Well Foundation and its Design
Well Foundation
Large, thick, ck, holl ollow cylindrical well sunk into the ground to transmit loads from super and substructure of bridge to the the found oundin ing g soi soil Suitable for very heavy vertical and lateral loadings as well as lar large scou scourr dep depths ths Commonly used and popular type of foundation in allu alluvi vial al plai plains ns in Sout South h Asia Asian n regio egion n Gene Generrally ally costl ostly y and and ine ineffecti ectiv ve in uti utiliz lization tion of material rialss in case ase of small all loads and and shallow depths. Not Not suit suitab able le for cla clays and and soil soilss cont contai aini ning ng lar large boul boulde derrs
Merit/Demerit of Well Foundation Merit It has a larger bearing area and section tion modulus as compared with the cross sectional area of the main body of pier and abutment. This This prov provid ides es bett better er load load dist distri ribu buti tion on and and good good late laterral resi resist stan ance ce.. Conc Concrrete is cast ast over grou ground nd and and has has bet better quali uality ty con control trol,, as well as good good dept depth h cont contrrol. ol. Skil Skille led d man man powe powerr and and tech techno nolo logy gy easi easily ly availa ailabl ble e
Demerit Obso Obsole lette techno chnolo logy gy in many any part partss of the the worl orld Unec Unecon onom omic ic for for smal smalle lerr brid bridge gess Prob Proble lem ms of til tilt and and shif shiftt Time Time cons consum umin ing g in cons constr truc ucti tion on Qual Qualit ity y con control trol prob proble lem m in bott bottom om plug pluggi ging ng of well ell
Types of Well
Open Well
Pneumatic Well
Floating Box Well
Types of Well
Components of Well Foundation
Pier Well Cap
Top Plug Well Steining Dredge Hole with Granular Granular Filling
Well Curb
Cutting Edge
Bottom Plug
Components of Well Foundations and their Functions Well Ste Well Steini ining ng Well steining is the main body of a well. It should be heavy enough to sink the whole well without excessive kentledge. It should be strong enough so that it is not to get damaged duri during ng sink sinkin ing g and and from from the the eart earth h pres pressu surre from from outs outsid ide e of well well..
Wel elll Cu Curb rb Lowest part of well steining to transfer load through the cutting edge to the ground. It is made ade of rich icher con concret crete e and and is hea heavil vily rein einforce orced. d. It withs ithsttand ands the force orce fro from bottom plug lug due to arch action. It is made with tapering side inside the well and the taper angle is around 60 degrees with the hori orizontal plane. Som Sometimes the curb is lined with steel plate through out its height insid side and outside to protect from damages due to boulders if any. Its top diameter(outer) is kept 5 to 15 cm higher than the outer dia. of the well steining to facilit facilitate ate sinking. sinking.
Cutt Cu tting ing Ed Edge ge Sharp cutting edge is provided at the end of well curb, where boulders are not expected. Wher Where e as stud stud nose nose cutt cuttin ing g edge edge is prov provid ided ed,, wher where e boul boulde derrs are are mix mixed with with soil soil.. Cutting ting edge dge shou should ld be stron trong g enou nough to resis sist cut cuttin ting pres pressu surre and and rigid igidly ly fix fixed with ith well ell curb.
Bottom Plu Bottom Plug g Concrete layer at end of curb to stop the sinking of well, transfer load of well in wider area area and and to contro trol the movement ent of und undergr ergro ound und water. It sho should be stron trong g enou nough to withstand large pressure and preferably in the shape of a bulb to produce arch action and and incr incre ease ase the the bear earing ing area area.. The The concr oncre ete sho should uld be rich icher and and with with abo about 15% more ore cemen ment content. It shou should ld be more ore work orkable able with ith slu slump abo about 150 to 200 mm.
Top Pl Plug ug Sand filling in the well is covered with top plug. It is usually made from lean concrete of 300 mm to 500 mm thickness. ss. Its function is to make a smoother base for well cap.
Wel elll Ca Cap p Well cap is a RC slab cast monolithically with the well steining and transfers load from sup superstru tructu cture / sub substru tructu cture to the the well stein eining. ing. Its Its diame iametter can be made ade large argerr by up to 1.0 m from the steining to accommodate the long abutment or pier. It should be stron trong g enou nough to with withsstand and the the pressur ssure e from abo above by slab slab acti actio on. The top level vel of well cap is usu usuall ally flu flushed shed with ith the the lowest water leve levell or at the riv river bed bed level.
Design of Well Foundation
Design of well foundation is carried out in the following steps. steps. 1.
Determine the depth of well foundation
2.
Determine the shape and size of well foundation
3.
Check the stability of well foundation Check stability at elastic state Check stability at at ul ultimate st state
4.
Perform structural design of well foundation Design well cap Design well steining Design well curb Design of bottom plug
Depth of Well Foundation
Shape and Size of Well Well Foundations Foundations
Shape and size of well depends on the size of substructure ,load and type of soil. Elongated shapes are used for long piers and abutments. Size of well is determined considering safe bearing capacity of soil at the founding level of well. The size of the dredge hole shall not be less than 2 m to facilitate dredging . Top diameter(outer) of curb should be higher than the outer diameter of the well steining to facilitate facilitate sinking. Usually curb offset offset is taken in the range of 50 to 150 mm Thickness of steining steining should be sufficient so that well can be sunk by its self weight . Minimum thickness of steining shall be 500 mm. Circular wells are most preferred because they are relatively strong, simple in construction , easy in sinking. Circular wells are not suitable for wide roads with wide substructures. Double D and rectangular types are commonly used wells after circular wells.
Load Loadss and and Load Loadss Comb Combin inat atio ions ns
(IRC:78-2 (IRC:78-2000, 000, Cl. 706.1.1) 706.1.1)
Load Lo adss an and d lo loa ads com omb bin ina ati tion on to be con onsi sid der ered ed in the de desi sign gn of wel elll ar are e
Dead lo Dead load adss fr from om su supe perrst stru ruct ctur ure e (G (G), ), se self lf wei eigh ghtt of su subs bstr truc uctu ture re in incl clud udin ing g we weig ight ht of sand filling (G), live load (Q) Q),, longitudinal force by braking (Fb), buo uoy yan ancy cy (Gb), force due to water current (Fwc), fr fric icti tion onal al for orce ce du due e to exp xpan ansi sion on//co cont ntrrac acti tion on of supe su pers rstr truc uctur ture e (Ff ), wind load (W), forces due to tilt and shift of the well (G), seis se ismi micc lo load ad fr from om su sup per ersstr tru uct ctur ure e an and d su subs bstr truc uctu turre (Feq), load due to back fill , Load due to snow (Gs), erection load (Fer), force due to water wave (Fwp), im impa pact ct due to floating bodies (Fim) an and d ce cen ntr triifu fug gal for orce ce (Fcf ) Impa Impact ct facto actorr is igno ignorred in the the desi design gn of found oundat atio ions ns.. Buoy Buoyan ancy cy is sepa separratel ately y cons consid ider ered ed for HFL HFL and and LWL. WL. Only Only 15% of the total otal buo buoyant ant force is take aken for the the depth epth belo elow max. max. scou scourr leve level. l. The loa loads and force orcess may be evaluated as per IRC IRC: 6 and their combinations for the purpos pose of the desig esign n of well ell will ill be as follo ollow ws:
Combination (I):
G + Q or Gs + Fwc + Ff ± Fb + Gb + Fcf + Fep
Combination (II):
(I) + W + Fwp or (I) + Feq + Fwp or (I ) + Fim + Fwp
Combination (III):
G + Fwc + Gb + Fep + Fer + Ff + W or Feq
Loads on Well Foundation W
W, H, M - Resultant vertical force, Horizontal force and Moment due to externally applied load
H
M Maximum scoured level
µ‘P
P-
Force due to net lateral earth pressure
µ’ P -
Frictional force along the embedded height of well
R-
Vertical reaction from base
M’-
Moment at base due to unequal distribution of base pressure
F-
Frictional force at base
P
M’
F R
Base of Well
Tilt and shift of well Soil stratum through which the wells are sunk are very rarely uniform and ther there efore ore, the the resis esisttance ance off offere ered by thes these e lay layers to the the sink sinkin ing g is dif different ent in different parts of the wells due to which tilt and shift of well my occur. The effect of til tilt and and shif shiftt is to cause ause ext xtrra found ounda ation tion pres presssure ure and and this this pres pressu surre shal shalll be cons consid ider ered ed in desi design gn..
IRC IR C 7878-200 2000 0 Pr Provi ovisio sion n
The well shall be shank vertically without any tilt and an d sh shif ifts. ts. However a tilt of 1 in 80 and shift of 150 mm due to translation in a dir ire ect ctiion whi hicch wi will ll cau ause se most severe effect shall be con onsi sid dered in desi sign gn of well.
Translational shift Total shift
Original C/L of Well Tilted C/L of Well Shifted C/L of Well
Base of Well
Stability of Well Stab Stabil ilit ity y of well ell und under the the acti action on of lat lateral eral load loadss depen epend ds on the the resist sistan ance ce of soil soil on its sides and base. For a given vertical load the deformation of load increases with the the incr increa ease se in lat lateral eral load loads, s, ther there efore ore resis esisttance ancess off offered ered by the sid sides and and the the base ase of well also change. The behaviour of the well at ultimate failure is different than at the elastic state. Therefore, in the design of well foundation, stability ity foundation ion (Ref (Ref.. IRC IRC 45) shou should ld be chec check ked at elas elasti ticc stat state e and and at ulti ultima mate te stat state e.
I.
Check Chec k the the stabi stabilit lity y of well well at elas elastic tic stat state e under under work working ing load load
Assumptions 1. Soil surr surround ounding ing the well and below below the the base is is perfe perfectly ctly elasti elastic. c. 2. Und Under er design design workin working g load unity unity soil reacti reaction on increases increases linea linearly rly with with increasing lateral deflection 3. Coe Coeffic fficient ient of of horizont horizontal al subgrade subgrade reacti reaction on increases increases linear linearly ly with the depth in cohesionless soil. 4. The well is assumed assumed to act act as a rigid rigid body body subjec subjected ted to unidirectional lateral load and moment at scour level of well.
Steps for checking stability of well at elastic state Step 1 Havi Ha ving ng de detter ermi mine ned d th the e gr grip ip le leng ngth th of we well ll,, ca calc lcul ulat ate e -Tot -T otal al do down wnwa warrd lo load ad con onssis isti ting ng of DL, LL ac acti ting ng on th the e ba base se of we well ll (W (W)) -Tot -T otal al la latter eral al lo load ad ap appl plie ied d ab abov ove e th the e sc scou ourr le leve vell (H (H)) -Total external moment applied at the base of well du due e to eccentricity of LL, tilt ti lt,, sh shif iftt et etcc (M (M). ).
Step2 Usin Us ing g th the e di dime mens nsio ions ns of we well ll ca calc lcul ulat ate e th the e fo foll llow owin ing g ge geom omet etri rica call pr prop oper erti ties es IB – MI of base section in the plane of bending about the axis perpendicular to the direction of lateral force
Iv – MI of vertical projected rectangle of well below scour level
B – Dia. of well L – Projected width of well in contact contact with soil offering passive resistance L=0. L=0.9 9×B m = Ratio of horizontal and vertical subgrade modulus at base level
,
μ - Coefficient of of friction between well sides and the soil
Angle of internal internal friction of soil soil φ – Angle
D f – Depth of grip of well
Step 3 Check the point of rotation of well lies at the base by ensuring that the frictional force at the base is sufficient to restrain the movement of well we ll for orwa ward rd or ba back ckwa ward rd
µ - coefficient of friction friction at base of well
Step 4 Check that the soil on sides remain elastic by ensuring the earth pressure below the pressure line γ – Unit wt. of soil (dry or submerged)
K A , K P – Coefficient of active and passive earth pressure
Step 5 Check the pressure at the base of well
σ 1 ,σ 2 – maximum and minimum base pressure
P – Total horizontal reaction from the side
A – Area Area of base section of of well qallow - Allowable Allowable bearing bearing capac capacity ity of soil
If the above conditions are not satisfied, the grip length of well shall be increased.
II. Check the stabilit stability y of well well at at ultimate ultimate sta state te under ultima ultimate te load load Steps for checking stability of well at ultimate state Step 1
Compute ultimate vertical load at base (Wu), Ultimate moment about the point of rotation of well which is taken at 0.2 Df from base (Mu) and ulti ul tim mate hor oriizon onttal lo load ad at the sc scou ourr le leve vell (Hu ) for va vari riou ouss ul ulti tim mat ate e loa oad d combination
Step 2
Check maximum pressure at base with allowable bearing pressure OR
Step 3
Check ultimate moment with total ultimate moment of resistance of well OR Mb – M.R. of base section Q – Shape factor Ms – M.R. Due to the well sides earth pressure M - M.R. due to to side frictio friction n
Structural Design of Well Well I. Design of of We Well Ca Cap
Critical section for BM and SF Pier/Abutment Well
Plan of Well Cap
D
Dia. of well cap (D) –
Dia iam meter of wel elll cap de dep pen ends ds on th the e si sizzes of ab abu utme men nt/ t/p pier an and d diameter of well. Diameter well cap is kept at least 150 mm larrge la gerr th than an we well ll an and d pi pier er//ab abut utme ment nt in al alll si side dess to ma main inttai ain n of offfse set. t.
Thick. of well cap (d) -
Thickness of well cap is critical crit ical secti section. on.
Area of steel (A st) -
Area of steel bars (Ast) required for well cap are designed for the BM found at the critical section of well cap.
determined to resist BM and SF at
II. II. Desi Design gn of Well ell Ste Stein inin ing g 1. Determine th the e th thiickne nesss of well steini ning ng Thickness of well steining should be such so that well is sunk by its self weight without exce ex cess ssiv ive e ken entl tled edge ge.. Th Thic ickn knes esss of st stei eini ning ng is fi fixe xed d ba base sed d on th the e fol ollo lowi wing ng co cons nsid ider erat atio ions ns.
d - Extern External al diamet diameter er of of well well (m) f – Skin friction of well k – Constant, which depends on the type of soil D – Depth of well below GL or LWL (m) γ c – Unit wt. of concrete t – Thickness of well steining
t
Well steining
2. Chec Check k the the pr pres essu sure re on wel welll steining during sinking Bed level Water level h’
Outside of well
KAγs h’ + KA γsub h
h
On the outside of well , the soil as well as water exert the pressure. On the inside of well, water exerts the pres essu surre, whi hich ch pa part rtly ly can ance cell lled ed th the e ou outs tsid ide e pr pres essu surre. The net pressure (p1 = KAγs h’ + KA γsub h - γw h) causes hoop str tres esse sess in th the e wel elll stei eini ning ng . Hoop compressive stress along the inner face (f 1) and outer face of steining (f2 ) should not exceed the allow all owabl able e com compr press essive ive str stress ess of con concr cret ete e
γw h
≤
Allowable compressive compressive stress of concrete
p1 -
Net pressure on ou outside of of well
r 1 , r 2 -
Inte Intern rnal al and and ext exter erna nall diam diamet eter er of of well well
K A –
Coefficient of active earth pressure
γ s , γ sub – Unit wt. of soil above water level and
submerged unit wt. of soil γ w –
Unit wt. of water
3. Che Check ck the the stress stresses es in well well ste steinin ining g due to to all possi possible ble loads loads Calcula te all the vertical forces, horizontal forces and moments at the level of maximum scour. Calculate scour. Find the maximum bending moment due to all possible loads at the section of zero shear and check stresses in well steining. x ’ distance from the MSL and found as follows. Section of zero shear lies at ‘ x
Scour level (MSL)
x
Section of zero shear force
Df B
Bottom of well
F – Factor of safety (F = 2) H – Resultant horizontal force at scour level γb – Submerged Submerged unit wt. of soil (γb = γsoil - γw) Mmax – Maximum value of BM at x level Mo – BM at scour level ka , kp – Coefficient of active and passive earth pressure B – Diameter of well Df – Grip of well
Stres Str esse sess in stein inin ing g of well ar are e fou oun nd at th the e se seccti tio on of max axim imum um momen entt and an d ch chec eck ked as fol ollo lows ws..
f 1 shall be less or equal to the allowable bending compressive stress of concrete f 2 shall be less or equal to the allowable bending tensile stress of concrete Where, VVerti ertica call load loadss at the the leve levell of X, i.e. i.e. at the the sect sectio ion n of zero ero SF Mmax - Ma Maxi ximu mum m mome moment nt at sect sectio ion n of of zer zero o SF SF ( Mmax = M0 + 2/3Hx) INet moment of inertia of the well section (I = I outer – I inner ) yDistance from the centroid to the outer face of well (y = outer radius of well) ANet cross se sectional area of the well steining ( A = π r2 outer - π r2 inner ) r outer, r inner - Outer and inner radius radius of well
4. Dete Determine rmine vertic vertical al and tra transver nsverse se reinf reinforce orcement ment of well ste steining ining To determine the vertical reinforcement, reinforcement, well is considered as column section subjected to axial load, shear and bending moment . However the amount of vertical reinforcement reinforcement provided in steining should not be less than 0.2% 0.2% of actual cross sectional area of the the steining. The transverse reinforcement in the steining should be provided in accordance with provision for a column but in no case should be less than 0.04% of the volume per unit length of the steining. steining.
III. III. Design Design of Well Well Curb Curb Well curb shall be designed for the loads subjected A. Wh Whil ile e sin sinki kin ng the the wel elll B. Whi While le the cur curb b rest rest on the bot bottom tom plu plug g of wel welll
A. De Desi sign gn of of curb curb whi while le sin sinki king ng th the e well well
Well Curb
N
N d
H θ
Q P
Q P
H
Where, d – Mean diameter of curb N – Weight or steining in KN/m θ – Angle of inclination of bevelling face of curb θ ≈ 600 μ - Coefficien Coefficientt of friction between between soil and concret concrete e of curb P – Force in KN/m acting normal to bevelling face of curb Q – Force in KN/m acting parallel to bevelling face of curb
B.
While Whi le the the curb curb re rest st on on the the bott bottom om plug plug of well well
River bed
Under the co con nditions when the cu cuttting edge is not able to move downwards , reaction can be resolved into hor oriz izon onttal and ver erti tica call com omp pon one ents ts.. For th the e co con ndi diti tion on hoop ho op te tens nsio ion n de deve velo lope ped d in cu curb rb is gi give ven n by Df
d
r
p2 b p1
In granular soil, the hoop tension ‘H’ is relieved by the active pressure around the curb.
At junction of the curb and steining , a moment ‘M 0‘ is developed due to the horizontal horizont al force H caused by bevelled action i.e.
IV. IV. Design Design of of Bottom Bottom Plug Plug For circular circular well, thickness of the seal ‘t’ ‘t ’ is given by the following relation
Reinforcement of Well Well Cap Well Steining
Sectional Elevation Well Curb
Sectional Plan
Reinforcement of Well Cap Pier
Well Cap
Bottom Reinforcement
Top Reinforcement
Reinforcement Detailing Well Curb
