practzce for use of masonry Structural use of unreznfmed masonry Structural use of reznforced and prestressed masonry Matemak and components, deszgn and workmanshzp Szlls and copzngs Speczfzcatzon for copzngs of precast concrete, cast stone, clayware, slate and natural stone Code of practzce for szte znvestzgatzons Precast concrete masonry unzts Speczfzcatzon for precast concrete mason?-# unzts Method for speczfyzng precast concrete masonry unzts Marzhme structures General m t e m a Deszgn of quay walls, jettzes and dolphzns Deszgn of dry docks, locks, slzpways and shzpbuzldzng berths, shzplzfts and dock and lock gates Deszgn of f e n d m n g and moomng systems Code of practzce for dredgzng and land reclamutzon Deszgn of znshore moorzngs and Jloatzng structures Guzde to the deszgn and constructzon of breakwaters Speczfzcatzon for reconstructed stone masonry unzts Spec@catzon for clay and calczum szlzcate modular bmcks Code of practzce for foundatzons Code of practzce for strengthened/reznforced sozk and other fzlk Code of practzce for deszgn of concrete structures for retaznzng aqueous lzquzds Code of practzce for ground anchorages Structural use of concrete Code of practzce for deszgn and constructzon Code of pructzce for spenal czrcurnstances SpecZfzcatzonfor hot rolled prducts of non-alloy structural steels and thew technzcal delzvery condztzons
Part 1 1992 Part 2 1985 Part 3 1985 Part 2
BS 5930 1981 BS 6073 BS 6073 Part BS 6073 Part BS 6349 BS 6349 Part BS 6349 Part BS 6349 Part BS 6349 Part BS 6349 Part BS 6349 Part BS 6349 Part BS 6457 1984 BS 6649 1985 BS 8004 1986 BS 80066) BS 8007 1987
1983
1 1981 2 1981 1 1984 2 1988 3 1988 4 5 6 7
1985 1991 1989 1991
BS 8081 1989 BS 8110 BS8110 Part 1 1985 BS 8110 Part 2 1985 BS EN 10025 1990
Informative references BSI publications BRITISH STANDARDS INSTITUTION, London
BS 1377 BS 1377 Part 1 1990 BS 1377 Part " 1990 BS 3921 1985 BS 5837 1991 BS 5950 BS 5950 Part 1 1990 BS 5950 Part 5 1987 BS 6031 1981 DD ENV 1992 DD ENV 1992-1-1 1992
Methods of test for sozk for czvzl engzneerzng purposes General requzrements and sample preparatzon Compactton-related tests Speczfzcatzon for clay brzcks Guzde for trees zn relatzon to constructzon Structural use of steelwork zn buzldzng Code of practzce for deszgn zn szmple and contznuous constructzon hot rolled sectzons Code of practzce for deszgn of cold formed sectzous Code of practzce for earthworks Enrocode 2 Deszgn of concrete structures General rules for buzldzngs (together wzth Unzted Kzngdom Natzonal Applzcatzon Document)
BSI - British Standards Institution BSI is the mdependent national body responsible for preparing Bntish Standards It presents the UK vlew on standards m Europe and at the mternational level It IS mcorporated by Royal Charter Contract requirements
A Bnt~shStandard does not purport to include all the necessary provlslons of a contract Users of Bnt~shStandards are responsible for thelr correct apphcation Revisions
B r ~ t ~ Standards sh are updated by amendment or revlsion Users of Bntish Standards should make sure that they possess the latest amendments or ed~t~ons Any person who finds an maccuracy or amb~gu~ty whde usmg t h s Bntish Standard should notlfy BSI wlthout delay so that the matter may be mvestigated swiftly BSI offers members an individual updatmg servlce called PLUS whch ensures that subscribers automat~callyrecewe the latest editions of standards Buying standards
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Orders for all BSI, mternational and fore~gnstandards publications should be addressed to Customer Serwces, Pubhations at Milton Keynes Tel 0908 221166, Fax 0908 322484 In response to orders for mternatlonal and European standards, a is BSI pohcy to supply the BSI implementat~onof those that have been published as Bntish Standards, unless otherwise requested Information on standards
BSI prov~desa wide range of information on nat~onal,European and international standards through its L~brary,the Standardhe Database, the BSI Informat~onTechnology Semce (BITS) and its Technical Help to Exporters Serv~ceContact Customer Serv~ces,Information at M~ltonKeynes Tel 0908 226888 Subscnbing members of BSI are kept up to date w ~ t hstandards developments and recewe substantial discounts on the purchase pnce of standards For deta~lsof these and other benefits contact Head of Customer Serv~ces, Membership Group at Milton Keynes Tel 0908 226777 Copyright
BSI 2 Park Street London WIA2BS
Copynght subs~stsm all BSI pubhcat~onsand no part may be reproduced m any form wlthout the pnor permlssion In writing of BSI This does not preclude the free use, in the course of implementing the standard of necessary details such as symbols and size, type or grade des~gnat~ons lncludmg use by mcorporatlon Into computer programs, but where these deta~lsare reproduced includmg w~thouthmitat~onm pnnted form, in computer programs or in any other form whatsoever, the permlssion m writing of BSI must be obtamed and ~f granted w l l be on terms mcludlng royalty, before the product 1s sold, hcensed or otherwise exploited for commerc~algain Enqumes about copynght should be made to the Copynght Manager, Publications at Mllton Keynes
BSI Lmford Wood Milton Keynes MK14 6LE 9404- 11- 1k- JWD
ISBN 0 580 22826 6
B1526
