1991_Recommendations Clouterre - English Translation

This method, first developed at the University of California at Davis, is a limit equilibrium method, as well. It assumes that the potential failure surfaces are vertical axis parabolas, the vertices of which are located at the bottom of the facing (figure 12). The soil is assumed to be homogeneous and without water, and the geometry of the wall is simple (vertical facing, horizontal soil surface at the top, parallel nail rows, equidistant and of the same length). The nails are assumed to work only in tension. As with the Stocker method, two blocks separated by a vertical line passing through the extremity of the nails are examined when the failure surface exits beyond the reinforced volume. To calculate the forces between these two blocks, a coefficient K is used, defined as the ratio of the horizontal and vertical stresses, and taken as equal to 0.4 in frictional soils and 0.5 for cohesive soils.

101

Soil Nailing Recommendations-1991 y

aH

c

B

H

x

Figure 12. Shen design method (1978).

The same global safety factor F is taken for both soil and nails, whereby: 't

= e/F

mob

't

mo

b

=

+

cr

tan g

cp/F

. (TF TF G

L

mln - , - ]

Thus, the minimum safety factor value corresponding to the most critical parabola can be calculated. The validity of this method, which is limited to very simple geometries and a specific failure mechanism, has been assessed by analyzing the failure surfaces observed in structures, as well as the heights of walls at failure observed on reduced scale models in centrifuge. The agreement between the forecast and experimental results appears acceptable for the few simple experiments conducted. The method has recently been modified to take account of more complex geometries.

102

Chapter 3: Conception and Design

2.4.2.3. Juran design method (1990)

Juran has developed a limit equilibrium method to calculate the failure point for soil nailed walls, which is similar to the one he developed for Reinforced Earth (Juran et al., 1977) and has been used as the basis for the current design methods of Reinforced Earth walls. The potential failure surfaces are taken to be logarithmic spirals intersecting the bottom of the wall. The nailed mass is divided into slices parallel to the nails. The assumption formulated is that the horizontal component EH of the force between any two slices remains constant (figure 13a). The soil is assumed to be homogeneous and without water. It is also assumed that the points of maximum traction and maximum shear force in the nail

rows coincide with the most critical potential failure surface, i.e., correspond to the minimum global safety factor. This method is interesting in that it can, by considering the local equilibrium in each slice, be used to calculate the tensile and the shear forces developed in each nail row at their point of intersection with the failure surface. Thus, the soil nailed wall can be designed to avoid any risk of progressive failure through the nail breakage beginning with rupture in one nail row. It does not, however, allow analysis of mixed failures. The shear forces are calculated, based on the assumption that the maximum shear force in a nail is mobilized at the point that coincides with the considered potential failure surface. The bending stiffness of the nails is also taken into account on the basis of the nondimensional parameter N defined as follows:

where ks is the lateral coefficient of subgrade reaction, [0 is the transfer length of the nail, and [0 = (4 E I / ks D?/4, and D is the nail diameter. The maximum tension T" and the maximum shear force Tc in a nail row are determined in accordance with two nondimensional parameters: TN

T" / y H Sv Sh

TS

T c / y H Sv Sh

Figure 13b shows the type of graph proposed to calculate T" and Tc knowing the value of the nondimensional parameter N. This design method was proposed (Juran et al., 1990) for designing soil nailed walls at a serviceability limit state. The assumption is that the peak shear resistance of the soil is mobilized under service conditions along the maximum tension line, irrespective of the value

103

Soil Nailing Recommendations-1991

of the wall's global safety factor. This assumption, which is based on an analysis of results from a few full-sized structures (Juran et al., 1988) needs to be justified, both theoretically and experimentally. However, the approach is interesting and merits being looked at in greater depth from the point of view of developing a method to cover the internal design of soil nailed walls for service limit states.

(a )

i= 15° 0.3f----+--

:z 0.2

"7=0 {3=0

1

0.08

f--"',;,---

(J)

I-

I--

0.06'----...... 0.11---0.04 t---~---+---~ ......---"<:-"d----j

O'--------'----L---...L-----l 0.05

0.10

0.15

0.20

o'------'------'---..L-------.JL---.....J 0.05

c/yH

C.IO

0.15

0.20

c/yH

(b ) Figure 13. Juran design method (1990): charts used to calculate Tn

104

= Tmax and Te.

0.25

Chapter 3: Conception and Design

3. GENERAL METHOD FOR STUDYING THE STABILITY OF A SOIL NAILED STRUCTURE

3.1.

Limit state design -

Assumptions and data

3.1.1. Principles of limit states design

The stability of a soil nailed structure is justified in terms of its ultimate limit state by looking at sufficient potential failure surfaces, whether these intersect the nails or not, to determine the most critical one (figure 14).

Figure 14. Different types of potential failure surfaces.

The application of limit equilibrium methods to soil nailed walls consists of comparing the forces or stresses resulting from the external actions, with the maximum resisting forces or stresses that can be mobilized in the soil nailed mass, for a series of potential failure surfaces.

3.1.1.1. Basic formula

For limit equilibrium methods of slices (Bishop or perturbation method), the equilibrium analysis can be presented using the following symbolic form:

where

r S3

:

method factor that takes account the approximations inherent in the design method

105

Soil Nailing Recommendations-1991

r m:

partial safety factors

In this symbolic form, the formula: 't(rcG + rQQ = rcwF w + r A FA + rTFT + rRFR) represents the force or stress on the potential failure surface resulting from the combination (marked +) of the actions shown between the parentheses, and where 'tmax (soil nailed) represents the resisting force or stress that can be mobilized in the nailed soil along the potential failure surface. The term 'tmax (soil nailed) thus "incorporates" any increase (or reduction) in the shear resistance of the soil along the potential failure surface due to the presence of nails, the effect of which is to increase (or reduce) the normal stresses on the potential failure surface. The following notations are used for the actions: G : Q: Gw : FA FT FR

permanent loads variable loads effects of water accidental loads ground anchors forces nail forces (reinforcement)

The actions are shown in the above formula with their representative value, which is either the characteristic value defined on a statistical basis or a value defined by a code (nominal value). The characteristic value is defined by the ratio of the most probable value and the dispersion coefficient. One takes for the most probable value its arithmetic average. The dispersion coefficient value is determined to ensure that the characteristic value, maximum or minimum, a minimal probability is not achieved (above or below). Nominal values are carefully fixed by codes on the assumption of known extreme values or on the basis of other values that might be reasonably considered. The way in which the representative values of each action are calculated is explained in paragraph 3.1.2. Each action is ascribed a load factor (rc, r Q, r cw, r A , rt , rr)' The resistance of a given material is expressed by its characteristic value that, in principle, shows an acceptable probability of not being reached. With regard to soils, the geotechnical engineer will give the characteristic values to be taken into account and that will be combined with the corresponding partial safety factor r m' 3.1.1.2. How to account for nails and prestressed ground anchors

The difference between external forces and resistances is conventional and could be subject to different interpretations depending on the design method used and the point of view

106

Chapter 3: Conception and Design

adopted regarding prestressed ground anchors and nails. In the form shown above, the forces in both nails and prestressed ground anchors are shown as external forces (F R and FT ) through their tangential components on the potential failure surface, and playa role in the resistance ['t max (soil nailed)] with the effect of their normal components on the potential failure surface. In any limit equilibrium method, the values of the forces in the nails FR are calculated for each potential failure surface. This is done by taking account of the nails' resistances and the soil-nail interaction (one might, for example, use the multicriteria approach [see paragraph 3.2.2. of this chapter]). This is why the load factor r R of Fr will be written as the inverse form of the partial safety factor of the nails: 1

r m,R Within the framework of ultimate limit state design, the current rules consider that the forces present in the prestressed ground anchors are external forces and are therefore known; they are independent of the potential failure surface considered. In an ultimate limit state calculation, it would be appropriate to take as the tensile value of a prestressed ground anchor the smallest value between the guaranteed elastic limit of the reinforcing bar and the anchor limit pull-out force. Moreover, steps should always be taken to verify the compatibility between the corresponding total displacement (soil + tie-back) and the deformations experienced by the nail and the soil at failure. Since this design procedure is recognized as complex, the value of the tension will be taken as equal to the lock-off tension, T b , at the same time bearing in mind the soil creep and the steel's "relaxing." The load factor r T of the tensile force FT in the prestressed ground anchor will be taken as being equal to I, given that the tension at failure will be higher than the lock-off tension. (See current regulations for details about lock-off tension values, Recommandations TA 86).

3.1.2. Actions 3.1.2.1. Types of actions

The actions to be considered are as follows: a) Permanent forces (G) of the soil's own weight, either in situ or brought in, and of any structures that form part of the site and/or that affect it, loads caused by buildings located in the structure's area of influence, and long-term surcharges. b) Variable forces (Q) might include:

107

Soil Nailing Recommendations-1991

The effects of rolling loads, vibrations, cyclic loads. Climatic effects (for example, effects of ice on the heads of the ground anchor rods). c) The effects of water (GJ resulting from pore water pressure in the soil nailed mass. In principle, soil nailed structures must, insofar as possible, have drainage facilities in their area of influence. If, in spite of everything, pore water pressure remains, this absolutely must be taken into account. d) Accidental forces (FA) may be caused by: Earthquakes and impacts. Exceptional hydraulic conditions (flooding). e) Forces in the nails (FR) f)

Forces in the ground anchors (FT)

3.1.2.2. Characteristic values of these actions

The minimum or maximum characteristic values given for a specific action will be chosen depending on whether its effect on the structure is to stabilize or destabilize it with respect to the considered potential failure mechanism. In the absence of any specifications to the contrary being given in the contract documents, the following characteristic values of actions will be used in the above formulae.

1) Permanent forces (G) •

Forces due to weight

For soils and other materials likely to be included in a soil nailed structure, the following characteristic unit weight values will be used: -

Soils

Unit weights are assessed on the basis of representative measurements. In the absence of such measurements, the following nominal values may be used on condition that these clearly result in improved safety for the structure (Table II).

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Chapter 3: Conception and Design

TABLE II. Nominal values of the density of some soils.

In situ State Soil Type Silt Clay Marl Sand Gravel Chalk Weathered rock

Reinforced concrete: Yb .

mm

Steel:

Ya . mm

=

Ya

max

Unit Weight (kN/m 3) loose

Unit Weight (kN/m 3 ) dense

17 17 20 18 18 17 20

20 19 22 20 21 19 22

= Ybmax = 25 kN/m 3

= 78.5 kN/m

3

The consistency principle requires that one material is considered to have the same typical unit weight values, whatever its effects, stabilizing or destabilizing, with respect to the considered potential failure surface. • Effects of water (G w) Pore water pressures are calculated from the most critical flow net by taking as the unit weight of the water: Ymin

= Ymax = 10 kN/m3

• Forces of prestressed ground anchors (FT ) The tensile forces FT in the prestressed ground anchor will be calculated on the basis of the procedure explained in paragraph 3.1.1.2. It is recommended that the following equation be adopted: FT,min = FT,max = Tb (lock-off load). • Forces of the nails (FR ) The characteristic values of the forces in a nail will be taken to be equal to the limit forces in that nail determined, for example, on the assumption of a multicriteria approach and using characteristic material resistance values (see paragraph 3.2.2. of this chapter). 2) Variable forces (Q) The characteristic values of variable loads are defined in the contract documents. If this is not the case, take as the characteristic values of the variable load on a platform:

109

Soil Nailing Recommendations-1991

a kPa

FQ,mzn. FQ,max

10 kPa

3) Accidental actions of type (FA) -

Seismic loading

Forces of a seismic origin, specified in the contract documents or by current regulations, are given by a nominal acceleration value (aN) and by a topographic coefficient (rot). From these values, one deduces the maximum characteristic values of the seismic coefficients to be applied to the actual weight of all or part of the structure. These coefficients are: K H = ± aN / g rot Kv

= + 0.5 K H

(horizontal component) (vertical component)

The plus sign corresponds respectively to an outward horizontal and a downward vertical. The minimum characteristic values are kH

= 0 and kv = a

3.1.2.3. Combination of actions and calculations

The combination of actions and calculations to be considered are:

•

Fundamental combinations: +r

•

51

G. mzn + GW +

r Q

Q

+

r

F T

T

R + _ F_ )

r

m,R

Accidental combinations: R

F -) 't(Gmax +G. mzn +Gw +Q+FA +rT FT +rm,R

where Gmax Gmin

r 51

r'51

permanent forces having a destabilizing effect, permanent forces having a stabilizing effect, load factor for the Gmax force, load safety factor for the Gmin force.

For reasons of simplification, only one basic variable force will be considered in all the combination of actions.

110

Chapter 3: Conception and Design

The load factors values of the actions are given in table III. It must be remembered that the forces of the nails FR are reduced by the partial safety factor

r m,R , •

which applies to the nail's failure criterion.

Remarks

1) r S1 = 1.05 for unfavorable gravitational forces and r'Sl forces.

= 0.95 for favorable gravitational

These values differ from those applied to other permanent forces (rS1 = 1.2 and r'Sl = 0.9), and this is justified because gravitational forces, which are dominant in soil nailed structures, are known with a fair degree of accuracy. Uncertainties may arise with the geometries involved (ground dimensions, excavation elevations, etc.). 2) The consistency principle, already referred to in paragraph 3.1.2.2. above, dictates that a single volume of soil be considered with the same characteristic unit weight value, as well as the same partial safety factors r s 1 or r'Sl , whatever the considered potential failure surface. 3) A partial safety factor equal to 1 will be used when determining the unit weight of water. The calculated buoyant unit weight value will be equivalent to:

4)

y'

= r S1 Y- Yw (overall destabilizing force)

Y'

= r'Sl - Yw (overall stabilizing force)

r GW =

1. This again refers to the consideration that, for simplification purposes and in the case of forces linked to pore water pressures or to flows of water that will have to be taken into account, it will be assumed that safety will have already been accounted in the representative Gw values.

3.1.3. Resistances 3.1.3.1. Failure criteria of materials

As indicated in paragraph 2.1. above, the application of limit equilibrium methods to soil nailed structures requires the compatibility of the deformations at failure of the soil and the nails (ductility of the nails, the soil and the soil-nail interface).

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Soil Nailing Recommendations-1991

TABLE III. Justification of the stability of the soil nailed wall at ultimate limit state. Partial load factors.

LOAD FACTORS NATURE OF FORCES 1)

Permanent forces, type G. Soil unit weight de-stabilizing force stabilizing force

NOTATION

3)

Accidental Combination

G

= =

1.05 0.95

1'51

= =

1.20 0.90

=

1.00

lSI 1'51

Other permanent forces unfavorable forces favorable forces

2)

Fundamental Combination

lSI

= =

1.0 1.0

1'51

= =

1.0 1.0

lew

=

1.0

lSI 1'51

lSI

Water pressures

Gw

lew

Nail force

FR

l / lmR*

Ground anchor force

FT

IT

=

1.00

IT

=

1.0

Variable forces, type Q (live loads, climatic forces)

Q

IQ

=

1.33

IQ

=

1.0

IFA

=

1.0

1 53

=

1.0

Accidental forces, type FA

l/lm,R*

= FA

Method factor

• See Table IV -

1 53

=

1.125

Design Resistances

It will be subsequently assumed in the presentation of design methods that this compatibility

exists both for standard nails and the majority of soils. All limit equilibrium methods take into account only the following failure criteria for the materials. •

Soil

The soil is characterized by a Mohr-Coulomb type criterium where

112

Chapter 3: Conception and Design

characteristics Cll and
Nails

The nails will be characterized by the three following resistances: RIJ Rc Mo -

•

resistance to simple tension, resistance to shear force, moment of plastification of the nail in pure bending, which will be determined as shown below.

Soil-nail interaction

With regard to soil-nail interaction, two criteria relating to the two modes of interaction will be examined: The limit skin friction, which will be characterized by the unit skin friction qs. The ultimate bearing pressure Pll under the soil on the nail, which will be taken to be equal to the limit pressuremeter pressure PI . 3.1.3.2. Characteristic values of strength parameters

As indicated in paragraph 3.1.1.1., the characteristic values of strength parameters of the soil and the soil-nail interactions will be taken to be equal to the most representative average values. •

Soil

It will be the geotechnical engineer's responsibility to define the characteristic values for the

shear strength parameters of soils. These must take account of the dispersion, quality, and representativeness of test results. With regard to the soil, one shall take the long-term characteristic values of the internal friction angle
Nail

Where nails include a metal reinforcing bar sealed in grout, the strength of the grout will not normally be taken into account, except where this can be specifically justified with the regulations on reinforced concrete (BAEL 83). The characteristic nail strength values (RIJ , Rc and M o) will be calculated on the basis of the guaranteed elastic limit cre of the steel where the nails include a metal reinforcing bar.

113

Soil Nailing Recommendations-1991

•

Soil-nail interaction

At the project design stage, the characteristic value for the soil nail unit skin friction qs will be determined based on the charts provided in the appendices of this chapter. At the construction stage, the value of qs will be determined from the compulsory pull-out tests in accordance with the procedure given in chapter 4. With regard to the resistance of the soil against the nail, the characteristic value of the ultimate lateral pressure Pu of the nail on the soil will be taken to be equal to the limit pressuremeter pressure PI .

3.1.3.3. Calculation values of strengths

Strength calculation values to be used for justifying the structure will be determined from the characteristic values by reducing them with a factor r m' called "partial safety factor": calculation value

= characteristic value / r m

The r m factor values, both in fundamental and accidental combinations, are shown in table IV overleaf. Please note the following additional comments. 1)

Shear resistance of the soil

For the shear resistance of the soil 't max = C + (j tamp, the partial safety factors r m,s and r m,

The coefficients

max

= c / r m,c

+

(j

tan

r m,
r m proposed for the shear resistance of the soil in particular take account of:

Any potential differences between the resistance values of the soil in the structure and those determined from the various tests carried out either in a laboratory or in situ. Any potential consequences for the structure from an area of soil having a local resistance lower than the characteristic values. It is appropriate to remember, as in the analysis of slope stability, that the design for a soil

nailed structure is extremely sensitive to the values taken to be characteristic of the shear strength of the soil, in particular its cohesion. This justifies the adoption of different partial safety factor values for both the angle of internal friction and the cohesion. 2) Normal soil-nail interaction a) The partial safety factor are:

114

r m'pl values proposed for the limit pressuremeter pressure pi

Chapter 3: Conception and Design

I'm'pl

=

1 for short-term loadings, in particular for excavation phases,

I'm,pl

=

2 for permanent loadings, which brings us back to the fact that the ultimate pressure of the soil in contact with the nail is close to the critical creep pressure.

b) The partial safety factor I'm E for the pressuremeter module EM , which plays a part in the determination of the subgraJ1e reaction coefficient ks will be taken to be equal to 1.0 for all combinations, always provided that the value used by the geotechnical engineer for E results from a sufficient number of representative tests. M 3) Soil-nail unit skin friction: qs

The values of I'm,qs will depend on the way of determining the characteristic soil-nail unit skin friction either from charts or from in situ pull-out tests. It will be noted that the values suggested for

are higher than those used for calculating deep foundations. This is due to the extreme sensitivity of this parameter to the conditions of installing the nails. 4)

I'm,qs

Steels

One shall take I'm cre = 1.15 for reinforced concrete bars and other steels with an elastic limit lower than 500 MPa, in accordance with existing regulations.

3.1.4. Situations

The whole of the structure must be justified for the situations described below.

-

In course of construction

This corresponds to the excavation and earthwork phases and the gradual installation of the reinforcements. One phase that, in particular, should be checked is where the earthworks for a section of excavation is completed, although neither the nails nor the facing have been installed. It will be noted that it may be necessary to look at a set of particular parameters when the

structure is in this phase of construction, Le., for different soil resistances and hydraulic considerations.

-

In Service

This refers to a finished structure.

-

In an "accidental" situation

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Soil Nailing Recommendations-1991

This situation might correspond to the following situations: Earthquake. Exceptional hydraulic conditions. One shall look at situations from the point of view of an accidental combination of actions.

Table IV.

PARTIAL SAFETY FACTORS

r m applied to characteristic values of the materials MATERIAL PROPERTIES Fundamental combinations

1)

2)

3)

Accidental combinations

standard

sensitive

standard

sensitive

Cu

rm,'I'11 rm,c r mc

1.20 1.50 1.30

1.30 1.65 1.40

1.10 1.40 1.20

1.20 1.50 1.30

MILD STEEL elasticity limit

crc

r mcr, e

1.15

1.15

1.00

1.00

SOIL-NAIL INTERACTION unit skin friction (tests) unit skin friction (charts)

q,

rm,q,

1.40 1.80

1.50 1.90

1.30 1.60

1.40 1.70

limit pressuremeter pressure pressuremeter modulus

Em

1.90 1.90

2.00 1.00

1.00 1.00

1.10 1.00

SOIL tangent of the effective friction angle effective cohesion undrained cohesion (
116

t amp l 1

c

PI

u

rm

r

,Pl

m,E

M

Chapter 3: Conception and Design

3.2.

Calculation methods

3.2.1. Stages of calculation

• First design (see paragraph 1.6) For the preliminary design of a structure, the most critical potential failure surface should be sought by simultaneously looking at internal, mixed, and external failures (see paragraph 3.1.1. and figure 14). • Iterations and optimizations Where a structure is unstable, the project will be modified as many times as is necessary, and its verification performed again. If the structure is stable, the design already verified shall be kept, or optimized by making modifications and reverifying it.

3.2.2. Determination of nail forces at failure ("multicriteria" rule) 3.2.2.1. Failure criteria and limit equilibrium methods

The forces in the nail at its point of intersection 0 with the potential failure surface can be represented by a system of forces: Til Tc

M

normal force,

= shear force, bending moment.

The determination of the forces and moment at which the nails fail, requires the consideration of four failure criteria for the constituents and their interaction with one another: Soil-nail friction interaction: 't ~ qs Soil-nail lateral pressure interaction: p ~ Pll Constituent material: 't ~ k where k is the maximum shear stress of the material from which the nail reinforcement is made.

1) For methods based on yield design theory, it is theoretically possible to take into account both the bending moment and the shear forces in the nails. To date, this aspect has not been developed and only the tensile strength of the nails has been studied. Such a calculation would lead to the determination of the maximum resistance of the nails being dependant first on the soil-nail interaction criteria:

117

Soil Nailing Recommendations-1991

lateral pressure: p:::; Pu and, second, by the failure criteria of the nails. Here one might, for example, use the following simplified formula (Anthoine, 1987): (TIt /R It )2 + (TC /R)2 + C

IM/ M o I - 1 :::; 0

which is slightly conservative when compared with Sobotka's formulae (1954, 1955):

or Neal's (1961):

which is also considerably easier to use. With regard to the criterion of lateral pressure (p :::; pJ, its formulation in relation to Tc and M requires, by contrast, an assumption to be made about the distribution of the pressure along the nail (uniform distributions, opposed from one side to another of the failure surface (figure 15), e.g., Brinch-Hansen type distribution). The validity of this assumption needs to be verified by experimental data.

A'

~TC

=0

A: Point of maximum moment 0: Point of maximum shear force Figure 15. Schematic distribution of the lateral pressure along the nail.

118

Chapter 3: Conception and Design

The maximum contribution that these criteria allow can be calculated in a similar way to that currently proposed for nails that work only in tension. In other words, by verifying the overall equilibrium of the cross section of the structure, which is bordered by a potential failure surface (Anthoine, 1990). At present, these methods are limited in that only the nail's tensile strength is taken into account. However, where the nails are to be implemented without the introduction of any additional assumptions, this would result in considering the potential failure surfaces as logarithmic spirals (of angle
2. For classical limit equilibrium methods, the combination of the materials' failure criteria and their interactions, as well as the relationships between Til , Tc , and M, have been studied within the framework of an elastoplastic behavior of the soil-nail system (chapter 2, paragraph 2.2.). The relevant multicriteria approach has been in use for several years to design real structures in reinforced soils approach (soil nailing and micro-piles), and has been the subject of experimental verification tests (chapter 3, paragraph 2.3). Four criteria that correspond to four nail failure modes will be considered. • The soil-nail skin friction criterion (el) This criterion, which corresponds to the structure's failure when the nails are pulled out, is represented for a homogenous soil as: T~q1tDL 11

where:

5

a

qs is the soil-nail unit skin friction.

D is the perimeter of the nail where D = Dc (borehole diameter) for grouted nails, and D = D a (equivalent diameter) for driven nails.

1t

La is the nail grouted length beyond the failure surface, except where there is no facing or liaison between the head of the nail and the facing. In this case La = L*, the length L* is the shorter of the two lengths between the failure surface and the facing or extremity of the nail in the structure (figure 16).

119

Soil Nailing Recommendations-1991

a

a. Nails connected to facing.

Q

La

= L to = the sma II er of (L I' L 2 ) b. Nails free at their heads.

Figure 16. Determination of pull-out length La.

• Soil-nail lateral pressure criterion (C2) The lateral pressure exerted by the nail on the soil is limited by the ultimate lateral pressure of the soil Pu' Failure by bearing pressure of the soil under a nail may be defined either when Pu is achieved at the single point 0 of maximum shear force (figure 17) (the most conservative assumption), or when the soil is plastification over a maximum length to be defined. In the first - the simplest and most conservative case - an analysis of the nail under combined

120

Chapter 3: Conception and Design

loading (normal force, shear force and bending moment) as shown in chapter 2, paragraph 2.2.1., gives us the following criterion: Te < T C2,max -

(e2)

T

with

where:

D C2,max

= _e

2

1

opu

De = diameter of the nail (grout and reinforcement), 10 = transfer length, pu = ultimate lateral pressure.

In the second case, it will be assumed that the extent of soil plastification under the nail is,

unless justified otherwise, limited to the value nl/2, which corresponds to the distance between the two points of maximum moment as determined by the elastic behavior of the nail and the soil (figure 17), In both cases the result is a criterion that focuses on the shear force Te of the type Te

--- ......

AI

::;

Tmax'

l

A

A: Point of maximum moment 0: Point of maximum shear force Figure 17. Schematic representation of the soil-nail interaction (elastic behavior).

• Criteria (3) and (4)

The two criteria (C3) and (C4) involve the forces and moment (Tn' Te , and M) created in the nail when it plastifies either by shearing at 0 (the point of maximum shear force), or by bending moment at A and A' (maximum moment points). To simplify matters, one should assume that the normal force Tn does not vary in the bending zone around the potential failure surface.

121

Soil Nailing Recommendations-1991

Use Anthoine's criterion (1987) to represent the actual resistance of the nail, since this is both simple and slightly conservative:

•

Criterion (C3)

Nail plastification by shearing occurs at the point of maximum shear force O. This corresponds (for reasons of symmetry) with the point of nail/failure surface intersection, provided the nail lengths between one side and another on the potential failure surface are at least longer than 310 • At the 0 point, the bending moment is zero (M = 0) and the failure criterion, based on the general failure criterion of the nail, can be written as:

(T/R/ + (TjR/ ~ 1

(C3)

One usually takes Rc

= Rn /2.

In the (Tn' TJ plane, (C3) is represented by an ellipse. •

Criterion (C4)

Using a simplified assumption (see chapter 2, paragraph 2.2.4.), nail plastification by bending moment occurs at the points of maximum moment A and A' located on both sides of the potential failure surface at a distance equal to Ip = 1t lj4 and calculated with an elastic behavior of both soil and nails. Plastification at those points (see figure 14) where the shear force is zero (Tc to the criterion:

M

with M max

= Mofl - (T,/R,/l

~

Mmax

determined by the nail failure criterion.

Based on this value, the following formula gives the shear force at point 0:

122

= 0), corresponds

Chapter 3: Conception and Design

Teo

a

~o [1 - (T/RJ] a

where A is a constant and is equal to 3.12. In practice, which demands that nails be ductile, plastification at the two maximum moment points A and A' in the initial phase does not imply the failure of the system. Plastification remains localized with two plastic hinges in the nail. These hinges, which initially occur at A and A', move as the nail continues to deform. The value of lp is initially equal to 1t lj4 but then varies in order to meet equilibrium equations and the nail's failure criterion. The calculation of lp in the elastoplastic phase is complicated, and certain experiments, such as the CEBTP No.1 soil nailed wall, tend to show that lp varies even inside the same structure. In the absence of any more detailed information, a simple assumption involves taking lp as constant and equal to 1t 1/4. At point 0, after the two plastic hinges at A and A' have developed, the plastification of the soil under the nail yields the following criterion: Tc

Te4,max

=

b(Mollo)

s

Tc4,

max

[1 - (T,/RS]

+

cDe 1a p11

where band c are two constants and equal respectively to 1.62 and 0.24. This criterion respects the equilibrium equations but not the nail's failure criterion. However, by combining (C4) and (C3), the latter criterion is met, and this is the (conservative) measure to be adopted when determining the multicritera approach. An assumption in which the distance lp is different would result in a similar criterion.

3.2.2.2.

Combinations of failure criteria

As indicated in paragraph 2.2.3. of chapter 2, the multicriteria rule (Schlosser, 1981, 1982, 1983) consists of representing the four criteria Cl, C2, C3 and C4 in the (Tn' TJ plane where Tn and Teare respectively the tensile and shear forces. Their intersection is then considered to be the resultant criterion for the forces in the nails at point O. The forces Tn ' Te are expressed by the calculating values that take account of the various coefficients (load factors rand partial safety coefficients r m)' The intersection of these criteria (figure 18) defines a convex domain of stability in which the representative point for the forces in the nail at failure at the point where it intersects with the potential failure surface can, at first, fall anywhere on the outer edge of the domain. It is interesting to note the important role played by the ultimate lateral pressure P1I in the maximum shear force value Te , max resulting from the multicriteria approach:

123

Soil Nailing Recommendations-1991

Tel,max

where Tel and Te2 are both functions of Pu. If Pu is sufficiently high, Te,max can reach the value of Re, and in this case the intersection of the criteria is reduced to C1 • This is the limit case for a nail placed in a rocky soil, sheared along a joint. Once the multicriterion has been determined, a rule needs to be chosen for calculating the forces in the nails at failure. 3.2.2.3. Rule for determining forces 1. Tension and compression of the nails

The axial force Tn in the nail can be tension force (Tn> 0) or compression force (Tn < 0). The distinction relies on the orientation of the nail compared with what is believed to be the potential failure surface. Experiments conducted on samples of sand reinforced with metal bars in a large shear box (Marchal, 1984, see chapter 2, paragraph 2.2) have shown that the nail is still strained in tension if it has an angle of incidence i of between -15° and + 90° compared to the normal to the potential failure surface (figure 19a). In practice, the negative limit on i, which corresponds to a nail inclined in the opposite direction to the shear, may be considered as being independent of the shape of the potential failure surface, depending on the relative soil nail stiffness; this angle i varies between _10° and - 20°.

124

Chapter 3: Conception and Design

INCLUSiON

"8 Potential fai lure surface

Stresses in the inclusion ((3)

Rn

R :-

c 2

-c-- ---

Lateral pressure interaction Soil first in a ( .) Inclusion first in (C4) plastic state C2o- 'Plastic state

..........

I

......

I ......

",

I I

,

I I

---~---

L

\

II

__

~-

Skin friction ..:...in..:...te.::....r.:;.ac..:...t_io_n

(C,)

To

~~=-.::..:.....::....~~~.-:-..:.--'-...:...-:...--'-. . .:. . .-:......-:---'-~ _ _~:-='-""'----i_ n

Tn!

/~:::::-~--.:::::::....

/

'J?7.~~~~~f'--

___

~

Potential failure surface

Figure 18. Combinations of failure criteria (multicriteria rule). Determination of the forces in the nails.

125

Soil Nailing Recommendations-1991

a )

Direct shear test (Marchal, 1984)

Tn <0

o

>0

8: Inclination of the nail from the horizontal

J..=d.+{3-..!-

2

angle between the nail and the normal to the potential failure surface

b)

Definitions

Figure 19. Nails put into tension or compression, depending on their orientations with respect to the failure surface.

In c1assicallimit equilibrium methods, the nails can be stretched or compressed along the potential failure surface, depending on the value of the angle i, as defined above (figure 19b). As a general rule, when analyzing the internal stability of a soil nailed wall, and whatever the method used for designing at failure, one will not take account of the compression forces in the nails on the potential failure surface studied.

126

Chapter 3: Conception and Design

2. Maximum work rule A rule currently used to determine forces in the nails is the so-called "maximum work rille." The application of this rule to soil nailing (Schlosser, 1981, 1982, 1983) involves the assumption that upon failure, the point P, which represents the forces in a nail, is located at the outer edge of the domain of stability described by the failure multicriteria. The position of this point P on the outer edge is chosen to maximize the work T' "t of the force in the nail, in_the considered failure mechanism, in comparison with the work T* ."t of any virtual force, T' satisfying the multicriteria rule in the nail for the same failure mechanism. This is equivalent to choosing the representative point P so that its projection on the displacement vector is maximized, with "tbeing the displacement of the point of the nail on the potential failure surface considered. In order to determine the force Tin the nail, the knowledge of only the direction of the "tvector is needed. This procedure is equivalent to choosing the point P such that the normal at P on the outer edge of the domain of stability is parallel to the displacement ~ In this second form, the maximum work rule can be interpreted as a normal rule. In practice (figure 18), the point P and the forces Tc ' Tn upon failure of the structure are determined by considering the displacement "t tangent at the point a to the potential failure surface. This means that one needs to look for the point P on the outer edge of the domain of stability, where the tangent is perpendicular to the direction B: NOTE: The limit equilibrium method with relative displacements, as used in nailing unstable

slopes and referred to at the beginning of this chapter, uses a similar procedure of maximization. The forces in the nail are assigned parameters in function of the displacement along the potential failure surface. This principle comes back to the consideration that from the moment one failure criterion is achieved (for an amplitude of displacement), the point representative of the forces on the outer edge of the domain of stability is displaced until it reaches (for an amplitude of displacement 02 > 01) the same position as shown in figure 18. This calculation method must therefore lead to the same results as would be achieved using classical limit equilibrium methods as long as they use failure criteria defined in the same way and with the same multicriteria. On the other hand, with this method it is possible to introduce other rules for calculating forces in the reinforcements based on displacements criteria or resistance thresholds. However, they are only valid when the zone of displacements prior to failure is influenced by a preexisting failure surface that'channels' any displacements. Of course, this is generally what happens to unstable slopes that have been stabilized with soil nailing.

127

Soil Nailing Recommendations-1991

3.2.3. Calculating the stability of the soil nailed structures

In classical limit equilibrium methods, the study of the internal mixed or external stability of a soil nailed structure is carried out in a similar manner to the studies of the stability of nonreinforced slopes; but forces brought about by the nails along the potential failure surface, as well as additional stresses in the soil, should be also taken into account.

It is necessary to ensure the equilibrium of that part of the structure bounded by the potential failure surface, by taking into account the nail's forces in the three static equilibrium equations. Any method adapted from the calculations of nonreinforced slopes that does not respect this condition is not recommended. Several calculation methods are available, among which the methods of slices (Fellenius, Bishop's simplified) and global methods (perturbation method) can be highlighted. Each is characterized by the type of additional assumptions used to obtain the number of equations necessary to resolve the problem. Among the methods of slices, the simplified form of Bishop's method is the one in which the additional assumptions give the most realistic results. Adapting it to reinforced structures involves the introduction of the forces of the inclusions through their projections (Tn' TJ on both the normal and the tangent to the potential failure surface at the bottom of every slice involved. This takes the form of components oN and oT which are going to be added or subtracted from the components Ni and Ti , which are calculated without the reinforcements in classic Bishop's method (figure 20). Equilibrium is then ensured when the following conditions are met:

In this formula,
128

Chapter 3: Conception and Design

since these give results that are too approximate, particularly in the case of purely cohesive soils. Whatever the method chosen, it is important to find the volume of reinforced soil showing the most critical stability conditions. Here it must be emphasized that this one can differ greatly from the critical volume of the nonreinforced soil. The study of the external stability is carried out in a similar manner to that of the overall stability of retaining structures. It uses the ultimate limit state methods with the values of the partial safety factors given in this chapter. In the particular instance of a soil nailed structure being built on a site where the initial stability, while ensured, is nevertheless low in comparison with the criterion used for the soil nailed wall, steps should be taken to verify that the latter is having no weakening effect on the initial stability of the site. Where a slope is present, it is useful to verify the stability of any potential failure surface that could occur along the slope, particularly up to a distance 3H from the facing, H being the height of the soil nailed structure.

Figure 20. Calculation of the internal stability of a soil nailed wall using the method of slices.

129

Soil Nailing Recommendations-1991

Figure 21. Specific potential failure surface passing under the toe of the facing.

3.2.4. Examples of calculations and design

Three examples of calculations made for soil nailed walls are presented below. Of the first two, one deals with an accidental failure due to the lack of friction, the other with failure caused by the breakage of the nails in the CEBTP No.1 experimental soil nailed wall, which formed part of the Project CLOUTERRE. These first two examples, details of which have already been published in the literature, are to some extent, a good check for the limit equilibrium methods with regard to the two most observed types of failure found in soil nailing. The third case involves fairly detailed design of a soil nailed wall subject to water and several different soil layers. 3.2.4.1. Example of the Eparris wall

The Eparris soil nailed wall (Schlosser and Guilloux, 1982), was built in February 1981 in a shallow clayey formation that included some areas of sand. Water was found several meters down and, at varying levels, this is evidence of a complicated hydrogeology. When the wall was constructed, several subhorizontal drains, 6 meters long, were installed in the wet zones. The tubes used to house the nails were vibrated into a predrilled borehole and then injected with grouting at a pressure of 100 to 200 kPa. The final equivalent nail diameter was 100 mm. In May 1981, following a period of very heavy rain, the wall failed. Figure 22 shows the kinematics of the failure, which is completely different to those resulting from the breakage of the nails. Pull-out tests on the nails, which were carried out following failure, showed that

130

Chapter 3: Conception and Design

the soil nail skin friction had a value well below that which had been adopted when planning the design (qs = 45.5 kPa on average, instead of qs = 100 kPa). This showed that the wall had failed due to a lack of soil nail friction. The wall was rebuilt immediately using the same type of nails (tubes), the same diameter of nail (Dc = 100 mm), and the same spacing arrangement (Sv = Sh = 2 m), but with a longer length of nail (L = 10 m). The facing also had a flatter slope (inclination from the vertical 11 = 30° instead of 20°). The structure has behaved perfectly ever since. The stability calculations of the failed wall were first made by taking all the partial safety coefficients as being equal to 1, as well as the load factors and the method factor r S3 ' With these values a coefficient r mill was calculated whereby:

r.mm

='tmax /'t

which, to some extent, represents the difference between the calculation method and the reality, or can be interpreted as the overall safety factor F as used in traditional calculation methods. It was later noted that a partial safety factor value I m,R higher than 1.0 would not alter I mill as

long as I m,R remained lower than or equal to the minimum for the ratio R,/TL , equal to 7.0 and calculated on the length of nails located beyond the most critical potential failure surface. This shows that the failure of the soil nailed wall has clearly been caused by a lack of soil nail friction. It should be noted that when formulating a pessimistic assumption for a water table at the surface of the soil on the slope, the value of the coefficient I mill is 0.71 (figure 22).

On the other hand, when formulating an assumption of perfect drainage and the absence of any pore water pressures in the domain of soil affected by the nailed mass, the r mill coefficient value is 1.0. The reality lies between these two extremes and is without doubt nearer to the value r mill = La, when it is remembered that subhorizontal drains had been installed. One should also note the uncertainty concerning the final diameter of the nails (Dc = 100 mm), since investigations carried out after failure have shown fairly strong variations in this parameter. This diameter, Dc, resulted from the drilled borehole expanding due to pressure from the injected grout; the heterogeneity of the soil and the different grouting pressures might explain the variations observed in this parameter, which is fundamental in any failure caused by lack of friction. A post-analysis calculation is able to clearly demonstrate the influence of the bending resistance of the nails on the factor I mill' If this resistance is ignored with an assumption based on the absence of pore water pressure, then the value of I mill is 0.92 instead of 1.00. This shows that here the effect of the bending resistance of the nails is to increase the overall

131

w

.....

I\)

28°

45,5 kPa

500 kPa

9150 kPa

tp'

qs

Pr

k s Dc

TALREN

Prgram

= 0.71

= 1.0

AOJ A9 '(\(j \lQI

STABILITY

0.73

0.73

76

THE

~~~~

Scale:

o

+

0.97

0.87

.

0.81

0.77

0.74

0 }3

+

072

2

EPARRIS WALL

Steel structural tubing 10 40mm ,00 49mm

SV=Sh=2m

+

0.85

. +

0.77

0.74

0.76 +

+

0.79

0,75

+

0.74

0:12

0.73

+

+

0.711

072

8

0.71

+

+

. 2

+

0

072

48

0.77

/8

078

+

074

+

+

0079

074

0.80

ANALYSIS OF

l%:~llilU*,~v~~,»~rn;;my;

~e\\\'O{C\ ~s\ee\ <\> '2.':>((\((\

"i\l\les

Steel structural ___ tubing. 10 40mm, 0049mm

f s 3

fm,r =7.0

r

fm,P = 1.0

f m , q s =1.0

fm,'f =\.0

f min

Figure 22. Analysis of the stability at ultimate limit state, for the Eparris wall.

TERRASOL

Method of perturbations

0

3

C'

I

20kN/m

N

Y

Soil

10m

I f-£

e:.,

00

0

........ 0

ro

~

S· '"

0

~

0

Pl

Pl

C

.....

::l.

~

o

::to

S Pl

()

Pl ........

()

OCI

S'

0-

~

o

"1j

00

nl

o"1

()

g-

§::

:is

~

o

;t"oo

~

§.g

()

8'

~

..... .-+

~

prro ~"1 ro ()

OO"1j

..... \0

s:-~

OCI

ro ~

~

ro ><

(if Pl ~"1j g;. ~

ro,<

::r' cr'

.-+"1

'<

2.0.

o Pl'

ro '<

~ ~

::r'

:is

....

l8

....

I

!II

::J

::!' O·

2-

::J

:3 :3 CIl

0

:a ~

lQ

s:

el.

!C!:

::::

CI) 0

Chapter 3: Conception and Design

3.2.4.2. Example of CEBTP Experiment No.1

The soil nailed wall involved in the first CEBTP experiment, which was carried out as part of the Project CLOUTERRE, was designed so that it would fail because of breakage of the nails. As indicated in paragraph 3.4. of chapter 1, failure was deliberately caused by partly saturating the soil nailed structure with water fed from a pond at the top of the structure. The design of the wall was worked out so as to obtain a small overall safety factor, which was calculated as being equal to 1.10 using the TALREN program and Bishop's method of slices. A few comparisons were made following its failure using the same computer software (Schlosser, 1989). In order to transform the basic inequality referred to in paragraph 3.1.1.1. into an equality, the coefficient r mill was introduced, such that:

Figure 23a shows the internal design calculations used for the wall. These calculations can be redone starting with the fundamental inequality with respect to a failure by breakage of the nails. By including in the calculations the previous inequality, then all coefficients are equal to 1.0, except r mill' which is to be determined, and r m,qs , which was taken to be equal to 4.3, (the value representing the minimum of the ratio TL / R as calculated for the length of the nails located beyond the most critical failure surface) one gets r mill = 1.10. This shows that this coefficient fully corresponds, in respect to these assumptions, to the global safety coefficient F = 1.10 found in traditional methods. Il

Figure 23b shows details of the study of the actual failure for which it was assumed that, following the introduction of the water, it would be possible to distinguish two layers of soil: one, virtually saturated at the base of the wall and having no cohesion (c = 0 kPa), the other not saturated and keeping its initial cohesion (c = 3 kPa). This schematization resulted from the observation at the moment of failure, in that a lower part of the facing had been soaked through to a great extent. The height h of the soaked zone had been taken as the calculation parameter, and the graph given in figure 24a shows the variation of the coefficient r mill at failure in the function of h. It will be noted that where r mill = 1.00, the value of h varies from 2.25 m when the bending resistance of the nails is not taken into account, to 3.25 m when this resistance is taken into account, while the observed value is around 2.5 m. This breakup in two layers of soil having different physical and mechanical characteristics represents an extreme case. Another approach would be to assume that the amount of water poured into the wall is shared uniformly throughout the full height of the zone of wall located under the pond. This assumption gives an average water content w = 19 percent while the initial value was W o = 10.7 percent.

133

Soil Nailing Recommendations-1991

Bishop's method

Soil N

I

y

16.6 kN 1m

f st

1.00

c

3 kPo

'f

38" 80 kPo

qs

~ \.\0

Pt

1200 kPo

Ks ' B

27500 kPo

f min , 1.0

\"(I'\{'

fm''f'I.O

::,<>(1'

,,>A .

Sv

= Sh'

fm,e' 1.0

1m

f m,Q5'4.3

,--£!..J-

fmp'I.O • !

.-£!-l-

CD

fm,r =1.0

~

f 53 '1.0

~

..-£!-2~ ~

,

1.

DESIGN / BREAKAGE OF NAILS

Scale

Bishop's method

2

I

Soil N y

a

16.6 kN 1m 19.3. kN/m

f st

1.00

c

3 kPa

1.00 o kPa

'f

38"

38"

qs

80 kPa

80 kPo

1200 kPa

1200 kPa

Pt

27500 kPa 27500 kPa

Ks'S

I"

Sv = Sh '1 m

I

5.00m

.

2.BOm

I

fm,'f= 1.0

R~'2.:'·:'(I'

I

.....---:

------

f min , 1.0

1«1,1\ -- \.cf)

fm,e' 1.0

..E-!.

f m,Q5'4.3 fm,p, =1.0

..-£!-2-

CD

f 53 =1.0

~

~

®

f m,. =1.0

..-£!-2-

b) STABILITY

~~ ~ ~ura h=225m soil ~.

ANALYSIS

ACTUAL

OF THE

FAILURE

Scale ,

•

~

TALREN

V2.0 du 12/03/91

TERRASOL Figure 23. Design and stability analysis of the actual failure of the first full-scale experimental soil nailed wall (CEBTP, French National Project CLOUTERRE, 1986).

134

Chapter 3: Conception and Design

h tml

I

....

...

3

I

'-,I ..... ..... I

: I

2 Without

...... ~With bending in - '.." the nails

-Y

""',

1

bending: in the nails :

-', "

I

,

I

o

1.10

1.05

0.95

fmin

( a )

w t%l

Average water content

t w = 29% ~

'''''...

I

Sr

= 100 1

~ With bending

....... ~

..... ....

20

__________

%

yt:J~_%

in the nails

....... .....

.........

, I

~

.... ....... : fmin =0,98:,

I

Without bending in the nai Is ..........

.......... ...

(Wo=10.7%,Sr=37 % ) : -.-.-.-.-.---·----------.. ------------r·---I---------------10 L-._ _---r ---r_ _-'---,-_._ _

'......

-._----~...

r

_ min _

-,-~loo.__.....,::~

0.85

0.90

1.05

0.95 (

~

1.10

b)

Figure 24. Variations of the factor r min with the average water content wof backfill (CEBTP wall NO.1 - calculations using the TALREN software program).

135

Soil Nailing Recommendations-1991

By assuming that the cohesion varies linearly as a function of the degree of saturation Sr , and the water content (where c = 0 kPa, Sr = 100 percent) one can trace two curves of the coefficient r min (figure 24b), one taking into account the bending stiffness of the nails, and the other not. It is noted that the value of the coefficient r min upon failure thus falls within 0.95 and 0.98. On the whole, and in spite of the uncertainties inherent in the type of loading used in this full-scale experiment, the results showed that the values r min calculated using classical limit equilibrium methods are a correct approach to reality.

3.2.4.3.

Example of a mixed soil nailed wall with surcharges and partial drainage in a layer system

Here we are looking at the case of a soil nailed wall with a vertical facing, a total height of H = 17 m, comprising five rows of nails and a row of prestressed ground anchors at the top. The nails are shorter at the base but are also placed closer together (figure 25). Prestressed ground anchors have been used in view of the sensitive nature of the structure (buildings and roadway to be constructed at the upper part of the wall) and to reduce the deformations undergone at the top of the structure to acceptable levels. Since the structure was situated in a slope with a water table, drainage of the soil nailed structure was provided. This drainage was effective for the prestressed anchors but only partly effective in the zone of the lowest nails. Figure 25 shows the results from calculations made using the TALREN software package. The typical features of the soils, the surcharges, the nails, and the ground anchors are shown in table V. When designing the structure, the characteristic parameters of the nails and the ground anchors were taken to be such that the factor r min linked to the required values for both the load factors and the partial safety factors had a value of 1.00. It will be noted (figure 25) that this value corresponds to two potential failure surfaces: a circle affecting mainly the nailed structure, and another encircling a large part of the nailed structure and passing through the centers of the prestressed ground anchors, and therefore more representative of the overall stability of the structure.

3.2.5. Simplified methods 3.2.5.1. Assumptions

These methods are referred to as "simplified" because they formulate the assumption that the nails work only in tension.

136

......

-...J

W

46000.0

26000.0

17000.0

0.0

0.0

!P1

Ks. B

0.01

5500.0

Sr 3

/

/

0990-INF-S

Fich:

0990infsy

SOIL NAILED WALL WITH GROUND ANCHORS SURCHAGES AND PARTIAL DRAINAGE

··~~·.I

hl

0.0 0.0

500.0

1+ 17

1,10

I.;: 07

It 08

1. 00

1,11

1+07

If 04

1,04

Nails

TERRASOL

I .. 10

I+. 07

1+ 06

1,02

1+. 01

1.(.01

'

f

"

.i?ni.: ..

= 125

fm'R = 1.15

f"'Q =150

fm,r =1.15

f m,Qs=1.50

fm,pe=z.OO

fm,c= 1.65

fm,'f= 1.30

fmin= 1.00

1.+08

l-i- 03

1,01

G:OO)

.:-: /ca~e.: ..... :...

\ l

\

~

1.; 04

1.;:02

1,17~,02

1,10

1,06

Ground anchors

If 06

1

1,07

I.;: 04

1,06

Calculation done by

C1 5

C1 4

Sl2..

C1 2

Ti 1

1 f 09

If 11

1,02

1.;: 02

1.(.03

Figure 25. Ultimate limit state design of a soil nailed wall with surcharges and partial drainage.

TERRASOL

Doted (12 March 1931

TALREN

»:-:~

..

Sr 4

Units in kN 1 meters, and degrees Calculation method' PERTURBATIONS

0.0

0.0

2000.0

1500.0

1000.0

80.0

0.0

0.0

30.0

40.0

36.0 200.0

35.0 160.0



35.0

0.0

20.0

200.0

30.0

20.0

10.0

10.0

120.0

0.0 1. 000

17 .0 1. 050

20.0

1. 050

19.0 1.050

19.0

1. 050

7

19.0

6

1. 050

4,' .

19.0

·3·

1. 050

2.

0.0

I

i<·

25.0

N

qscl

rs1

y

Soil

:::J

cQ'

~

~

Q.

g

::::!"

j

Co) "

...CD

9 ~

Soil Nailing Recommendations-1991

TABLE V. Values of the parameters in the design example.

SOILS Soil No.

y

r si

c


qs

pI

ks

1 2 3 4 5 6 7

19.0 19.0 19.0 19.0 20.0 17.0 0.0

1.05 1.05 1.05 1.05 1.05 1.05 1.00

10.0 10.0 20.0 30.0 200.0 20.0 0.0

25.0 35.0 35.0 36.0 40.0 30.0 0.0

0.0 120.0 160.0 200.0 0.0 80.0 0.0

0 1000 1500 2000 0 500 0

0 17000 26000 46000 0 5500 0

SURCHARGES PER UNIT LENGTH OF SURFACE N2

Sr Sr Sr Sr

1 2 3 4

0'1

0'2

rq

20.0 110.0 22.5 45.0

20.0 110.0 22.5 30.0

1.330 1.200 1.000 1.200

NAILS N2

Tc

Sh

Elevation

L

i

Dc

Mo

EI

C12 C13 C14 CIS C16

400 400 400 400 400

2.00 2.00 2.00 2.00 2.00

10.50 8.00 5.50 3.50 1.50

13.0 13.0 13.0 12.0 10.0

10.0 10.0 10.0 10.0 10.0

0.13 0.13 0.13 0.13 0.13

1.9 1.9 1.9 1.9 1.9

26.4 26.4 26.4 26.4 26.4

GROUND ANCHORS N2

Tg

Sh

Elevation

L free

L grouted

i

TL

Ti 1

900

2.00

13.00

13.0

12.0

10.0

1350

units in kN, meters and degrees

138

Chapter 3: Conception and Design

Under these circumstances, there remain only the nail failure criterion and the soil-nail skin friction criterion ('t < qs)' which are detailed below.

a) Skin friction criterion

qs 1tD

La D D

= Dc

= Da

unit skin friction perimeter of the nail anchorage length of the nails beyond the failure surface for grouted nails for driven nails

b) Nail failure criterion This is reduced to: T It

RIt

=

~

RIt

tensile strength of the reinforcing element

The tension force for calculation is then expressed as:

< . _ mm

T

[q s 1t D La ,

It

r m qs r m cre

r m, q,

r:,'J

partial safety factor for the unit skin friction. =

partial safety factor for the tensile strength of the reinforcing bar.

Values for these factors are given in table IV. With the exception of this assumption on the behavior of nails in "tension only," the conditions for the study of the stability of the soil nailed structure are identical to those found in general cases. The assumption of a nail working in tension only can be justified for the reasons given below. •

It corresponds to the way the nails work in the majority of structures in service. In fact,

the mechanism of skin friction interaction is preponderant in soil nailed retaining structures and develops with small deformations, before the mechanism of lateral pressure, which is only observed in the immediate vicinity of the failure.

139

Soil Nailing Recommendations-1991

•

For certain techniques (driven steel bars), the nails used in retaining structures have a section and inertia small enough, such that the results of calculations with the assumption of "tension only" differ slightly from those where shearing and bending have been taken into account.

Regarding this point, the following comment must, however, be added. On the theoretical level, when using classical limit equilibrium methods, there is not always a beneficial effect from taking account of bending and shearing, in fact, as shown in the multicriteria principle, the value of the force at failure in pure tension RIJ can be significantly higher than the component at failure tension TIJ that results from the combination of tension and shearing. However, in methods based on yield design theories, taking bending and shearing into account, would always be beneficial. One must be careful, given present knowledge, to consider that the "simplified" calculation provides systematically a conservative approach of conditions of stability.

3.2.5.2. Conditions of use

The methods that account for nails working in tension are only applicable to structures where tension is largely predominant when compared with the shear forces and bending moment, and which fulfills the following conditions: Small inertia of the nails. Only slight inclination of bars on the horizontal (never exceeding 20 0 ). Any deformation in the structure result primarily from a lateral soil unloading. No heavy surcharge on top of the structure. With certain nailing techniques, these conditions are fairly well met.

3.3.

Safety considerations

3.3.1. Practical rules

Each parameter is directly included in the fundamental formula (see paragraph 3.1.1.1.) with its "calculation value." •

The calculation value of the actions is the product of the characteristic value defined in paragraph 3.1.2.2. by a load factor r, which varies depending on the actions and on the considered combination of actions, as shown in table III.

•

The calculation value of the resistances of the soil and the nails is the quotient of the characteristic value given in paragraph 3.1.3.2. by a partial safety factor that varies depending on the material and the combination of actions involved. See table IV.

140

Chapter 3: Conception and Design

The calculation method used for design (method of slices, perturbation method, etc.) is ascribed a factor r S3 , the so-called method factor, which appears in the fundamental formula and has its minimum values:

r S3 r S3

1.125 in fundamental combination =

1.0 in accidental combination

In theory, this method factor should vary depending on how realistic is the chosen failure mechanism and how well results compare with reality. The more simple the type of failure surface, a plane, for example, the more the method factor would have to be raised; but at the present time and in the absence of any in-depth study about this matter, the minimum values given above, provided steps are taken to carefully examine as many circular surfaces as possible (c1assicallimit equilibrium methods) or as many logarithmic spirals as possible (approach based on yield design theory). The distinction between short-, medium-, and longterm structures (see chapter 6) is taken into account during the design by calculating the extra thickness of steel needed to compensate for the corrosion of the steel present at the end of the service life of the structure. Details of this procedure are given in chapter 6.

3.3.2. Illustration In appendix 3, the calculation at ultimate limit state of a wedge held by a nail is presented. This was done to familiarize the reader with the use of load factors r and partial safety factors r m in calculations at ultimate limit state. Equivalences become apparent between these factors and the traditional global safety factor.

4. JUSTIFYING THE FACING

4.1. The mechanical role of the facing -

Modelling for the calculations

The facing has several functions. a) It provides a lateral confinement for the soil by ensuring equilibrium between the local pressure p of the soil (assumed in the calculations to be uniform) and the tension To at the head of the nails: p

To

In practice, however, the local pressure p of the soil between the nails is not uniform. It depends on the deformability and the local displacement of the facing. There is a tendency for arching effects to develop between the nails, which concentrates pressures locally in the vicinity of the nails.

141

Soil Nailing Recommendations-1991

b) It can support certain exterior loads, such as facing panels or vehicle barriers. This paragraph will not deal with this type of loads. The facing is designed at ultimate limit states. The tensions To at the head of the nails and the pressures resulting from the soil on the facing are therefore considered as permanent and external forces on the facing. Also remember that calculations of the stability of the soil nailed mass at ultimate limit state are made without account taken of the facing's resistance at the point where, what is regarded as the potential failure surface, intercepts the facing. This is justified by the fact that during the construction of the structure, the lowest point of the most critical potential failure surface generally intersects the excavation phase. Designing the facing involves the following steps. 1) Determine the forces at the head of the nails and of the ground pressure on the facing, and 2) Check the facing resistance.

4.2. Determining the forces applied on facing In an attempt to simplify matters, it is generally assumed that the system of forces at the nail facing connection is reduced to the tensile force To and that the ground pressure p is uniform. Two approaches are possible. One can either deduce p by calculating the tension To on the assumption of the maximum tensions that can be mobilized in the nails, or one can calculate p as a local earth pressure using an appropriate failure mechanism and then deduce To' In the absence of any reliable data on the way earth pressure is distributed along the facing, it is recommended that the first approach described above be used. The value of To is deduced from the value of the maximum tension force T max' which can be mobilized by considering a single value for the ratio To / T max in the wall. An assessment of T max in a row of nails is made by taking the minimum value of either the tensile strength or the pull-out resistance of the nails as calculated on an anchorage length La defined in figure 3.16, and weighed by the relevant partial safety factor whereby:

T

max

=

.

mIn

1t DL R] an [q s -----

r ' rm, cr, m, q,

where D = Dc for a grouted nail, D = Da for a driven nail, La = length of anchoring (figure 16). In reality, the experimental results from in-service structures generally show To / T max ratios lower than those observed in Reinforced Earth walls. This is understandable if one remembers that the soil is unloaded during the construction of a soil nailed wall. The values

142

Chapter 3: Conception and Design

of To / T max depend on a certain number of parameters (the stiffness of the soil, rigidity of the facing, rigidity of the nails, depth and spacing of nails). The most important of these is the spacing between the nails. Bearing in mind the results available from experiments conducted to date, it is strongly recommended to adopt the maximum value of the ratio To / T max given by the following empirical formula:

= 0.5

where 5

To / Tmax

=

To / T

=

max

+

5 - 05

5

.

when 1 ::; 5 ::; 3 m

0.6 when 5 ::; 1 m 1 when 5 ~ 3 m

= max of (5 v and 5il ), expressed in meters.

This formula is derived from an analysis of measurements of the distribution of tensions in walls where the nails were set out on a rectangular grid where 5v ::; 5il • In the other case, the formula will need to be adapted conservatively. Except in special circumstances, the facing will be designed using Tmax values that correspond to the final structure phase, and no design of the facing will be done during the excavation phases.

4.3. Design of the facing The facing is in equilibrium under the effect of its weight, the tensions To at the head of the nails, the pressures and the shear stresses exerted by the soil. It is during the very first phases of excavation that the vertical equilibrium of the facing can be most critical (chapter 5, paragraph 2.3.2.3, figure 10). During the subsequent phases, the skin friction mobilized at the interface ensures equilibrium. When designing the facing of a finished structure, one does not usually take into account neither the weight of the facing of the shear stresses in the soil or the shear forces at the head of the nails.

Figure 26. Different models for designing the facing.

143

Soil Nailing Recommendations-1991

It is assumed that pressure is distributed uniformly on the tributary area of each nail. This is

conservative when compared with actual conditions when calculating bending in the facing. The calculation model for the facing is similar to a slab loaded perpendicularly and supporting concentrated loads at the heads of the nails (figure 26). Depending on how the facing is built, the following schematic models apply. 1) Facing built with continuity of reinforcing bars (and standard overlapping lengths)

a) Short term wall (duration of service less than or equal to 18 months): Here the facing is calculated as a continuous slab in both directions, ignoring any cracks that might occur. The construction conditions (installation of reinforcing bars and shotcrete) will need to take into account the actual continuity of the facing (figure 26, la). b) Medium-term wall (from 18 months to 30 years) or long-term wall (more than 30 years). When making the calculations, take into account the fragile nature of the concrete zone between the construction phases by introducing hinges into the model (figure 26, Ib)

2) Facing built without continuity of reinforcing bars or with minimum construction continuity. Here the facing is discontinuous and considered as cut by horizontal bands. Each band is then calculated as a slab that is independent of its neighbors. Sometimes they can be reduced to a one dimensional model and each horizontal band calculated as a beam (figure 26-2).

4.4.

Justifications of the resistance

Justification of the facing takes two forms: Justification under bending. Justification against punching of the facing around the nail head. All the justifications are carried out in accordance with the current regulations for reinforced concrete (BAEL 83). The most essential points are highlighted below.

4.4.1. Justification under bending

The forces used when making calculations for the facing are detailed in paragraph 4.2 and are ultimate limit state forces. The forces are those resulting from the calculations defined in the previous paragraph 4.3.

a) Facing for a short-term soil nailed wall

144

Chapter 3: Conception and Design

Calculations for the reinforced concrete used in facing sections are made exclusively at ultimate limit states. The following factors apply: permanent loads r G = 1.35 to be applied to the tensile forces To at the head of the nails concrete: r m,b = 1.50 r m,s = l.15 steel:

b) Facing for a medium- or long-term soil nailed wall The reinforced concrete calculations for the sections of facing are made at both ultimate limit and serviceability limit states. The calculation at ultimate limit states is performed in exactly the same way as for a short-term soil nailed wall. The calculations at serviceability limit states are done when cracking is taken into account. In this event, the permanent forces in the nails at the facing will have To values as defined in paragraph 4.2. and are not weighted. The forces thus obtained are examined according to the specifications of Article A.4.5 of BAEL 83. 4.4.2. Justification of a punching mode of failure around the nail head

In order to account for the concentrations of earth pressure p on the facing around the nails when justifying a punching mode of failure of the facing, it is recommended to adopt To values such that To / Tmax = 1, whatever the layout arrangement. To forces and p pressures will be treated as Qu loads in the sense of Article A.5.2.4 of BAEL 83.

In addition to the articles referred to above, the rest of the BAEL regulations will need to be equally respected, particularly those relating to nonfragility and minimum percentages.

145

Soil Nailing Recommendations-1991

BIBLIOGRAPHY ANTHOINE, A. (1990). Une methode pour Ie dimensionnement a la rupture des ouvrages en sols renforces. Revue Fran<;aise de Geotechnique No.50, p.5-17. A method for designing at failure reinforced soil structures. BANGRATZ, J.L., and GIGAN, J.P. (1984). Methode rapide de calcul des massifs doues. Proc. of International Colloquium on In Situ Reinforcement of Soils and Rocks, ENPC Press, Paris, p.293-299. Rapid calculation method of nailed structures. BLONDEAU, F., CHRISTIANSEN, M., GUILLOUX, A., and SCHLOSSER, F. (1984). TALREN, methode de calcul des ouvrages en terre renforcee, Proc. of International Colloquium on the In Situ Reinforcement of Soils and Rocks, ENPC Press, Paris, p.219-224. TALREN, Calculation method of reinforced soil structures. DE BUHAN, P., and SALEN\=ON, J. (1987). Analyse de la stabilite des ouvrages en sols renforces par une methode d'homogeneisation. Revue Fran<;aise de Geotechnique NoA1, p.29-43. Analysis of the stability of reinforced soil structures by a homogenization method. DE BUHAN, P., MARIGIAVACCHI, R, NOVA, R, PELLEGRINI, G., and SALEN\=ON, J. (1989). Field design of Reinforced Earth Walls by a homogenization method. Geotechnique No.39,2, p.189-201. . DELMAS, Ph., BERCHE, J.C, CARTIER, G., and ABDELHEDI, A. (1986). Une nouvelle methode de dimensionnement du douage des pentes: programme PROSPER Bulletin de Liaison des laboratoires des Ponts et Chaussees No.141, Janvier-Fevrier 1986. A new method for designing slope nailing: PROSPER program. FAU, D. (1987). Le douage des sols, application au soutenement de fouille, etude experimentale et dimensionnement. Doctoral thesis, ENPC Soil nailing, application to excavation retaining structures, experimental study and design. GASSLER, G., and GUDEHUS, G. (1981). Soil Nailing, some aspects of a new technique. Proc. 10th ICSMFE, Stockholm (3) pp.665-670. GASSLER, G., and GUDEHUS, G. (1983). Soil Nailing. Statistical Design. Proc. 8th ECSMFE, Helsinki (2) ppA01-494. GIGAN, J.P. (1986). Application du clouage en soutenement: parametres de conception et de dimensionnement des ouvrages. Bulletin de Liaison des Laboratoires des Ponts et Chaussees No.143, Mai-Juin. Application of soil nailing for retaining structures: conception and design parameters for structures.

146

Chapter 3: Conception and Design

GIGAN, J.P., and DELMAS, P. (1987). Mobilisation des efforts dans les ouvrages cloues. Etude comparative des differentes methodes de calcul. Bulletin de Liaison deslaboratoires des Ponts et Chaussees No.144.

Mobilization of loads in soil nailed structures. Comparative study of different design methods. GUILLOUX, A., and SCHLOSSER, F. (1982). Soil nailing: practical applications. Proc. of Symposium on soil and rock improvement techniques, including geotextiles, Reinforced Earth and modern piling methods, Bangkok, November-December. JURAN, 1., BAUDRAND, G., FARRAG, K., and ELIAS, V. (1990). Kinematical limit analysis for design of soil nailed structures. Journal of Geotech. Div. ASCE, vol. 116, Janvier 90, pp.54-72. RAJOT, J.P. (1983). Introduction du clouage dans une methode globale de calcul de stabilite des pentes. Travail de fin d'etudes, Ecole nationale des Travaux Publics de p. l'Etat, 177.

Introduction of nailing in a global slope stability analysis method. SCHLOSSER, F. (1982). Behavior and design of soil nailing. Proc. of Symposium on soil and rock improvement techniques, including geotextiles, Reinforced Earth and modern piling methods, Bangkok, November-December 1982. SCHLOSSER, F. (1982). Le clouage des sols: comportement et dimensionnement. Stage sur Ie renforcement des sols. Formation continue ENPC.

Nailing of Soils: behavior and design. SCHLOSSER, F. (1983). Analogies et differences dans Ie comportement et Ie calcul des ouvrages de soutenement en Terre Armee et par clouage du sol. ITBTP Journal, No.418, p.8-23, Oct.

Anologies and differences in the behavior and the design of Reinforced Earth and soil nailing. SCHLOSSER, F. (1989). Le Projet National CLOUTERRE. ITBTP Journal, No.473, Mars-Avril.

The National Project CLOUTERRE. SCHLOSSER, F., and LONG, N.T. (1972). Comportement de la Terre Armee dans les ouvrages de soutenement. Proceedings of the 5th European Conference on Soil Mechanics and Foundation Engineering, Volume I, p.299-306, Madrid.

Behavior of Reinforced Earth in Retaining Structures. Instruction technique sur les directives communes de 1979 relatives au calcul des constructions (DC 79). Bulletin officiel du ministere de l'Environnement et du Cadre de Vie et du ministere des Transports. Fasc. special 79-12 bis, circulaire No.79-25 du 13 mars 1979.

Technical Instruction on Community Directives (1979) relating to calculations of buildings (DC79).

BUREAU SECURITAS (1986). Recommandations concernant la conception, Ie calcul, l'execution et Ie contr6le des tirants d'ancrage (TA86), Ed. Eyrolles.

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Soil Nailing Recommendations-1991

Recommendations regarding the design, calculation, implementation, and inspection of ground anchors (TA 86). Regles techniques de conception et de calcul des ouvrages et constructions en beton arme suivant la methode des etats-limites BAEL 83. Bulletin officiel fasc. special No.83-45 bis, fasc No.62 titre 1er, section 1. Technical regulations on the design and calculations for structures made of reinforced concrete using the BAEL 83 limit-states method. Regles techniques de conception et de calcul des foundations des ouvrages de genie civil. Facs 62-titre 5 (document provisoire). Technical regulations on the design and calculations offoundations for civil engineering works. Eurocode 7, Geotechnics - Preliminary draft for the European Communities.

148

Chapter 3: Conception and Design

APPENDIX1

CHARTS GIVING THE UNIT SKIN FRICTION qs FOR PRELIMINARY DESIGN OF THE NAILS

1. DATA BANK As part of the French National Project CLOUTERRE, the CEBTP was charged with the collection of the results from pull-out tests carried out on nails installed by several members of the project, either on construction sites or on experimental sites. These tests, some 450 in all, were distributed throughout 36 different construction sites (figure 1). However, if we consider a project as one site, one type of soil or one single construction technique, then the tests were performed on 87 different sites.

, :) \

0 00

\.

0 0

"t

\_~/:

,'.

,/" ,' ....

,

.

,

... - ........

o o

......... ";"'\"'<9 "'Q.

Figure 1. Location of test sites in France.

149

Soil Nailing Recommendations-1991

The results of the tests now constitute a data bank (French National Project CLOUTERRE, CEBTP, January and March 1989) and for test purposes the following information was retained: The The The The The

owner. site. soil characteristics. nail characteristics. test results.

The total number of tests for various soil types is shown in figure 2. Sands. Gravel and debris. Marls and Chalks. Clays and Silts. Weathered rocks. Numbers of tests

120

100

'" . :..:::.:.;. [2] ....

....,

Sands

.... ~

Grovel and debris

~ ~

Marls and chalky soils

~ ~

Cloy and silt

l:'7l ~

Weathered rocks

:",

"

80

60

40

20

Figure 2. Distribution of tests as function of soil types; all nail types being put together.

As far as the installation methods were concerned, gravity injection was the most used technique (60 percent of cases as opposed to 30 percent for the driving method and 10 percent for low pressure injection).

150

Chapter 3: Conception and Design

On the various sites where nail pull-out tests were performed, the soil data were obtained from: Pressuremeter tests (and laboratory tests) Laboratory tests Unreported data

86% 9% 5%

The pull-out tests were conducted in accordance with two installation modes: The majority of the tests were performed under controlled force (creep steps) similar to regular pull-out tests on ground anchors, but somewhat simplified. The rest of the tests were performed under controlled displacement (at constant speed). From all the tests carried out, 27 percent were not taken up to pull-out failure for two reasons: 1) The test objective was to verify the service load of the nail. 2) The elastic limit of the reinforcing bar was lower than the failure load of the anchoring grout.

2. PRELIMINARY DESIGN CHARTS

By establishing a data bank, we are now able to suggest preliminary design charts that (as for piles and prestressed ground anchors) give a correlation between the pressure limit PI as measured with the pressuremeter and the unit skin friction qs' As with piles and prestressed ground anchors, the scattering of the test results is fairly wide (French National Project CLOUTERRE, CEBTP, January and March 1989, Bel Hadj Amor, 1990). The use of the preliminary design charts does not alleviate the user from performing routine tests (see chapter 4): conformity tests prior to construction, and inspection tests during construction. The charts have been drawn for five types of soil for which we have a significant number of test results: Sand, Gravel, Clay and silt, Marl-chalk, Weathered rock. Two construction techniques were retained: nails drilled and grouted under gravity, and nails directly driven into the soil. Another chart has been prepared for nails grouted with low

151

Soil Nailing Recommendations-1991

pressure into gravel (injection pressure at the nail head is generally between 0.2 and 0.5 MPa). Some adjustments have been made using hyperbolic relationships given by the results of shear tests conducted by the Institute of Mechanics in Grenoble (Boulon et al. 1986). The results have shown a good agreement for this type of curve, particularly where granular soils are involved.

3. DESIGN METHOD

For a nail grouted along a given length La' the limit pull-out force computed for a given skin friction, is equal to:

where: D D

= Dc:

borehole diameter for grouted nails,

= D s : equivalent diameter for driven nails (usually steel angles).

The unit skin frictions qs values are taken from the following table and reported in figures 3 to 7. Correspondence between the charts, the soils, and the construction techniques.

SOILS

CORRESPONDING CHARTS

CONSTRUCTION TECHNIQUES Gravity Grouting

Sand

Figure 3

SI

Gravel

Figure 4

Gl

Clay/Silt

Figure 5

Al

Figure 6

Ml

Figure 7

Rl

Marl Marl-Chalk Weathered to fragmented chalk Weathered Rock

152

Low pressure Grouting

Drivin g S3

G2

G3

Chapter 3: Conception and Design

qs (MPa) 0.3

I

SAND 0.25

0.2

0.15

I

0.1

~ \---

~ ....... V v

...

~

0.05

-

~

--- i-\ i-$1

- -

~

$3

Pe (MPa) o

3

2

1

Figure 3. Chart to estimate the unit skin friction qs for sand.

qs (MPa) 0.3

I

I

CLAY 0.25

0.2

0.15 I~

0.1

V /'

0.05

o

V

/

~

~ I--

I o

Pe(MPa)1 1

2

3

4

Figure 4. Chart to estimate the unit skin friction qs for clay.

153

Soil Nailing Recommendations-1991

qs (MPo) 0.7

GRAVEL 0.6

0.5

V

0.4

G2 ~

0.3 0.2

0.1

:,....

o

....

....

1

G

""

'---

-

1---

~.- 1 - ' -

/

,/

V V....

/

v

I~-

v G3

.- .- .L ' -

1-.-

2

3

4

Pe(MPo)

I

5

Figure 5. Chart to estimate the unit skin friction qs for gravel.

qs(MPa) 0.5

MARL-CHALK 0,4

0.3

0.2

0.1

o o

---I--

~

~

f--

~

2

P~ (MPa) I 3

Figure 6. Chart to estimate the unit skin friction qs for marl-chalk

154

4

--

Chapter 3: Conception and Design

qs (MPo ) 0.6

WEATHERED ROCK 0.5

0.4

/"

0.3

0.2

V

/

/

/

/'

/

V

V

0.1

o

Pe (MPo) 2

3

4

5

Figure 7. Chart to estimate the unit skin friction qs for weathered rock.

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Soil Nailing Recommendations-1991

156

Chapter 3: Conception and Design

APPENDIX

2

STABILITY CHARTS FOR PRELIMINARY DESIGN OF SOIL NAILED WALLS

1. NAILING DENSITY

The nailing density d is expressed by the following nondimensional expression:

d

where: t(kN/m) skin friction force per unit length of nail, y(kN/m 3) soil unit weight, Sv and Sh (m) : vertical and horizontal spacings between nails. This parameter characterizes the reinforcement force developed by the soil nail skin friction interaction as a function of a unit volume of the soil. The value of this parameter shows the amount of reinforcement in the soil and has been evaluated for a large number of structures. Depending on the soil types, the values vary from 0.5 to 1.5 in structures with grouted nails.

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Soil Nailing Recommendations-1991

2. DESIGN CHARTS

Characterization of the soil reinforcement by a single parameter can be useful for the preparation of the design charts for homogeneous soils (Bangratz and Gigan, 1984). The principle is illustrated in figure 9, chapter 3. For a given geometry of a structure defined by the ratio L I H (nail length over wall height), the stability charts are given for different values of nailing densities, d. The charts are drawn in a coordinate system: n = ely H versus tamp, where q> and e are the strength soil parameters. Linearity of the relationship between the different parameters leads to a chart of the global safety coefficient, such as F = OMIOA, M being the representative soil point. Inversely, for a predetermined global safety coefficient F, the charts give the nailing density, and thus the nail spacing can be deduced. The facing, with height H, is vertical and the soil surface at the top of the wall is horizontal and without any surcharge. Two nail distributions have been studied: a) nails having a constant length L I H: 0.6 - 0.8 - 1.0 - 1.2 b) nails with decreasing length linearly with depth, such as the lengths located beyond the sliding block (inclined to 1t + ! on the horizontal) are constant. 4 2 The lengths midway up the facing are such that: L

I H = 0.6 - 0.8 - 1.0 - 1.2

The charts (figures I and 2) have been prepared for nails with a constant length and for nails inclined below the horizontal e = 20 0 • The results can be corrected for other inclinations by using the charts in figure 3. The nailing density varies from d

= 0.1 to d = 1.0.

3. USING THE CHARTS 1) Choose the chart in accordance with the ratio L I H and the geometry involved (constant nail length or decreasing length). 2) Locate point M representing the soil in the coordinates system (N

158

= e I yH versus tanq»

Chapter 3: Conception and Design

3) Calculate the nailing density: d

where:

't

= P qs P

perimeter of the nail section,

qs : unit skin friction.

4) The straight line OM intersects the stability curve corresponding to the value of d at point A. The global safety coefficient is therefore equal to the ratio OM/OA.

159

Soil Nailing Recommendations-1991 N =o-f.-

yH 03

-----;:::===~;:::::===;---I-----:--;::~~-I TL L / H 0.6

I"

=0

d= - - ' = - - -

yShSv L

L

~ l;J

H

A / /

/ / /

/ / /

/

c

N=o- 0.3

yH

L/H

=0

0.8

L

~ C=O

I

-~~

2

1

tan

H

l..f

Figure 1. Design chart for preliminary design of soil nailed walls with L / H = 0.6 and 0.8 (8 = 20°).

160

Chapter 3: Conception and Design

c

N=yH 0 . 3 . - - - - - - - - - - - - - - , - - - - - - - - - 1 L /H = 1 L

H

2 ton

'f

N=~

yH 0 . 3 . - - - - - - - - - - - - , - - - - - - - - - - 1 L/H

O.II---\-\\r\----J..-~

= 1.2

__---+----:::::,.....,~------____j

2

ian Figure 2. Stability charts for preliminary design of soil nailed walls for L / H = 1.0 and 1.2 (e = 20°).

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Soil Nailing Recommendations-1991

F Itan,\,

4 3

I

FIN

L=O.6 H

I

I L =H r ---1,-----:-_ __ _

F/tan'f

FIN

tan'f=O

----l----T----~----~

I I

I I

I I

~

4

I I

-!I

3 1---....-::-----;.12

,

,0

I

0'------1-----'-----'-----'--__ 30 40 10 20 Inclination

I

. . . . -. -.

I---tl_---t----+--_ 10.3l 2 1---;---;---+----!' 0.1 N Iton'f

0(

o

10

Ion 'f =0

1-I

-.

- --

i

g.1 1N/tan'f

-ra. 3

I

I

I

I

I

I I

I

I

I I

I

20

30

I

40

Inclination
(0)

Figure 3. Corrections for the charts for inclinations other than 20°.

4. SAFETY VERIFICATIONS 4.1. Traditional method The value of qs is taken to be equal to the allowable value of the unit skin friction. The global safety factor of the soil nailed wall is F = OM/OA.

4.2. Calculating at ultimate limit state The coordinates of point M are in this case equal to (tamp / r m,
r m,e

The nailing density is calculated as:

t

d

r m, qs yr s 5J

V

Sh

The straight line OM intersects the corresponding design curve for a value of d at point A, which then gives:

r S3

162

~

OM/OA

Chapter 3: Conception and Design

APPENDIX

3

STABILITY OF A WEDGE OF SOIL REINFORCED BY ONE NAIL

Calculating the stability of a soil nailed mass resting on an inclined potential failure plane is presented for academic purposes. This type of calculation is not recommended in practice, as already stated in paragraph 3.2.3. The assumptions and notations are shown in figure 1. The forces acting on the sliding wedge are given in the sketch.

eto

H

~fO

Figure I. Equilibrium of a wedge reinforced upported by one nail. Notations and acting forces on the wedge.

In the event of a plane potential failure surface, the equilibrium condition to be verified is:

163

Soil Nailing Recommendations-1991

where: T T max

r S3

projection of the resulting forces on the sliding plane, shear resistance along this plane, method coefficient.

The equilibrium equations yield: T

(Wcosa

Tmax

where: a X

Wsina

=

+

- Tn cos X

Tn sin X) tamp

+

cH / sina

inclination of the potential failure plane on the horizontal, inclination of the nail with respect to the potential failure plane.

The basic formula, using the calculation values, can be rewritten as:

r S3 (Wsina Using the load factors

r S3 [rSI

Tn COSX) ~ (Wcosa

Tn sin X) tamp

+

+

cH/sina

r and the partial safety coefficients r m' we obtain:

J

· r m,R Wsma - - cos X < -

r m,R

r SI wcosa

tamp -

r

m,~

+

. Xtamp -Tn- sm -

r m,R

r

m,~

cH

+ --

r m, C

.

sma

It is observed as discussed in paragraph 3.4.1., that the nail forces playa role both as active

forces (with the projection TIl cos X of the effort Tn ) and as resisting forces (with the increase of resisting force induced by the normal component Tn sinx tan
Comments: Relationship between the load factors, the partial safety factors and the traditional safety factor F. In a traditional calculation, the safety factors are taken into account in the various nail failure criteria (they can be expressed by FR ) and then on the global stability when the structure slides (global safety factor F). For this example of a sliding nailed wedge, the global safety factor F is expressed as:

F

(T/FR ) sin X tan

Wcosa tan
+

R

)

If we put these two formulae together we can illustrate the following equivalences, which are not equal:

164

Chapter 3: Conception and Design

TRADITIONAL SAFETY (FACTOR)

LOAD FACTORS AND PARTIAL SAFETY FACTORS

FR

rSl .

rm,R

F on tamp

r S3 ·

rm,
F on c

r S3 ·

r Sl • rm,c

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Soil Nailing Recommendations-1991

166

CHAPTER

4

INVESTIGATION AND TESTS

1.

GEOTECHNICAL STUDY

The aim of the geotechnical study is to supply data relating to the behavior of the soil; this will be needed for the design and construction stages of soil nailed walls and should also allow for the monitoring of the interaction between the walls and existing structures.

1.1. Preliminary investigation The owner will prepare a report giving complete details of the site, showing site topography, access roads, the locations of adjacent wall structures and their foundations, and all roads and utilities. The plans, along with cross sections and elevations, will show all structures and facilities on the site, as well, as a perimeter area extending around the soil nailed structure at least 1.5 times the depth of the excavation. All documents will specify the service life of the supporting structure. A preliminary site visit and a review of geological maps will allow the engineer to predict the nature of the soil to be nailed (see chapter I, paragraph 2.4.). In all classes of soils (difficult or not), the field site report must recommend a geotechnical investigation program.

1.2. Soil Investigation From the geotechnical investigation program, the subsurface stratigraphy will be developed for every 200 to 600 m 2 of soil nailed wall. Depending on the complexity and project size or for any significant changes found in the site geology, the stratigraphy for a cross section will cover a distance from the face equal to one and a half times the height of the wall. It will also extend at least two meters below its base, unless it can be demonstrated that the soil at the deeper levels has at least the same quality (Figure la). In sloping ground, the distance from the wall face will be taken as three times the height of the wall (Figure Ib).

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Soil Nailing Recommendations-1991

The exploratory borings and the soil sampling (disturbed and undisturbed samples) should be carried out in accordance with current French standards.

L ~ 1,5H

-

ISj I

I, 2m

L> 3H

I---------------------=~

I I I I I

I H

..

I;

II

I

H

2m

® Figure 1. Area of investigation.

For large sites, the soil stratigraphy can be confirmed with destructive production drilling techniques with appropriate data recordings. These drilling parameters will be calibrated with the conventional geotechnical boring advancement techniques used at the site. The development of the stratigraphy with .conventional methods may be supplemented with pressuremeter tests and/ or cone penetrometer tests. The use of dynamic penetrometer tests will enable the penetration of any intermediate compact soil layers and the foundation stratum to be found. It will also provide data on the possibilities of installing metal nails using the percussion method.

1.3. Laboratory and in situ tests The objectives of these tests are: 1) To identify the soil. 2) To determine the soil strength parameters

168

S

n

Chapter 4: Investigations and Tests

3) To assess the unit skin friction qs based on pressuremeter tests. 1.3.1. Cohesionless soils

1) The identification will consist of performing a sieve analysis, including the fines fraction and determining the natural water content. 2) In determining the strength parameters,

The standard penetration test (SPT), or cone penetrometer tests (CPT), may be used with the usual correlations to estimate the value of the internal friction angle
1) The identification will consist of determining the Atterberg limits, the total unit weight, the natural water content w, and the dry unit weight Yd' 2) For parameters

= 0 since

• Long-term strength parameters of saturated soil One will determine the intrinsic parameters
characteristics are almost equal and the angle of internal friction is close to
169

Soil Nailing Recommendations-1991

1.3.3. Characteristic values of strength parameters

The geotechnical engineer is responsible for providing the most probable values, based on in situ and laboratory tests performed on a sufficient number of samples. The characteristic value of the unit skin friction qs is given either by charts (see the appendix to this chapter) or based on the results of in situ tests (see paragraph 2.8.).

1.4. Determination of the soil corrosion potential Chapter 6 - Durability of the structures - gives a global corrosion index. This index depends on the soil's resistivity and pH level.

1.5. Hydrogeological study In order to find the water table and follow its variations, a system must be installed to measure the site hydraulic regime. Depending on the type of soil, the frequency of measurements of the water table variations will need to be increased. Generally, standpipe piezometers will be used. If one or more water tables is present, the permeability of each layer of soil that intersects the excavations needs to be determined.

2.

NAIL TESTS

2.1. Tests objective Since the stability of a soil nailed wall is studied at its ultimate limit state, the main objective of nail tests is to determine the unit skin friction qs. However, it is recommended that during the test, steps be taken to measure all the unit skin friction curves as a function of the relative nail! soil displacement.

2.2. Different types of tests Depending on the objectives of the tests and at what stage of the building schedule they are performed, one can distinguish among: Preliminary tests at the planning stage, even before construction works have started. Conformity tests when work begins on the site. Inspection tests during the construction. All the tests are identical and attempt to prove the quality of the soil nail friction. This is done by applying a static tension at the head of a nail until movement by lack of friction

170

Chapter 4: Investigations and Tests

occurs. The selection of the cross section is chosen to avoid breakage of the nail in tension. None of the nails used in these three types of test can be reused or incorporated into the structure.

2.3. Objectives of the different tests 2.3.1. Preliminary tests

In principle, preliminary tests are reserved for soils outside the normal range of applications defined in chapter 1, for validating a new nailing technique, for an important construction site, or if demanded by the owner. They can be carried out by a company other than the one in charge of the construction. The objective is to determine a maximum tension. Preliminary tests are, of course, carried out several weeks in advance of any service nails being installed. The preliminary tests are performed on specific nails installed in test plots. The minimum number of preliminary tests to be conducted account for the wide dispersion observed on real sites. Table I gives the minimum number of tests specified for each type of soil, depending on the facing area that concerns that soil type (figure 2).

TABLE I.

m 2 of facing

No

m 2 of facing

No

Up to 800

6

4 000 to 8 000

15

800 to 2 000

9

8 000 to 16 000

18

2 000 to 4 000

12

16 000 to 40 000

25

171

Soil Nailing Recommendations-1991

100m

I-

~I

1000 m 2 of facing N=9

N° 1

10 m

400 m2 of facing

4m

N=6

Figure 2. Example showing the number of tests to be carried out for each different type of soil encountered.

2.3.2. Conformity tests at the beginning of the construction

The objective of these tests is to check the validity of the assumptions on the soil nail unit skin friction value qs taken at the planning stage. The conformity tests are conducted for each different soil layer once the appropriate excavation depth is reached. The wall facing may be used as a reaction block. In that case, one should check that the facing will not be damaged when applying the maximum pull-out force. If no preliminary tests are carried out, the conformity tests are compulsory for all soil nailed

walls. Table 11 gives the minimum number of conformity tests specified for each type of soil, depending on the facing area concerned with that soil type. It is important to distribute the total number of tests evenly throughout the whole structure.

TABLE II.

m 2 of facing

172

No

m 2 of facing

No

Up to 800

6

4 000 to 8 000

15

800 to 2 000

9

8 000 to 16 000

18

2 000 to 4 000

12

6 000 to 40 000

25

Chapter 4: Investigations and Tests

If the company conducting the preliminary tests is not in charge of the construction, conformity tests will still have to be carried out at the beginning of construction. However, if the same nailing technique is used with the same installation procedure, the number of tests could be divided by 2. If the company conducting the preliminary tests is in charge of the construction, no conformity test is required.

2.3.3. Inspection tests during construction

Inspection tests are carried out on nails chosen in advance and for which the cross section has been dimensioned so that failure by pull-out will be reached without causing the nail to rupture. Inspection tests are compulsory for all sites. For each soil layer encountered a minimum number N is fixed as five tests up to 1 000 m 2, with a minimum of one test for each excavation stage. Over 1 000 m 2, the number of tests will be increased by one for each additional 200 m 2• The total number of tests should be distributed evenly throughout the whole structure.

2.4. Contractor's duties If the owner believes the contractor can be exempted from the preliminary tests as defined in

paragraph 2.2.1., the reasons for exemption will be specified in the technical documents. The contractor is responsible for the execution and interpretation of the preliminary, conformity, and inspection tests. The results are given to the owner or his representative who should give agreement before any work can be carried out or continued.

2.5. Nail tests and reaction forms 2.5.1. Nail tests location

The nails to be used in the preliminary tests will be installed in the ground and grouted in the soil layer for which the interaction between the soil and nail is to be measured. The nails used in conformity tests will be installed in the concrete facing when work begins on the soil nailed wall. Tests will be conducted on each soil type or where soil differences occur. The nails to be used in the inspection tests will be installed between the service nails in the soil nailed wall.

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Soil Nailing Recommendations-1991

2.5.2. Reaction forms •

Preliminary tests: Special reaction forms will be built for test nails and the design should

prevent any appreciable rotation of the support. •

Conformity tests: The facing will be used as the reaction form and special attention must be given to the design of the facing to ensure no damage occurs during the tests.

2.5.3. Nail installation for the three types of tests

The procedure for placing the nail (inclination, drilling, installation, and grouting, in the case of a grouted nail) should follow exactly the same procedure as the one planned for the construction work. However, the reinforcing bar can have a higher tensile resistance, such that the nail can fail through lack of adhesion rather than breakage of the nail. The grouted or driven length will be equal to either the actual length of the nails used in the structure, or 5 meters. A minimum free length of 1 meter will be available in order to avoid boundary effects when the supporting plate bears directly on the soil around the nail head (see figure 5).

Test nails

Soil

I

nO 1

Soil nO 2

-

Figure 3. Layout arrangement of nails used in preliminary tests.

If no free length has been reserved, the stresses induced by the jacking system should be transferred far away from the nail with a whaler-type device. The steel section of the reinforcing bar will be designed so that the maximum tension remains less than O.9Tc . The free length can be much longer for preliminary tests in order to simulate the real overburden conditions of the structure (figure 3).

174

Chapter 4: Investigations and Tests

2.6. Nail tests procedure The testing procedures for the three types of tests (preliminary, conformity, and inspection) are identical. 2.6.1. Choice of the procedure

Based on results of research and studies conducted as part of the Project CLOUTERRE (French National Project CLOUTERRE, CEBTP, June 1988 and December 1989; French National Project CLOUTERRE, CERMES, December 1989), the standard pull-out test used is the controlled displacement test (constant speed) with additional controlled force tests (creep steps). With the controlled displacement pull-out test, it is possible to determine the maximum pullout force Tu the residual force, as well as the value of the initial slope of the force displacement curve (figure 4). The maximum unit skin friction, the residual unit skin friction and the initial slope of the skin friction mobilization curve can be determined from the force displacement curve (see appendix 1 to this chapter).

Force at head

I

To Peak

~

Residual

------_.~--~

-

Displacement at head Yo Figure 4. Determination of the maximum pull-out force.

175

Soil Nailing Recommendations-1991

With controlled force test, the critical creep tension Tc and eventually the limit tensile force T L can be measured. Results from a wide number of controlled force tests conducted within the National Project CLOUTERRE or supplied by members of the Project (French National Project CLOUTERRE, CEBTP, June 1988, March 1989, December 1989) have allowed the development of correlations between the limit tensile force T L and the critical creep tension Tc • These ratios are summarized in table III, which shows the order of magnitude of k = TLIT(I as a function of the type of soil and the installation method used.

TABLE III.

k

= TIITc

GRAVITY

Sands

1.2

INJECTION

Clays

1.3

Marls and Chalks

1.3

Sands

1.4

DRIVING

For preliminary and conformity tests, irrespective of the type of soil found, an equal number of controlled displacement pull-out tests and controlled force pull-out tests will be conducted. Inspection tests will include controlled displacement pull-out tests for noncreeping soils (Ip < 20) and an equal number of controlled displacement and controlled force pull-out tests for creeping soils (Ip 2 20) (see table IV). TABLE IV. Inspection tests.

Soil

All types of structure

Ip < 20

100% controlled displacement pull-out tests

Ip 220

50% controlled displacement pull-out tests 50% controlled force pull-out tests

2.6.2. Choice of the maximum load capacity for the nail tests

For all three types of tests, the real unit skin friction value qs will be determined for each type of soil and nail to check the unit skin friction value estimated on the basis of the pressuremeter test results or some other method by the construction company or the

176

Chapter 4: Investigations and Tests

consulting engineer. Where the unit skin friction value measured is higher than the estimated value, tests will be performed until the nail has been extracted fully. The reinforcing bar will be designed such that:

TG

elastic limit of the bar used in the test,

T\E

nail's pull-out tension estimated on the basis of geotechnical data or the contractor's experience.

2.6.3. Materials and equipment used during the tests

The equipment used comprises (figure 5) an adjusting wedge to apply the tensile force along the bar axis, a ring-shaped jack and its pump, and a load cell. The load cell ring should indicate the forces with an accuracy of at least 1 percent of the maximum force. It must be possible to reproduce every measurement, particularly those taken during the controlled force tests (creep stages), with an error less than 0.1 percent of the maximum force during each creep stage, independently of the temperature variations. The displacement at the head of the nail should be measured in relation to a fixed point, a system embedded in the ground (figure 5) or any other device that ensures a very stable base. Using this type of equipment, which is simple and quickly installed on a construction site, a sufficiently large supporting plate will be fixed to the nail so that the tip of the sensor capable of measuring to 1/100 mm remains permanently in contact throughout the duration of the test. ..

. '

...

'

.

'<1

c·~ ;

"

. . :", '.

~c

Pacemeter (1mm/mn)

Support rod

Figure 5. Pull-out test set up for a nail.

177

Soil Nailing Recommendations-1991

2.7. Controlled displacement pull-out tests (constant speed) 2.7.1. Test procedure Before putting the nail into tension the following points need to be verified: The free length of the nail must be protected by a tube and sufficient space provided between the tube and the supporting plate to avoid any contact throughout the total duration of the test. Where a free length does not exist, jack forces will be taken by a whaler system supporting the loads at a distance of 1 meter away from the nail axis. The system, which comprises the supporting plate, jack, and load cell must be set up in such a way that the bar will not bend prior to testing, thus providing a reliable loaddisplacement curve. The nail will be put into tension at a speed of 1 mm/min, preferably using a manual pump. The margin of error on the speed must be ± 10 percent. This speed can be controlled, for instance, using a pacemeter and a dial graduated every 1/100 mm. The stem should allow minimum travel of 50 mm. The test will be stopped once the tension force has either passed a maximum or has stabilized. It is recommended to plot the force displacement curve. Readings of loads will be made every 1/10 mm (or every 6 seconds) up to 5 mm, thereafter every 1/2 mm (or every 30 seconds) up to the residual load. If no residual load appears, the test will be continued until the tensile force at the head of the nail varies less than 1 percent for a 1 mm displacement.

To

TL

.6F

-------------

-------;,----

F

~

F

y

y

Figure 6. Failure criteria used in pull-out tests.

178

y

-<

Chapter 4: Investigations and Tests

During unloading, force versus displacement readings will be made at each tenth of the maximum force. The maximum pull-out force T L will be the maximum force value reached during the test. This value, Tu will correspond on the force-displacement curve to either the peak value, the residual value, or the value such that the variation in force per 1 mm is less than 1 percent, or the value for a maximum displacement of 30 mm. 2.7.2. Interpretation of test result

If during the test only the forces are measured, the result obtained will be the maximum pull-out TL • If during the course of the test, both the forces and the displacements at the head of the nail are measured, then not only can the maximum pull-out force be obtained (useful for calculating the ultimate limit state) but also the law which describes the soil nail interaction. This is useful for estimating the displacements in the soil nailed wall, and will be vital for future calculations at serviceability limit state. The interpretation of the results can be based on the skin friction mobilization law presented in figure 7. Eventually, the strength of the grout may be taken into account in the calculations (French National Project CLOUTERRE, CEBTP, June 1988). The calculations of the displacements along the nail during a pull-out test are developed in the appendix of this chapter. The following significant results will be helpful for the interpretation of the test. Distinction will be made between two cases. The behavior of a nail in a soil nailed wall under service conditions far away from the failure and the behavior of a nail at ultimate limit state, i.e., at failure.

I

qs 2

k,G /5

I I I I

y

Figure 7. Skin friction mobilization law (Frank and Zhao, 1982).

179

Soil Nailing Recommendations-1991

2.7.2.1.

Generalities

If a tensile force To is applied at the nail head, the nail moves with respect to the soil and

mobilizes the skin friction, which balances the force To' Mobilization of the skin friction is made gradually from the nail head toward the nail tip, the shorter the nail the more rapidly the mobilization occurs (figures 8a, 8b, and 8c).

\

15(X)

\

,, \

,,

\

\ \

\",,{

Ultimate

limit state (failure)

\

1000

\

\

\ \

\

\

" ....

" ",

\

Service state

c =£1 + £2 2

500

£,

:_-----11

--1.....

1.:=2

5(X)

a)

12 m

.I

I(XX)

Distribution of the deformations along a nail measured from tests.

Figure 8. Experimental and theoretical distributions of tension forces and deformations along a nail.

180

Chapter 4: Investigations and Tests

100 '- '-

'-

'-",-f .....

.....

'-

Ultimate

limit state (failure)

Service

state

'-

50

--I

I..

50

b)

100

150

200

I

.1

3 m

250

Theoretical distribution of tension forces along a 3-m-long nail.

To (kN)

400 Ultimate' limit state (failure) 300

Service state

200 ~'-_ _----II

I...

/2 m

.1

100

L (em)

o 000 c)

Theoretical distribution of tension forces along a 12-m nail.

Figure 8. Experimental and theoretical distributions of tension forces and deformations along a nail.

181

Soil Nailing Recommendations-1991

When the To force is increased, the skin friction begins to fully mobilize at the head ('t and full mobilization progresses from the head toward the tip (figures 9a and 9b).

2.7.2.2.

= qs),

Behavior of nail under service conditions (To < TL)

The skin friction is still in the course of being mobilized (figures 8 and 9). The tip of a short nail will have a displacement similar to that of the head, while for a long nail, the tip displacement will be negligible compared to that at the head (figure 10). Experiments and modelling of the behavior have established that the displacement of the nail head Yo is proportional to the head force To, as long as the Yo displacement does not exceed the Yl value corresponding to the first bend of the skin friction mobilization law (figure 7). With this condition Yo < Yl' the following relationships can be demonstrated and are developed in the appendix: To

To

displacement at nail head, displacement at nail tip, force at nail head, length (in contact with the soil).

a

with P k~

perimeter of the nail, initial slope on the skin friction mobilization curve.

When the displacement of the nail head Yo exceeds Yl' the Yo displacement does not remain proportional to the To head force, although the skin friction is not fully mobilized along the nail and the nail is still a long way from failure (figure 11).

182

Chapter 4: Investigations and Tests

?

(kPa)

---------- ~ -- ---

Ultimate limit state (failure)

state

=_1....-----1

----

10

--------------

1_

3m

..

I

L (em)

O'-------.----r-----.----.--------.----r-~

250

300

a. Short nail.

7' (kPa) Ultimate limit state (failure)

60

----

1- - - - - - - - - -

50 40 30

2 12 m

..

I

10

L (em)

500

1000

b. Long nail. Figure 9. Distribution of shear stress along a nail.

183

Soil Nailing Recommendations-1991

To (kN) Load at head of nail

Displacement of the tip Displacement of the head

-



u 0:; '

E

- --

La=4m

c::::::r-

ilic:::b-

0

y

r.n

0

0

r.n

E

~

;!::

:::)

(f)

10

5

Displacement

y (mm)

To (kN)

Load at the head of the nail ",.,."""'"

400

/'

I

/

~~----------------

""..-

~

Displacement of the tip Displacement of the head

300

--(l)

o

( /)

E

200

(l)

--(l)

0

I

( /)

Jr

E

100

L=12 m

(l)

u > 1-

'

J---.. I

(l)

Cf)

o 5

10

15

20

25

Displacement

y (mm)

Figure 10. Force/displacement curves at the head and tip of both a short and a long nail.

184

o

E

Chapter 4: Investigations and Tests

To (kN)

Load at head of nail

L L= 12 m

250

=

12m

l

~;r

,,

x

,X l

x

200

"

,,x.

,x

-x--x--x-

,

,X

150

~ Calculation of

the force-displacement at the head of the nail assuming that the nail tip is fixed and T'= constant

-+-+-T-? curve

,,

'i

• \ Calculation of the force-displacement

•

f

-0-

o-? curve at the head of the nail ta king into accunt the unit skin friction mobilization low

100

50 I I

: -0

/1 /

:,

---- ---

L=2m --------

~---~

I I I

I I

5

I

K)

Displacement of the head of nail

I

15

y 2

2b

Yo

(mm)

...

Figure 11. Theoretical comparison of force-displacement curves at the nail head (based on two different sets of assumptions).

2.7.2.3.

Behavior of nail at failure To

=T

L

Skin friction is fully mobilized (figures 8 and 9). At failure, the theoretical displacements at the head and tip of the nail will be equal to:

where Yo YLs

displacement of the nail head at failure, displacement of the nail tip at failure,

185

Soil Nailing Recommendations-1991

p

qs ES Ls

nail perimeter, unit skin friction, nail stiffness, length in contact with the soil.

The experimental measurements are in agreement with the theoretical calculations (see appendix to this chapter). At failure, every nail has a displacement at its tip interaction law.

2.7.2.4.

YLS

equal to the value Yz of the soil nail

Calculations of the unit skin friction qs

The objective of a pull-out test is to find the maximum pull-out force TL in order to determine the unit skin friction qs' The unit skin friction qs will be calculated using the TL value given in paragraph 2.7.1. and figure 6.

where p Ls

nail perimeter, length in contact with the soil.

NOTE: Even as a first approximation, the comparison of the real force-displacement curve with the theoretical one based on the following two assumptions: The nail tip is fixed and the shear stress is constant along the nail, whatever the pulling force, is wrong. The real forcedisplacement curve can be well-approximated using the skin friction mobilization law. Figure 11 shows simulations for both models and clearly indicates that the first approach is inappropriate, Le., the shorter the nail, the greater the degree of inaccuracy.

186

Chapter 4: Investigations and Tests

Yo (mm)

Cumulated displacements

8

7 6

0.9 T max

4

3l-_------------- 0 .8 T max

L----------------=-:~-0 6 max 2 l--l - - _ - - - - - - - - - - - - - 0.5. TTmax ~--------------0.4

~============== I10

max

0.7 T

TmlJ.( max

0.3 T 0.2 Tma<.

60

100

Log t (mi n )

Figure 12. Creep curves of a pull-out test.

2.8.

Controlled force tests (creep steps)

In tests carried out under rigorous experimental conditions on a same type of soil not susceptible to creep, both controlled force pull-out tests and controlled displacement pull-out tests have shown the same maximum pull-out force Tu 2.8.1. Procedure for controlled force tests

The preparatory work will be the same as for the controlled displacement pull-out test, but the rate of movement will not be measured. Before carrying out a controlled force test, it will be necessary to estimate the limit pull-out force Tw which is used for the determination of the step values. Usually the limit pull-out force TLE is assessed from the controlled displacement pull-out tests (which are always carried out first). The nail is gradually subjected to a pull-out force, which increases up to the estimated limit pull-out force Tw and which must be lower than 0.6 TG so as to limit creep in the steel. Successive loading steps are maintained during 60 minutes, except the 0.7 T LE step, which is maintained during 3 hours. The first loading step is applied at 0.2 TLE •

187

Soil Nailing Recommendations-1991

The first displacement reading is taken at 0.1 T w during a continuous loading between 0 to 0.2 T LE • The following steps are applied every 0.1 T LE • At each load step, the creep displacement measurement is performed as follows: The tensile force imposed on the nail is measured using a load cell. The level of this force is kept rigorously constant throughout each loading step, however, a variation in the force value of up to 0.1 percent of the estimated limit pull-out force TLE is acceptable. The importance of carrying out real creep tests (at constant force) is emphasized. If this is not done, variations in the levels of force applied to the nail during the course of the loading steps can mask creep phenomena, especially during the first loading steps. During the first steps, the creep should not exceed 1 to 2 tenths of a millimeter. The start of time to is taken at the instant when the load level of the corresponding step is reached. Displacement measurements are taken at each loading step and performed at the following time intervals, to: 1, 2, 3, 4, 5, 8, 10, 15, 20, 25, 30, 45, 60 minutes, and then every 15 minutes for the 0.7 TLE loading step, which is then maintained for 3 hours. If necessary, the jack pressure can be adjusted to maintain the force at the required loading step value. Temperature measurements are made at the beginning and the end of each step.

If the soil-grout bond does not fail at the estimated limit pull-out force Tw it will be possible to continue the test in 0.1 TLE increments until the 15th step (not exceeding 0.9 Tc )' After the last step, unloading will be carried out and reading of displacements will be taken every tenth of the limit pull-out force reached during the test. 2.8.2. Interpretation of the controlled force pull-out test results 2.8.2.1.

Drawing of the creep curves

For each creep step, the x axis will be the time (decimal logarithmic scale) between to + 1 min and to + 60 min (or t1 + 180 min for the 0.7 TLE loading step) and the a axis will be arithmetical scale - the displacement (figure 12). For each loading step, the creep curve is characterized by the slope of the tangent to this curve at to + 60 min (or to + 180 min for the 0.7 TLE step). Generally, for the first loading steps, the creep curves are straight lines; at higher loads the creep curves are no longer straight, but in some cases, even for loading steps close to the limit pull-out force, the creep will be represented by straight lines.

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Chapter 4: Investigations and Tests

Slope ex (mm /cycle of log t (min))

100

50

T/Tmox

o 0.5 Figure 13. Determination of the critical creep tension.

2.8.2.2. Determination of the critical creep tension

The slopes a are reported in graph form using arithmetical co-ordinates where: x axis: the tension values T corresponding to each loading step y axis: the slope a. The critical creep tension Tc is obtained from the plot of the a values (figure 13). The first part of this plot is nearly linear and the second part is concave upward. The critical creep tension Tc corresponds to the last loading step before the curve bends. One can also define a value Tc', as the intersection of the two straight lines. Numerous tests have shown that: Tc '" 0.9 T/c

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Soil Nailing Recommendations-1991

2.8.2.3. Drawing of the force-displacement curve

The force-displacement curve will be plotted taking account of any creep displacement observed at each loading step. The limit pull-out force is generally reached when it becomes necessary to continually activate the jack to maintain the load constant.

2.9.

Determination of the characteristic limit pull-out force

The unit skin friction value qs used in soil nailed wall design will be determined based on the limit pull-out force TL • The minimum number of tests to be carried out is six (see paragraph 2.2) and the most representative average value from the controlled displacement pull-out tests and controlled force pull-out tests will be retained as T L. The parameter qs value is given by the following formula:

qs = where p Ls

TL P Ls

nail perimeter, length of the nail in contact with the soil.

A characteristic unit skin friction value qs can then be deduced from the qs values. The surface of the nail is calculated on the basis of the theoretical length in contact with the soil, Le., by subtracting a possible free length. When controlled force pull-out tests do not allow determination of the pull-out limit force Tu then this value can be estimated from the critical creep tension Te, using the ratio shown in table III of this chapter.

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Chapter 4: Investigations and Tests

BIBLIOGRAPHY

Internal Reports of French National Project CLOUTERRE CEBTP (1988). Essais de traction en vraie grandeur de differents types de dous dans du sable de Fontainebleau. Juin 1988.

Full size pull-out tests using different types of nails in Fontainebleau sand. CEBTP (1988). Rapport final interaction sol-dou. Juin 1988.

Final report on soil-nail interaction. CEBTP (1989). Banque de donnees des essais d'arrachement de clous et autres indusions rigides (comparaison avec DTU 13.2 et SETRA 1985, doc No.3 bis), Mars 1989. Data base of pull-out tests on nails and other rigid inclusions (comparison with DTU 13.2 and SETRA 1985, doc. No.3). CERMES (1989). Interaction sol-dou. Etude en laboratoire. Decembre 1989.

Soil-nail interaction. Laboratory study. CEBTP (1989). Determination du mode operatoire de l'essai de traction sur dou. Test sur deux types de dous suivant un essai a vitesse constante et un essai par palier de fluage. Decembre 1989.

Determining the operational procedure for pull-out tests on nails. Pull-out tests carried out on two types of nails using a controlled displacement and a controlled force test.

PUBLICATIONS BUREAU SECURITAS (1986). Recommandations concernant la conception, Ie calcul, l'execution et Ie contraIe des tirants d'ancrage (TA 86). Ed. Eyrolles.

Recommendations for designing, calculating, contracting, and controlling ground anchors. PLUMELLE, C. (1979). Etude experimentale du comportement des tirants d'ancrage. These de Docteur-Ingenieur, Universite P. et M. Curie, 1979.

Experimental study of ground anchors. GASNIER and PLUMELLE, C. (1984). Etude experimentale en vraie grandeur de tirants d'ancrage. c.R. Colloque international renforcement en place des sols et des roches. Presses ENPC, Paris.

Full-scale experimental studies of ground anchors.

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Chapter 4: Investigation and Tests

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Chapter 4: Investigations and Tests

APPENDIX

CALCULATIONS OF NAIL DISPLACEMENTS 1. GENERALITIES Contrary to the notations used in mechanics, the conventions traditionally used in soil mechanics will be adopted when calculating displacements for such inclusions as piles, micropiles, anchors, nails, etc. The displacement y and the coordinate x will be taken positive from the head toward the tip of the nail. If a force To is applied at the nail head, the nail moves with respect to the soil and mobilizes the skin friction that balances the force To in accordance with the skin friction mobilization law shown in figure 1. If the nail was perfectly rigid, compared with the soil, the displacements at both the head and the tip would be identical. Conversely, if the nail was infinitely extensible, compared with the soil, the displacement at the tip would be zero. The real behavior is generally in between, depending, in particular, on the length of the nail, which is generally short in pull-out tests. 1:(kPo)

90

45

-------------------------------~------

I

I

LI ikp = 25kFb/mm y (mm)

Figure 1. Skin friction mobilization law. (Frank and Zhao, 1982.)

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Soil Nailing Recommendations-1991

Accurate calculations of the displacements in the nail versus the applied force at the head of the nail depend on: Nail elasticity - Hooke's law. Skin friction mobilization law defined in figure 1. Length of the nail. The most general case must take into account the progressive skin friction mobilization along the nail as a function of the pull-out force applied at the nail head (figure 2).

5

4

5

Yz

Y

• Yo:

displacement of head

Initial position

A Ye:

displacement of tip

Position of nail under a force To

Figure 2. Gradual mobilization of shear stress at both head and tip of the nail as To increases.

1:

Yo < Yl

The displacements at the head and tip of the nail are located on the first linear part of the curve. The displacement at the head is located on the second linear part while the displacement at the tip is located on the first part.

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Yo> Y2 0< Yl < Yl

The displacement at the head is on the residual part of the curve, the skin friction at the head is fully mobilized, the displacement at the tip is located on the first linear section.

3:

Yl < Yo < Y2 Yl < Yl < Y2

The displacements at both head and tip are located on the second linear part of the curve.

4:

Yo> Y2 Yl < Yz < Y2

The displacement at the head is on the residual part of the curve; the skin friction at the head is fully mobilized and the displacement at the base is located on the second linear part.

5:

2.

The displacement at the head is already on the residual part and the displacement at the tip has reached the residual part. The limit pull-out force is reached, but the nail fails through lack of friction.

CALCULATION OF DISPLACEMENTS

Among all the phases of mobilization of the skin friction along the nail, two are of particular interest. The first is the stage at which the displacement at the head is still lower than Yl' This can be regarded as the "elastic" phase. The second is the limit pull-out force, which is reached when the nail tip reaches Y2'

2.1.

First phase (Yo < Yt)

Assumption: Small displacements and small strains are considered. The reference state will be the initial state. The example is based on a nail driven so that the effects of the grout will not be considered.

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Soil Nailing Recommendations-1991

e(x)

cr(x)

=

small strains

dy dx

= E e(x)

Hooke's elasticity law of the bar

T(x) = E e(x) ·5

dy

T

dx

5 E

2

5: Section of bar

d y

dT

1

2

dx

5 E

dx

dT

- 't P dx

p: Perimeter of bar in friction

- k~ Y

't 2

d y dx

E: Young's modulus of bar

2

P

5 E

k~ Y

Using the parameter a =

k

~ __~ 5 E

y

= M 1 ch(a x) + N 1 shea x)

T

= a E 5 [M1 shea x) + N1 ch(a x)]

The boundary conditions are:

as homogeneous to

t 1, we can express y(x) and T(x):

x=O x =1

The displacements and the forces can be computed at any point x in the function of the pullout force To and the soil and bar characteristics:

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Chapter 4: Investigations and Tests

Y

T

ch (a (L - x)) shea I)

To E 5 a

To sh

(a (L - x))

shea l)

displacement at the head (under the conventions adopted, Yo is negative). To E 5 a thea I)

-

displacement at the tip:

To E 5 a shea l)

The modelling shows that, in theory, there is some displacement at the tip when a force is applied to the head; this corresponds well with the results of tests performed on inclusion of different lengths (from 2 to 12 m). However, the displacement at the tip of a short nail starts to occur at the beginning of the pull-out test (figure lOa of this chapter), whereas in a long nail, the displacement of the tip can only be measured when the force is close to the limit pull-out force (figure lIb of this chapter). It should be noticed that if only the "elastic" phase 0 < Yo < Yl is to be used, there is an

anchor limit length beyond which the nail does not transmit any force to the soil.

If Yo = Yl is considered as the limit of the "elastic domain"

where, th(2)

= 0.964 # 1 then: a I I = 2 II = 2/a

I "2 I

J

E S P k~

II increases with the stiffness E 5 of the bar and decreases with the stiffness of the soil represents the length of transfer of the tension force from the nail to the soil.

k~.

It

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Soil Nailing Recommendations-1991

NOTE: The present simulation has been done for a test conducted up to failure, therefore there is no reason to limit the pull-out capacity of the nail to its "elastic" phase. There is equilibrium as long as the displacement at the tip of the nail does not reach the residual part of the skin friction mobilization curve.

2.2.

Second phase: at failure

This phase is reached when displacement at the tip is equal to Y2:

At this stage the unit skin friction value becomes constant along the whole length of the nail and is equal to qs: 't

= qs

T=T a - P qs x and Ta dy dx

p qs 1

T 5 E

(according to the conventions adopted, y is negative) Taking y at its absolute value:

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Chapter 4: Investigations and Tests

In the first equation, the expression Tal /2 E 5 represents the lengthening of the nail, for which the tip is clamped, and the unit skin friction is mobilized and constant along the nail. The value Yl' representing the displacement at its tip, must be added to obtain the exact displacement at the nail head. For the same type of nail and the same type of soil, at failure, the tip displacement is obviously the same whatever the nail length. On the other hand, the lengthening of the nail corresponding to the difference between the head and tip displacements is proportional to the square of the length. The theoretical results have been verified by full-scale tests conducted with 2 to 12 m long nails (Deguillaume 1981, Plumelle 1984). The results shown in figure 3 give the theoretical and experimental displacements at the head and tip of the nail at the critical creep tension Te •

2.3.

Example of a full calculation

Accurate calculations at each phase of mobilization were made using a steel angle of cross section 70 mm x 70 mm x 7 mm. The mobilization of the skin friction was studied without taking into account the possible yielding of the steel above the elastic limit. The skin friction mobilization law is shown in figure 4. Results from theoretical calculations are given for two lengths - 2 m and 12 m - in figures 5 and 6. For each phase, the respective positions of the displacements that occurred at the head and the tip of the nail are shown on the force versus displacement curves.

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Soil Nailing Recommendations-1991

Nail displacement (mm)

10

Yo

theoretical displacement of nail head

• 5

• e:theoretical displacement

Y {

•

0--0'--0---_

o

2

•

3

4

6

of nail tip

_

o

----0

9

12

Length of nail (m)

Experimental displacement at head of nail

o Experimental displacement

at

tip of nail

Figure 3. Comparisons between the experimental and theoretical displacements of both the head and the tip of 2- to 12-m long nails at the critical creep tension Te .

The rigid behavior of the short (2 m) steel angle will be evident in comparison to the more "elastic" behavior of the long steel angle (12 m).

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Chapter 4: Investigations and Tests

To (kN)

280 Experimental Curve

240 7; (kPo)

200

160 k,t3=25kPo/mm

120

8 60

1,8

10,8

Yf

Y2

Y (mm)

SKIN FRICTION MOBILIZATION CURVE

Linear part of curve

Displacement at head of nail 5

10

15

20

Y (mm) o

Figure 4. Theoretical force versus displacement curve at the head of the nail.

In both cases the "elastic" phase is clearly limited to a 1.8 mm soil-nail relative displacement, relative to the value of Yl' For instance, a straight line has been plotted on each graph that corresponds to a simulated head displacement where the tip is deemed to be fixed and a unit skin friction fully mobilized and constant along the nail. Comparisons with the exact curves clearly show that it is wrong and dangerous to try to use this type of simulation.

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Soil Nailing Recommendations-1991

Load at head

150

.-------------------,

1

Steel angle nails : 70mm x 70mm x 7rrrn L=2m

100 at head yo and tip

DisPlacements

YC

of the nail are about equal

50

~~~-:----

\

y ( mm

Displacement

)

O~--+----'-----r----------,----l------.--------

15

5

Figure 5. Comparison between two simulated force versus displacement curves for both the head and foot of a short nail. To (kN)

Load at head

1

To

Yo = z'Es-'

e ~ ,x '/

250

00-0--0

I

."..0 ............

j'k==y

'/ 1/

,;0

/,;.

/.

2

/

/0 o

150

/

I I

cf )'' "

/

I-

I I

1

./

/

I I I

I 1

/

I

f

/

1

o

10

/

I

Steel angle nails: 70mm x 70mm x 7 mm

" 1

I/

;:.

o

-0---0-

1

Displacement Ye at tip

of nail

I 1

Displacement yo at head of nail

Displacement y (mm)

o

5

101

15

20

25

1 I

YZ

18.8

Figure 6. Comparison between two simulated force versus displacement curves for both head and tip of a long nail.

202

C HAP T E R

5

_

WALL STRUCTURES CONSTRUCTION

1.

GENERAL PROVISIONS

1.1. General Aspects -

Principle governing the construction of the structures

The construction of a soil nailed structure appears as a succession of earthworks, between which operations for inserting reinforcement bars in the in situ soil and installing a reinforced concrete facing - generally shotcrete - takes place. With current technology, the inclusions are generally made of metal and installed by driving straight into the ground or inside a borehole. In the latter case, they are always sealed to the ground by means of cement grout or mortar. Earthworks operations are carried out in phases (generally horizontally) of sufficiently low height to ensure general and local stability of the soil mass in conditions acceptable for the structure itself and for its environment (figure 1). The need for permanently ensuring local stability, especially after each earthworks phase, means that the use of the technique is, in principle, only applicable to soils with sufficient cohesion - at least in the short-term - and without the presence of a water table (eventually after lowering the water table, if possible). Attention must be drawn to the fact that the soil nailing technique discussed here is passive nailing, even if, for various reasons (to ensure the stability of the facing before starting earthworks, for example), the inclusions can be pressed against the facing with some tension. The association of prestressed anchors with soil nailing is not excluded and is a commonly used technique, especially when environmental constraints impose a fairly strict limit on the displacements of the structure. However, the installation of this type of anchor, which is extensively described in the Recornrnandations TA 86 on ground anchors, will not be discussed below.

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Soil Nailing Recommendations-1991

Part of structure completed

/

Adjoining excavation

/

/ Genera I excavation

----

/

./

/

/

Figure 1. Excavation and stability.

1.2. Earthworks As work progresses, it is imperative that the conditions of construction of the structures, as set up and taken into account in the justifications by the consulting engineer, be strictly observed. It is essential that good coordination exists during construction; this coordination must also apply to general earthwork operations, or at least part of them, since they can influence structural stability. It is therefore recommended that earthwork operations, including a certain part of the general earthworks, while not carried out by the company responsible for the construction of the nailed structure, must be under its direct control and guidance.

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Chapter 5: Wall Structures Construction

1.3. Choice of an installation method for the inclusions The techniques most generally used for installing the inclusions in the ground are driving and drilling. There are, however, some special methods that have not yet been widely developed, but in any case, the rate at which nailing techniques are being developed does not exclude possible improvements to existing techniques, even the introduction of new methods in this field. Generally, the selection of the method of installation is practically one of design data. Frequently, this selection is imposed by certain constraints linked to the nature and characteristics of the ground or to certain requirements relating to the structure itself (geometric characteristics, short-, medium- or long-term nature of the structure and length/characteristics of the inclusions). In other cases it is not uncommon for the design of the structure to be the result of the selection of the technique beforehand. •

Driving

Normally, when driving is used to install the inclusions, the latter are not secured by grout injection, the pull-out strength of the inclusions being obtained by adhesion of the inclusions to the ground. However, some special techniques do allow the sealing of an inclusion installed by driving to be carried out with cement, grout, or mortar. Driving is a technique that can be particularly effective when used in loose, noncohesive soil that contains neither hard obstacles nor too many large blocks. Theoretically, driving is best adapted to small- or medium-sized inclusions (not exceeding approximately 8 meters) mainly because of the space taken up by the equipment when the metal device is in one piece, as often is the case, and because of the power of the driving equipment with regard to the rigidity of the inclusions. In any case, it is recommended that the latter be correctly guided during driving. The main disadvantage of driving exists in its limited uses. Its advantages, when used judicially, are speed of installation, flexibility of use, and, especially, that the efficiency of the inclusion is immediate when pull-out strength is obtained directly from adhesion of the inclusion to the ground (no securing with grout is required), as is often the case. •

Drilling

When the inclusions are installed in a borehole, they are always sealed to the ground, generally by means of a cement grout or mortar. The main advantages of drilling are the wide field of use of the technique, which covers practically any type of ground as long as the means used are well-adapted. The possibility of always obtaining the required length and, very often, of obtaining the pull-out resistance sought, using, if necessary, sealing injection under pressure.

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Soil Nailing Recommendations-1991

Among the disadvantages of the technique are the necessity of very often having to adapt the drilling technique as a function of variation in the nature and characteristics of the ground, and the fact that this can make the technique somewhat cumbersome. The pull-out strength of the inclusions sealed with cement grout can be fully mobilized only when the grout has reached a certain strength. This can be awkward, particularly in fairly loose soils or for small sites (slowing down the rate of progress of the earthworks). Special sealing conditions can partially remedy this disadvantage. Because of construction requirements (drilling and injection of the sealing grout), inclusions installed in a borehole are inclined on the horizontal (toward the bottom). As a general rule, this inclination must not be smaller than approximately 100. If the inclination of the bars is smaller than this value, particular care must be taken when carrying out sealing operations.

1.4. Selection of the method to be used to lay the concrete of the facing As a general rule, facings of soil nailed structures are made of reinforced concrete: The concrete can be cast-in-place or it can be sprayed with a gun (shotcrete). In some cases, precast concrete or metal elements have been used, but they are very special applications that have not been really developed. Cast-in-place concrete can be suitable when no short-term stability problems exist (ground of sufficient cohesion or the previous placing of a shotcrete protective curtain, for example). This placing technique is adapted when a large amount of reinforcement is used or when the facing must look like shuttered concrete. Generally, this technique is carried out using excavations in slots. Shotcrete is well-adapted to the nailing technique; it is flexible and permits rapid "protection" of the facing built by earthworks. Initially, only dry spraying was recommended for direct application in situ (see AFTES Recommandations). Recent experience has shown that wet shotcrete can be equally effective. However, dry shotcrete is more flexible to use; it allows more immediate use when it is needed, for instance, when local collapse of the ground is likely to evolve rapidly.

1.5. Resources used -

Equipment

One of the main conditions for the successful construction of soil nailed structures is the ability of the construction company to respond quickly to eventual adaptations of the design caused by risks linked to construction conditions of the structures in ground where the heterogeneity has not always been recognized. This means the use of qualified and experienced staff, who are capable of evaluating certain foreseeable risks and of reacting promptly and efficiently in case of need. It also means the use of well-adapted equipment capable of intervening rapidly (partial refilling, rapid spraying of concrete, pumping) or the possibility of eventually adapting working conditions

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Chapter 5: Wall Structures Construction

if the need arises, e.g., change in drilling technique or even method of inserting the inclusions. For fairly large structures, or in the case of highly heterogeneous ground, it is recommended that the possibility of changing the technique of installation of the inclusions be allowed during construction and, in particular, the ability to switch to drilling when driving had been planned. Such an arrangement must be foreseen from the moment construction work is planned (see paragraph 1.7. of this chapter) insofar as late improvisation would have serious consequences, at least on completion time.

1.6. Controls -

Monitoring deformations of the structures

The construction of soil nailed structures - be they short-, medium-, or long-term - is accompanied by controls that, in practice, concern nearly all construction stages: Controls are therefore needed before work begins to check that the specifications concerning materials and products have been respected. Control of the conformity of the methods and conditions of construction decided upon to obtain the required results, specially for grout, concrete, and pull-out strength of the inclusions. Controls during construction address the nature and characteristics of the ground encountered (earthworks, drilling or driving investigations), the quality of the grout and concrete applied, pull-out strength of the inclusions, and respect of tolerances. The behavior of soil nailed walls is also controlled in all cases during construction and service (particularly the monitoring of the displacements and deformations of the wall and, eventually, of the tension in some of the inclusions). The nature of these controls, their consistency and the frequency with which they must be carried out, are defined in chapters 4 and 7. However, it is necessary to recall that provisions relating to these controls must be defined before work begins and stated in the specifications or in quality assurance plans.

1.7. Program for the construction of the structures For the reasons already mentioned, and because of the special constraints imposed by the conditions of laying the materials used in the structures to structural stability during construction, the design and installation of the latter are closely linked. It is therefore important for conditions of construction to be set up in close cooperation with the consulting engineer at the same time as the construction drawings are prepared.

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Soil Nailing Recommendations-1991

For short-, medium-, and long-term structures, it is important to establish a program and method statement for the work before the installation begins. Precise data should be given on: The location and the conditions of installation of the inclusions for preliminary tests or conformity tests and technology of these inclusions. The selection of the method(s) of installing the inclusions as a function of the nature and characteristics of the ground concerned. General conditions of construction of the structure and, in particular, different phases of earthworks and the succession of operations carried out at each of these phases. Particular conditions of installation (reinforcement, concrete, phases planned, special zones) and methods envisioned as a function of the constructional provisions appearing on the plans showing the conduct of the work. Controls (resistance of inclusions, sealing grout, concrete), special recordings and measurements (monitoring of the movements of the wall), their frequencies, construction techniques, and names of the people (and eventually organizations) responsible for these different tasks (see paragraph 1.6. of this chapter). Names and qualifications of the managerial staff and construction staff allocated to these different tasks, and the means planned for the execution of the work.

2.

CONDUCT OF THE WORK

2.1. Earthworks .It is imperative to strictly observe the conditions laid out for the conduct of the earthworks

(phasing, height of earthworks at each phase, length of sections, possible delay before the start of new earthworks phase), and the way in which they were planned and taken into account in the justifications provided by the consulting engineer. They should only be modified with the agreement of the consulting engineer who will ensure beforehand their relevance to the stability of the structure and define, if necessary, the provisions to be adopted to ensure that this stability fulfills the normal conditions of safety. Experience has shown that these provisions concern both earthworks contiguous to the structure and general earthworks, the influence of which on the stability of the mass often appears less obvious. 2.1.1. Height of earthworks

Earthworks contiguous to the structure are carried out in phases. The selection of the maximum height of earthworks at each phase is dependent on the justifications during the

208

Chapter 5: Wall Structures Construction

phases of the work and on the evaluation of the local stability of the ground. It depends on many factors, especially on the nature and characteristics of the ground, on the more or less thorough knowledge of the latter, on the eventual presence of water (preliminary site investigations, similar work already completed on site or in the vicinity in the same type of formation, previous general earthworks) on environmental conditions, and on the inclination of the facing. At present there exists no general rule for evaluating the maximum height of earthworks. As an example is the soil nailed wall of the CEBTP experiment No.1 (French National Project CLOUTERRE 1986) where earthworks/1m high, in Fontainebleau sand, were stable with an apparent cohesion in the order of 4 kPa. Height must be adapted as a function of the risks of local collapse (figure 2). Thus, in terrain subject to collapse (loose granular soil without apparent cohesion), it could be necessary to use another technique. However, some particular provisions can limit these risks of local collapse (see below). It is recommended that the height of the initial phases of earthworks be reduced (in

particular of the first) if eventual difficulties have not been properly evaluated at the beginning of the work, as it is often difficult to react to them quickly. It is also recommended to lay at least one row of inclusions during each works phase.

Height of excavation

Loca Iized failurG

_ and its possible development over time

Figure 2. Height of earthworks and local stability.

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Soil Nailing Recommendations-1991

2.1.2. Special provisions

Certain special provisions can offer a solution to problems of stability of the mass or local stability:

•

Construction of the structure in small continuous or alternate slots (photograph of figure 3).

Such provision, more especially adapted to the solution of structural stability problems during construction, is generally decided upon before work begins, but it is constraining as it does not allow a rapid rate of progress and necessitates very good organization of the site. However, in good quality ground, it makes it possible to plan greater height of earthworks (figure 3).

Figure 3. Building a soil nailed wall by alternating excavation sections.

•

Installation of the inclusions before earthworks operations (a berm must be reserved)

The inclusions can be either inclusions already decided upon or additional inclusions, generally of shorter length. Such an arrangement enables the local behavior of the soil to be strengthened and reduces the time between earthworks and the placing of concrete. It can only prove really effective if the inclusions are placed sufficiently close to each other, and it appears better adapted and easier to install if the inclusions are driven directly into the soil (figure 4).

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Chapter 5: Wall Structures Construction

•

Spreading a thin protective concrete layer immediately after earthworks are completed

The goal is mainly to limit the risks of local collapse by reducing decompaction and weathering of the ground, which is at the origin of this collapse (figure 4).

Protective berm

Protective shotcrete 1f"";;;;~~~~~;;;;;;;;;;::,. ski n -----
-Figure 4. Examples of construction provisions to limit local failures.

When the wall is comprised of angles, the latter are generally built during the same phase when they are "reentrant" and, for reasons of stability, in two phases when they are salient (figure 5). When the already-constructed part of a wall is comprised of prestressed ground anchors, as is often the case in the vicinity of existing structures, there is reason to examine whether special provisions must be made to prevent the action of these anchors - notably that of the vertical forces they induce - from having a harmful influence during the construction of the underlying parts of the structure. 2.1.3. Construction time It is imperative to respect the time specified between the end of construction of one part of

the structure and the succeeding earthworks phase. Indeed, the aim at this time is to enable the parts of the structure already built (facing, inclusions) to reach the strength taken into account in the justifications relating to the earthworks phase. This provision concerns more particularly the concrete of the facing (strength of facing and of the inclusion/ facing bond). When it applies to the sealing ground of the inclusions, (pull-out strength of the reinforcement in its grout), it is necessary to have determined beforehand by means of conformity tests and controls during construction - that both concrete and grout will be able to reach the strength required at this phase of construction.

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Soil Nailing Recommendations-1991

• •

• 1\

• • • • • •

• •

Re-entrant" angle

Figure 5. Construction of angles areas.

As a general rule, it is necessary to allow a minimum of 24 hours at least for a grout, mortar, or concrete to reach a certain strength. Even if the overall stability of the mass is assured, it is not recommended that a new phase of earthworks adjacent to the facing be started if the corresponding part of the structure cannot be completed in the same day (installation of the inclusions, reinforcement and concreting of the facing). The aim of such a provision is to prevent a local collapse. In the event of local collapse, if remedial measures are not taken rapidly, the development of far more serious disorders, such as collapse behind the overlying concrete layer, can occur.

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Chapter 5: Wall Structures Construction

2.1.4. Anomalies -

Incidents

In case of an anomaly, notably if the nature or characteristics of the ground are significantly different from those envisioned as affecting structural design, or if conditions of construction have to be modified, the consulting engineer must be warned so that new provisions to be adopted are determined. In case of incidents (abnormal displacement of the wall or collapse), it is essential to take immediate measures to prevent disorders from getting worse.

2.2. Protection against percolation water As a general rule, earthworks must not be carried out below the level of a water table discovered during preliminary site investigation or during construction unless the water table has been efficiently lowered beforehand by pumping wells or deep drains.

Figure 6. Installation of weepholes.

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Soil Nailing Recommendations-1991

It is also advisable to protect the soil mass against runoff water that can migrate toward the

facing and infiltrate along the latter. This water can be collected and drained by gutters beyond the zones concerned. However, in spite of such provisions and even in the absence of a recognized watertable, the ground can be subject to water percolation (seepage water) sometimes inherent to building construction methods, such as wet drilling. In these cases, it is necessary to ensure the protection of the "slopes" against water percolation. Protection consists, generally, of positioning subhorizontal drains to collect and drain this water. To prevent eventual pressure from being applied to the still fresh concrete of the facing, it is also necessary to plan the installation of weepholes (figure 6). This arrangement can be completed with the laying of additional drainage systems at the ground/concrete interface. However, it is necessary to ensure that the presence of these systems does not harm the quality of the facing concrete, even if they are correctly fixed in the ground. In any case, it is recommended that these systems, if they are planned, should consist of discontinuous strips or elements.

2.3. Installation of inclusions They are generally positioned after the corresponding earthworks phase. However, the order in which these operations are being carried out can be modified (see paragraph 2.1.2. of this chapter). When this is the case, it is imperative that the final earthwork phase (trimming of the slope or face) be carried out manually in the vicinity of the inclusions to prevent any damage. Generally, the procedure of the different operations necessary for the installation of the inclusions is determined at the end of the reference tests (preliminary or conformity tests), which are carried out before the beginning of the work or at the very beginning of the work. During the course of construction, any change in these conditions that can affect the behavior of the inclusions (in particular, their pull-out resistance) will make it necessary to carry out additional tests to evaluate the influence of these modifications on the behavior of the inclusions and, if necessary, to determine the new provisions to be adopted. NOTE: Many types of inclusion exist that differ in character and/or installation method. The

most commonly used are inclusions directly driven into the soil, including a steel member and inclusions installed in a borehole and sealed in the ground by means of cement grout or mortar, incorporating a high-adhesion reinforcement as used in reinforced concrete or metal tubes. The following provisions concern only these two types. If inclusions differ in nature and characteristics from their constitutive materials (glassfiber, for example), their technology (protective sheaths, separators, injection tubes), or their mode of installation (method of inserting the inclusion in the soil, sealing method), it will be necessary to refer to the rules governing them or to define these rules before work starts in agreement with the consulting

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engineer and control organizations. In the latter case, preliminary and conformity tests are essential. 2.3.1. Inclusions directly driven into the soil 2.3.1.1.

Constitution of the inclusions

Generally, metal inclusions are made in one piece. If they are not, provisions for joining the separate elements are defined by the consulting engineer and are carried out according to accepted practice. These provisions must ensure the mechanical and geometric continuity (straightness among other things) of the inclusions and must not affect the adhesion to the soil. Inclusions driven directly into the soil, must be sufficiently rigid with regard to their length, the nature and state of compaction of the soil and the power of the driving tools used to avoid buckling. The risk of deviation must be limited to allow the inclusion a good penetration capacity. 2.3.1.2.

Installation (figure 7)

The inclusions are guided while being driven, which ensures more efficient driving while limiting risks of deviation or buckling of the inclusions. It is recommended that the driving time for each inclusion be recorded, as a sudden drop in

time could indicate the presence of zones with weaker characteristics than anticipated. If such an anomaly occurs (which can show itself by insufficient adhesion between inclusion and soil) and is very localized, it is generally possible to apply a simple remedy, for example, driving an additional inclusion in the immediate vicinity of the one whose pull-out resistance is presumed to be insufficient. If the anomaly concerns a large zone, the pull-out strength of the inclusions must be checked and, if an insufficient value is obtained, new provisions should be adopted but must be determined jointly with the consulting engineer (the simplest provision being to increase the density of the inclusions, i.e., their number). The same step can be taken in case of premature driving resistance (impossibility of driving the inclusions or deviation of the latter on hard obstacles) due, for example, to the presence of hard natural or added obstacles or to the presence of soil with a very high state of compaction. However, if the anomaly occurs over a very large area, it might be necessary to adopt a different method for the installation of the inclusions. In any case, one must strongly advise against pulling out an already driven inclusion. It is recommended to completely fill with grout or mortar an inclusion consisting of a tube with an open or obturated base.

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Figure 7. Placing a driven inclusion.

2.3.1.3.

Blocking the inclusion against (or in) the facing

The provisions adopted to ensure that the inclusions, consisting generally of steel members, are attached to the facing (or in the facing), are closely linked to the type of section used. These provisions must be correctly designed, justified, and carried out in strict observance of accepted practice. Furthermore, they must not hinder the correct placing of the concrete facing (risk of shadow effect if shotcrete is used; see paragraph 2.5. of this chapter). 2.3.2. Inclusion secured in a borehole 2.3.2.1.

Characteristic of the inclusions

Inclusions consist generally of high-adhesion bars for reinforced concrete (sometimes steel tubes) secured in the ground by cement grout or mortar. As a general rule, except for special provisions that must be justified, the fastening of the reinforcement to the facing is carried out with a nut screwed at the extremity of the nail and tightened against a metal support plate.

If necessary, the nail sections are joined end to end by means of coupling sleeves; the mechanical and geometric (straightness) continuity of the reinforcement must be ensured.

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The inclusions placed in a borehole must be equipped with centralizers equally spaced (at least one every 3 meters) and preferably nonmetallic for permanent structures and in sufficient numbers to ensure a uniform sealing. In the case of small diameter reinforcements not equipped with centralizers, the quality of the installation of the nails will have to be justified by additional pull-out tests, the number to be determined by the engineer. The nail can also be equipped with a device allowing injection "at the source," Le., injection at the tip of the borehole, generally carried out by means of a small tube attached to the bar when the cement grout or mortar is injected after positioning the nail in its borehole. 2.3.2.2.

Installation

The installation of a nail sealed in a borehole necessitates the following operations: Drilling of the hole. Positioning of the nail. Securing the nail in the borehole. As a general rule, nailing, as opposed to the installation of prestressed ground anchors, requires only subhorizontal boreholes of modest dimensions (length rarely exceeding 15 to 18 m and diameter normally between 60 and 120 mm) carried out where there is no water table (at least at the head of the borehole). The nail is generally simply secured in the ground using a gravity grout technique. However, in spite of the apparent simplicity of the installation of securing reinforcement in a borehole, this installation remains a delicate operation requiring qualified and experienced staff. The choice of the drilling methods and equipment to be used is mostly conditioned by the nature and characteristics of the ground in which drilling is carried out. It is also conditioned by the dimensions of the boreholes, by the rate of progress determined beforehand and by: Ensuring the satisfactory behavior of the hole until the bar is sealed. Limiting disturbances in the wall of the borehole. Creating no disruption in the ground in the vicinity of the structure or in the structure itself. The methods and equipment used must fulfill these requirements for all formations encountered. Drilling can be carried out by rotation, percussion, or rotopercussion. Rotary drilling can be executed with: •

A continuous drill auger (figure 8) in fine soil with low compaction containing no blocks and for short drilling lengths.

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•

A disintegration device (tricone, device fitted with blades, chisel-type tool) fixed at the end of a rod, when there is no risk of collapse of the wall of the borehole.

•

An open tube, when the risk of collapse or narrowing of the wall of the borehole makes it necessary to hold up the wall and in the absence of blocks.

Figure 8. Drilling using a continuous auger.

•

A tube or casing (figure 9) and a rod fitted with a tricone or chisel-type tool to destroy hard elements in the ground.

In most cases, drilling is carried out using a drilling fluid (air, water, mud, cement grout, or

foam) and the aim is to raise the sediments and, sometimes, ensure that the wall of the borehole is held up. The choice of drilling fluid is important insofar as the complementary requirements mentioned above depend greatly on this choice. As a general rule, however, and except in specific cases (rocky ground or collapse), wet drilling should be avoided. Percussion drilling alone is used, in principle, to drive casings with lost tips, in coarse ground of low compaction. In practice, it is hardly used but, it can prove extremely welladapted in the (rare) cases where the nature and characteristics of the ground allow it to be used. Rotary percussion enables rotation or percussion to be used separately or to be combined, if necessary. It is the method most often used and best adapted to heterogeneous or varied ground because it is so versatile.

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Figure 9. Drilling using a casing.

Once the borehole has been completed, the nail is positioned in it and grout or mortar is injected or poured, depending on the provisions established initially. These provisions make it possible to ensure the satisfactory behavior of the wall of the borehole and the continuous coating of the nail. When the nail is introduced after the grout has been poured, it must be lowered sufficiently slowly to prevent soil collapsing from the wall of the borehole (especially by the piston effect). In all cases, the grout or sealing mortar is injected "at the source," that is to say at the tip of the borehole, either using the stand of the drill pipe (or casing) or using a small tube (fitted to the nail before it is inserted into the borehole). The grout sealing the nail is generally cement based stabilized with a small quantity of bentonite and having, as a rule, a cement/water ratio, C/E between 1.7 and 2.4. The mortar sealing the nail is generally a fluid mortar without sedimentation (containing fine, poorly graded sand) with a cement/water ratio, C/E, generally between 1.7 and 2.4. In some cases, special grout or mortar can be used to comply with some particular characteristics of the ground (grout with additives or quick setting grout in open ground, for example). However, attention is drawn to the fact that it may be preferable to treat ground of that type with injection beforehand.

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The need for meeting requirements that are sometimes contradictory, such as having a grout sufficiently fluid to correctly coat the reinforcement and sufficiently stiff to prevent important losses (which could prove harmful as regards securing and coating of the reinforcement) can lead to the adoption of special provisions. Thus, in very open rocks (such as rock with large fractures, caving of slopes, or karst terrain), correct sealing of the nail can prove extremely difficult, even impossible, under normal conditions. One of the techniques sometimes used to remedy this situation is to position a very flexible sheath around the bar before inserting it in the borehole. Recall that, generally speaking, all the requirements for carrying out the different operations needed for installing the inclusions are defined before work starts (at the end of the preliminary tests or, eventually, when work starts). In case of modification in the procedure of the work, or if anomalies are observed during drilling (significant variations in some drilling parameters, important losses of drilling fluid) or during the placing of the grout, tests will have to be conducted to control the behavior of the inclusion. If that behavior is not acceptable, special provisions will have to be adopted in agreement with the consulting engineer. 2.3.2.3.

Attaching the inclusion against the facing

The provisions adopted to ensure the attaching of the inclusion against the facing (or in the facing) must be correctly designed, justified, and carried out with strict observance of accepted practice; but they must not hinder the correct installation of the facing concrete. Generally, except for special provisions, the device for blocking the inclusions consists simply of a nut screwed at the extremity of the nails resting on a metal plate. This plate must bear perfectly on the concrete facing. When shotcrete is used, it cannot be placed under the support plate in the presence of the latter (shadow effect).

If partial tension of the inclusion is being considered (if only to ensure that the facing is in contact with the ground), it is essential that the sealing grout and the facing concrete have the required strengths (see paragraph 2.1.3. of this chapter) and that provision be made to allow the separation of the facing reinforcement and the ground for the desired length. The most common provision consists of placing a watertight sheath closed at its base around the reinforcement (figure 10).

2.4. Installation of facing reinforcing bars In all cases, it is recommended to reinforce the concrete facing, even if the facing is only subject to low stresses and if the nails are closely placed. The reinforcement consists of round reinforcing bars or wire mesh generally positioned in layers parallel to the average plane of the facing. The general provisions relating to the reinforcement of reinforced concrete structures (regulation provisions and accepted practice) are applicable and completed or modified by the special provisions defined below.

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--- ---

--- --------- -Risk of the separation of facing from the soil

--

_

==t>-

Increase density of the nai Is (driven nails)

~ \\\~h

Protective sheath

--

bars

Partial pretensioning

Free leng til of the ba r

Figure 10. Blocking the nails against the facing

When shotcrete is used, any provision liable to cause a shadow effect and thus promote the formation of voids behind the reinforcement during spreading is to be avoided. The distance between two parallel bars must be equal to or greater than 10 cm and, as far as possible, one must avoid the installation of the bars in bunches. If two layers of reinforcements are planned, it is advisable to install them one after the other, the outer layer being placed only after the inner layer has been completely covered with concrete. The plates supporting the inclusions, as well as any other obstacle, must be positioned only after shotcrete has been spread (see paragraph 2.3. of this chapter). The bars must be held by sufficient packing and wedging to help keep the concrete in position when the latter is being spread without being susceptible to displacements or deformations exceeding the allowed tolerances. To this effect, the reinforcement is held together with tie wires or cross welding in principle at all crossing points. The elements thus formed are fixed rigidly both to the adjoining parts of the facing already built, which allows for some reinforcement to be left exposed in those

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parts, and to the wall of the excavation, which allows for a sufficient number of fixing systems. These fixing systems can consist of rigid sections of bar (round reinforcing bars or small metal sections) fixed beforehand on the wall of the excavation (by driving, for example). They can also consist of the metal bar of the inclusions, subject, however, to the fact that no tension, even partial, has been planned. In principle, such provision is therefore only possible when the inclusions are closely spaced metal inclusions. For long-term structures, the minimum cover of the reinforcement is 3 cm. The thickness is 2 cm when the outer wall of the facing is shuttered.

2.5. Placing the concrete of the facing The concrete of the facing can be cast-in-place behind a shutter (cast-in-place concrete) or shotcrete can be used. The selection of one of these techniques depends on many factors (see paragraph 1.4. of this chapter) and especially on the building conditions decided upon, on the thickness and reinforcement of the facing, and on the final appearance sought. In principle, this selection is made when plans for the execution of the work are set up as it can be closely linked with the building provisions planned. Shotcrete is the most commonly used solution for the installation of facings, mainly because of the flexibility of this technique. Concrete must be placed as quickly as possible after the ground has been laid bare. If these two operations cannot be completed in a single day, they should be postponed. Where no other solution is possible, and when the ground and environmental conditions allow, the placing of the concrete can be deferred as long as protection of the exposed face is ensured, for example, with a thin shotcrete layer. As a general rule, this protective layer is not taken into account in the strength of the facing.

In all cases, the general rules of concreting relating to the technique adopted will have to be respected, in particular as they regard cold weather concreting, curing of concrete, and resumption of concreting. Regarding resumption, note that it will be generally very difficult, and very often impossible, to ensure real resumption of concreting (mechanical continuity of the facing) at the level of the longitudinal and transverse joints between the different phases or sections because the surfaces become dirty and it is impossible to treat them correctly, Le., to clean and make them rough (photograph in figure 11). 2.5.1. Cast-in-place concrete

Because of the impossibility of subjecting concrete to vibration, it is recommended to give the latter sufficient workability to allow correct placing and satisfactory coating of the reinforcement. Furthermore, it is recommended that concreting openings (areas where the concrete is inserted by the shutter) be provided in sufficient number to allow a slight load of the concrete being placed against the hardened concrete of the upper part of the facing already built.

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Figure 11. Resumption of concreting in successive phases.

2.5.2. Shotcrete

General specifications for shotcrete are defined in the documents referenced in paragraph 1.1. of this chapter. Some essential rules are quoted below giving, where needed, some specific guidelines for the use of shotcrete in the construction of facings for soil nailed structures.

•

Qualification of the staff

One condition essential to the satisfactory conduct of the work is the use of qualified and experienced staff.

•

Conformity tests

As a general rule, controls must be carried out before work begins to ensure that the qualities required will be obtained under the building conditions set up (concrete mix design, shotcrete technique, equipment, spreading method). This is the object of the conformity tests (see paragraph 1.5.1.3., chapter 7). For long-term structures, particularly when conditions of work are difficult (great thickness, large amount of reinforcement), it is recommended to set up real test sections.

•

Installation of concrete

As a general rule, the spray lance is held perpendicular to the surface to be treated. However, slight inclinations can facilitate better coating of the reinforcement, particularly when the steel reinforcement is quite sizeable.

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In principle, the thickness required is obtained in several layers, the spraying of a new layer

taking place before the end of the setting of the previous layer. No preparation for resumption is then needed as long as the previous layer has not become dirty. When spraying takes place after the setting of the previous layer, it is only necessary to sprinkle the latter with water if its strength is not taken into account in the strength of the facing (a protective membrane, for example), but, if this is not the case, it must be treated to ensure good resumption of the concreting. At each layer, the progression of concrete takes place from bottom to top to limit the risks of the concrete being subject to slump. Concrete that has been dropped must not be reused.

2.6. Stability of the facing It is necessary to make provisions to avoid any risk of vertical displacement of the overlying

facing already built when starting on the following earthworks phase (figure 10). The seriousness of this risk depends mainly on some design factors (inclination of the facing, its thickness, type and number of inclusions) and construction factors (length of section, reinforcement, and overlapping of the reinforcement with that of the neighboring sections), on the characteristics of the ground in contact with the facing, and on the strength of the concrete of the facing when underlying earthworks start. With current technology, this risk is far less acute with inclusions directly driven into the ground, generally densely placed, with thinner facing than with drilled inclusions, which are much more widely spaced and require a thicker facing. Certain constructional provisions can limit this risk. In particular one could: Improve the adhesion of the facing to the ground, for example, by adding some short metal elements driven beforehand in the ground. Improve the "hooking up" of the facing element to adjoining elements by providing in the latter some reinforcement left exposed. Build the structure in "primary-secondary" slots. Build an additional horizontal one meter wide facing at the top of the wall. Partial pretensioning of the nails. The last of these provisions, well-adapted to drilled inclusions, demands that both concrete and grout have sufficient strength to stand the stresses applied and that the nails have a certain free length at their heads (see paragraph 2.3.2.).

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REFERENCES TA Recommandations 86. Ground Anchors Specifications 86. Recommandations AFTES, publiees dans Ie numero special d'avril1981 de la revue de l'AFTES Tunnels et ouvrages souterrains. Recommendations of Earthquake Engineering concerning tunnels and underground structures. Recommandations STRRES et AFB (September 1985). STRRES and AFB specifications. Fascicule 65 du CCTC (execution des ouvrages de genie civil en beton pre-constraint). Construction of Civil Engineering structure using reinforced or prestressed concrete.

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226

C HAP T E R

6

_

DURABILITY OF STRUCTURES

1.

INTRODUCTION AND AREA OF APPLICATION

The aim of this chapter is to explain the design and design rules that must be applied to soil nailed structures insofar as they affect the aging process of the materials used, Le., reinforcement (bar and grout) and facing. These recommendations only apply, of course, to structures whose service life and the environment in which they are situated are such that the phenomena of aging will have time to develop fully. Distinctions should also be made among: Short-term structures, whose service life is less than or equal to 18 months. Medium-term structures, whose service life is between 18 months and 30 years. Long-term structures, whose service life is between 30 and 100 years. These recommendations only apply to medium- or long-term structures since obviously the aging phenomena over an 18-month period will, in reality, be negligible, and short-term structures only need protection in particularly aggressive sites (see paragraph 4). However, where structures have been built using nails with prestressing steel strands for the nail reinforcement, the standards for ground anchors (in France, Recommandations TA 86) should be used for corrosion protection. Because the majority of soil nailed walls have been built using nonalloyed steel nails and a concrete facing, these recommendations apply principally to these two types of materials. It should also be noted that an experimental wall using fiberglass nails was built as part of

the Project CLOUTERRE, but the studies concentrated mainly on questions of soil nail friction and not on the structure's durability (French National Project CLOUTERRE, Scetauroute-SAPRR March 1988). Generally speaking, the aging process in the materials used can be taken into account in one of the following three ways:

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• Protection by providing additional thickness (passive) Here the phenomenon of aging is left to take its natural course. Sizing of the structure's nails is based on the estimated residual nail cross section at the end of the structure's service life.

•

Protection by coating the nail

Here attempts are made to slow the aging process by protecting the nail against attack from aggressive substances by coating them (galvanization, paint, etc.). This has the effect of delaying the aging process, although it is not able to fully overcome it. The nail's residual resistance will again be taken into account (this can be higher than the level provided by the previous case, although lower than for noncorroded materials).

•

Protection by sleeve (active)

The attempt here is to eliminate totally the aging process by surrounding the reinforcing material with other materials that play practically no mechanical role, but perform the role of barrier. The long-term resistance is the same as that of the original material, without reduction. NOTE: Cathodic protection has been excluded on technical grounds and because its installation is not compatible with soil nailing techniques.

2.

CLASSIFYING STRUCTURES ACCORDING TO THEIR PLANNED USE

With permanent structures, the purpose for which the structure is planned and the consequences of failure will influence the designer's choice of safety factor (will it be subjected to surcharges or harsh environmental conditions, such as water laden with de-icing salts, etc.). In practice, an index C is defined, and this is used to classify the structure. This classification will be the basis for designing protective measures (see paragraph 4.2.1. of this chapter). There are two main types of structure:

• Critical structures: Soil nailed walls that show at least one of the following features: Very high

(~

10 m).

Structure supporting heavy surcharges: graffic, railways, concentrated loads (pylons, etc.). Aggressive environments: polluted aquifer, structures under roadways treated with de-icing salts. Structures where excessive deformations or failure might lead to wide-scale damage: sensitive buildings and service networks nearby (located at a horizontal distance less than

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three times the height of the soil nailed structure; this distance includes a margin of safety), excavations running alongside heavily used traffic routes (freeways, major roads and railways). One chooses C ~ 2 for critical structures (2 is the minimum recommended value; the owner or client can use a higher value, but this will need to be shown in the construction specifications).

•

Standard structures: These are soil nailed walls that do not have any of the previously mentioned characteristics.

One chooses C

3.

= 0 for standard structures.

CLASSIFYING SOILS ACCORDING TO THEIR AGGRESSIVITY

3.1. Objective and area of application This section seeks to describe a method for assessing the likelihood of corrosion in soil nailed structures using nonalloyed steel bars that come into direct contact with the soil or that are likely to do so if the cement grouting should crack. The likelihood of these elements corroding not only depends on the properties of the metal materials used and the corrosive factors of the soil in question but also on the heterogeneity of the ground through which one single nail passes, plus external electrical influences. Given the wide range of factors influencing the corrosion phenomena, this document tries to present an appreciation of the probability of corrosion occurring, although where doubt exists, the matter should be referred to a specialist. This chapter also applies to structures built on soil sites, whether or not there is pure or brackish water present.

3.2. General principles Steel reinforcements in nails can, should they come into contact with soil and water, suffer two different types of corrosion. Uniform corrosion results in an average reduction in the thickness of the nails. Localized corrosion at specific points: this takes the form of pitting, sometimes deep, sometimes not, which can be altogether more severe than uniform corrosion.

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3.3. Evaluating the corrosiveness of the soil 3.3.1. Overall corrosiveness index

The table given below specifies the weighing ascribed to each of the four main assessment parameters used to evaluate the corrosiveness of soils. It was compiled on the findings from an analysis of buried metal culverts (reference CEFRACOR).

TABLE I. Overall corrosiveness index.

Criterion Type of Soil

Features • Texture heavy, plastic, sticky impermeable clayey-sand light, permeable, sandy, cohesionless soils

-

• Peat and bog/marshlands • Industrial waste - clinker, cinders, coal - builders' waste (plaster, bricks) • Polluted liquids - waste water, industrial - water containing de-icing salts

Weight A of Criterion

2 1 0 8 8 4

6 8 Resistivity

p < 1000 Q cm 1 000 < P < 2 000 2000
Moisture Content

Water table - brackish water (variable or permanent) Water table - pure water (variable or permanent) Above water table moist soil (water content> 20%) Above water table - dry soil (water content < 20%)

pH

<4 4il5 5il6 >6 Global index

230

5 3 2 0 8

4 2 0 4 3 2 0 Sum of above: LA

Chapter 6: Durability of Structures

The value of the weight of criterion "A" chosen in "Types of Soil," for example, would be the maximum value applicable to that soil from the subgroups "Texture," "Peat," "Industrial Waste," or "Polluted Liquids." The maximum weight for each of the four criteria is less or equal to eight. The corrosiveness of the soil is shown as a global index LA, which is obtained from adding together the weighing values set for each of the four evaluation criteria. 3.3.2. Electrolytic effect of electric currents: "stray" currents

Some electric currents, direct or alternating with very low frequencies (16.67 Hz = 50/3), circulate in the soil. These may come from industrial installations, neighboring cathodic protection installations, etc., and can result in corrosion caused by electrolytic action. This risk must be evaluated in a special study.

3.4.

Determining which characteristics will be used when assessing the corrosiveness of soils

3.4.1. Type of soil

This is first looked at from the point of view of the local geology. Examination will be limited to distinguishing between fine soils, typified by a high percentage of clay present (plastic, heavy, sticky or impermeable soils), and primarily granular soils that have few colloids and have weak cohesion, and are permeable and friable. As a first breakdown, "heavy" textured soils are fine grained soils classified by the LPC as having more than 50 percent by weight of the particles smaller than 80 rtm). "Light" textured or granular soils are those with less than 50 percent by weight of the particles smaller than 80 rtm.

If no identification tests have been carried out, one simple method for assessing the texture of a soil and its level of compaction is to manually form a sample of slightly damp soil into a small ball: Heavy textured soil will form a cohesive ball.

3.4.2. Resistivity

Resistivity measurements use the classical methods for measuring the global resistivity of the soil through a depth more or less equivalent to the distance between the electrodes (see appendix of this chapter). The inspection depth should correspond to the height of the soil nailed wall. Other measuring methods can also be used (for example, penetration using a probe, the magneto-acid method, etc.).

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3.5. Phreatic water table The presence of a permanent or variable phreatic water table affects the corrosion process because of its effect on the permeability to air, resistivity, etc. Geotechnical investigations (see chapter 4) will generally identify their presence. In addition, during the soil sampling operation, it will usually be possible to observe water infiltration into the boring after 10 or more minutes.

If there is no water table, the water content in the soil will be measured from the sample taken. The water content in a soil sample is found using the standard methods: Either in a laboratory using the oven dried moisture content test. Or, on site using alcohol drying.

3.6. pH The pH level in most natural soils is between 5 and 9. Natural soils, where the pH is lower than 6, will only be found in peats or siliceous soils from primary geological formation stages. Highly acid or highly alkaline pHs values are usually an indication of pollution originating from industrial processes (cinders, slag, clinker, industrial tipping, and miscellaneous waste, etc.). The pH measurement will be found in accordance with the test procedure shown in the appendix.

3.7. Interpreting the results On the basis of the overall corrosiveness index LA obtained from table I, soils can be classified as follows: TABLE II.

Index LA

Classification

Highly corrosive

I

9 to 12

Corrosive

II

5 to 8

Average corrosiveness

III

Slightly corrosive

IV

> 13

<4

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4. PROTECTION FOR METALLIC REINFORCEMENTS

4.1. Generalities The phenomena of corrosion, and thus the protective devices required to counter these, differ depending on the type of steel used ("standards," those with a high elastic limit, alloys, or steels that can be made passive). Nails are either installed by driving or by laying them inside a borehole and then anchoring them into the earth using a cement grout. This cement grout will be subjected to tensile stresses, so it is likely that some fine and regular cracking will occur. Since it is impossible to assess with any degree of accuracy the width of this cracking, and even though it might only be slight (in the order of tenths of millimeters), it cannot be proved absolutely that the cement grout will form a watertight barrier between the reinforcing bar and the soil. If cracking does occur, the reinforcing nail can come into contact with the electrolyte carried by the moisture in the soil. Concentration of the electrolyte should not, however, be feared, and by the same token, the cracking in the cement grout would not be able to generate any more severe corrosion phenomena than those resulting from the reinforcement coming into direct contact with the soil. The cracking of the cement grout does not, therefore, necessarily constitute an aggravating factor in terms of its triggering or encouraging corrosion phenomena. In conclusion, whatever the technological method used, the hypothesis will be formulated that any reinforcement that is not protected by some sort of covering will be in direct contact with the soil.

4.2. Standard steels (elastic limit

O"e :::;

SOD MPa)

Generally speaking, any metal part manufactured from standard steel that comes into contact with the soil corrodes in a more or less homogeneous manner. This manifests itself through a gradual and relatively uniform reduction in thickness of any exposed steel surface. The dissolved steel is replaced by the compact and more expansive ferric hydroxide. The tradition of using standard steel for elements to be installed in soil of any kind is widespread and dates back many years (piles, culverts and cables, sheet-piling, etc.). Laboratory studies, together with observations made on actual structures, have given us a good understanding of the corrosion phenomena that can affect these steels. It is therefore possible nowadays to make relatively reliable predictions for the behavior of

such steels when they are buried in soil, if we know the degree of aggressivity of the surrounding environment, the length of time they will be exposed, and what type of protection is being planned. Generally speaking, such predictions must deal with a loss of thickness equivalent to the thickness neutralized through corrosion. In fact, because these losses in thickness are nonuniform, the ratio "relative loss of resistance/relative loss of

233

Soil Nailing Recommendations-1991

thickness" is higher than 1. In practice, this coefficient is directly integrated into the loss of thickness values given below (which also include the average "geometric" loss of thickness and the effect of it being nonuniform). Protective systems can be classified into four categories: Thickness sacrificed to corrosion. Protection from plastic or steel sheathing. Galvanization. Nonmetallic protective coating. The first two methods are currently in use, but there has been little practical experience with the other two in soil nailing. 4.2.1. Thickness sacrificed to corrosion

This is the most simple and most widely used system of protection. The products of corrosion that appear over time form a protective coating between the steel and its immediate surroundings. This coating does not form a barrier around the steel in the mechanical sense; instead, by modifying the environment immediately surrounding the reinforcing bar, it changes the kinetics of the chemical reactions, which in turn manifests itself through a slowing down in the rate of corrosion. To determine the thickness of steel to be sacrificed to corrosion, the following steps will need to be followed: 1) Find the index C, which typifies the class of structure (see paragraph 2.). 2) Find the global corrosiveness index LA of the soil (see paragraph 3.). 3) In the light of the overall index I = LA + C, use the table below to find the additional thickness needed when sizing nonprotected steel in accordance with the required service life (total reduction of diameter or thickness, including both sides). TABLE III.

Overall Index I I Classification

Short-term ::::; 18 months

Medium-term 1.5 to ::::; 30 years

Long-term 30 to ::::; 100 years

::::; 4/IV

0

2mm

4mm

5 to 8/III

0

4mm

8mm

9 to 12/II

2mm

8mm

plastic sheath*

~

13/1

Protective plastic sheath must be provided*

•A metal casing is not recommended unless there are special reasons for using it.

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Chapter 6: Durability of Structures

4.2.2. Protection using plastic sheath

This system involves the installation of a casing made of some corrugated plastic material (polypropylene, polyethylene or similar, or, more rarely, steel). The annular space between the casing and the reinforcing bar is filled with cement grout or some other kind of sealant, which is nonaggressive to either the steel or the casing. This system is not recommended for driven nails because of the risks of damage when they are installed. This system, then, is used to prevent any contact between the soil and the reinforcement. The steel from which the reinforcement is made will not, therefore, be able to deteriorate. In order for this system to be fully efficient, it is important for the casing to be absolutely watertight and totally sealed at the base and up to the facing. Moreover, the casings should be capable of resisting the stresses to which they will be subjected. Corrugated plastic casings are generally used to make sure that the tensile stress to which the bar (steel) is subjected can be transferred. For further information see Recommandations sur les tirants d'ancrage - TA 86, (French recommendations on Ground Anchors). With this plastic sheath, which is strongly recommended for "corrosive" soils (class II), and vital for "highly corrosive" soils (class I), no thickness to be sacrificed to corrosion will be taken into account. NOTE: If metallic casings are used they must be at least equivalent to half the additional

thickness shown in table III. This is to ensure they will fulfill their protective role during the whole of the structure's life.

4.2.3

Galvanization

This type of protection is currently in low demand in soil nailed structures. The principle is as follows: In the case of galvanized steel, the corrosion by-products of zinc (zinc hydroxides in particular) initially form a protective screen. The zinc coating initially delays the appearance of any corrosion in the steel and, subsequently, slows its development once the zinc has been transformed to dry oxide. Zinc is more highly electronegative than steel. If steel is unprotected in places (as a result of accidental damage during handling or because of deterioration caused by corrosion), it forms an electrochemical battery and any adjacent zinc is "sacrificed" in order to protect the iron. As a result of the phenomenon of spontaneous cathodic protection, the zinc also assures some uniformity of corrosion. The thickness of the zinc must be sufficient (80 11m minimum) to guarantee efficient protection, but not too thick because it must adhere to the steel properly. Whatever the circumstances, the zinc coating must conform to French Standard NFA 91121 - Hot Galvanizing (Galvanization a chaud).

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Soil Nailing Recommendations-1991

However, zinc protection, albeit readily valid in principle, has not yet had the benefit of being widely used in the area of soil nailing. Also, it requires that the same type of steel be used throughout the structure, particularly when it comes to anchorage heads and connecting sleeves (thread). There is also serious risk of damage during installation, particularly for driven nails. As a result, given our present level of understanding and technology, the same additional thicknesses used for bare steel will be used here (see paragraph 4.2.1.). 4.2.4. Protection with nonmetallic coating

This involves a coating of paint, such as thick bituminous paints, traditional tar, or tar improved with resins, epoxy-resins, etc. These coatings form a screen that will give some protection to the steel but only for a limited period, generally about 10 years or so. Unlike galvanization, the by-products of the coatings, when breaking down, form no protective barrier likely to alter its electrolytic nature upon coming into contact with the reinforcing steel. Furthermore, this type of coating poses problems of adhesion and the threat of damage when the nails are installed. For all these reasons, current thinking is that where long-term structures are concerned, the efficiency of a painted coating will be nil (in practice, therefore, the same additional thickness values will be used as for bare steel - see paragraph 4.2.1. of this chapter).

4.3. High strength steels with a high elastic limit (a e > 500 MPa) This type of steel is characterized by its strong susceptibility to stress corrosion when put into tension. This mainly applies to steels whose elastic limit is higher than 900 MPa, and even at low working stress levels. In order to ensure the steel is kept free from this sort of problem in long-term structures, the only effective solution is to prevent any contact between the steel and the electrolyte. Therefore, protective casings are systematically used - see Recommandations TA 86 for the precautions to be taken.

4.4. Alloyed materials and passive steels Here we are concerned with metals such as stainless steel or aluminum alloys. When they come into contact with their environment, these materials become coated with a protective layer of oxides that prevent any attack on the base metal. However, when this protective layer is locally destroyed (either mechanically or chemically), an electrochemical battery is formed where the layer of oxide plays the role of the cathode and the base metal that of the anode (reverse phenomenon to that produced in the case of galvanized steel).

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Chapter 6: Durability of Structures

This battery then becomes the subject of intense current corrosion that, beginning at a one single pin size perforation, results in the rapid destruction of the base metal. If certain chemicals are present in the soil, particularly chlorides, the probability of this sort of damage occurring is heightened. The phenomenon of corrosion in these materials is difficult to forecast, therefore, it is impossible to keep it in check. These materials should not, therefore, be used unless they are 100 percent insulated from the electrolyte. In fact, lack of experience in the use of this type of material in soil nailed sites means that, for the moment, they have to be rejected altogether.

5. REINFORCEMENT PROTECTION USING "SYNTHETIC MATERIALS" Based on our current level of knowledge, several types of material are now being proposed as reinforcements for soil nailing: Fiberglass. Carbon fiber. Plastic materials. Kevlar (aramid). Generally speaking, lack of experience in the use of these materials means that greater care must be exercised. Given our current level of knowledge, we suggest they should be reserved for experiments and standard structures (as defined in paragraph 2 of this chapter). Under certain conditions, however, there can be situations where protection of the nails will be provided by plastic materials. The stipulations of the British Standard BSI DD 81-1982 allow for the use of plastic materials (polypropylene and polyethylene) for the protection of long-term ground anchors, provided the protective measures are carried out in the factory, and the use of UV-sensitive plastic, provided it includes carbon black.

6.

DURABILITY OF THE FACING

In long-term structures, the facing forms an integral part of the soil nailing process and its durability must be taken into account just as for the nail itself.

6.1. Concrete facing

• Shotcrete: see the recommendations on the application of shotcrete to underground structures, published in the AFTES Review (July 1982), as well as the technical regulations contained in BAEL 83.

237

Soil Nailing Recommendations-1991

In particular: The cements used must comply with AFNOR Standards NFP 15-300 to 305 - 311 - 350. It should be noted that pure Portland cements (CPA) are not appropriate for aggressive soils. Potential additives (particularly accelerated setting agents) must be compatible with the cement used, be free of chlorides and must not corrode the reinforcing bars. The position of the reinforcements (usually welded mesh) in the layer of shotcrete, as well as the subsequent cover of the reinforcements, must correspond to the recommendations and regulations referred to above. The minimum cover is 30 mm.

• Cast-in-place concrete - please see the standard regulations regarding reinforced concrete for the concrete itself as well as any reinforcings. In particular, the cement used should correspond to the same criteria as for shotcrete. A minimum cover of 20 mm will be provided. Attention is also drawn, however, to the problem of cold joints in the concrete which are always difficult to achieve correctly (see chapter 5) and where fissuring often occurs. This can encourage corrosion of the concrete reinforcements. The problem can be dealt with if special care is taken when these joints are matched (injection of finishing agent, etc.).

• Prefabricated concrete panels: the same regulations as above apply with the addition of the following: Minimum thickness must be respected (50 mm). Sufficient curing before installation to avoid the risk of splitting and/ or fissuring. Thermal expansion must be taken into account.

6.2. Facing with steel panels These panels can be made from standard steel, but provision must be made for the anticipated loss of thickness due to corrosion in keeping with the service life expected of the structure and the problems of thermal expansion (when joints are installed).

6.3. Facing with other materials With regard to durability, plastic materials pose problems with certain soils and cement-based materials with which the panels come into contact. The additional problem of aging brought about through atmospheric conditions must be considered with certain plastic materials used as protective facing or lagging. Compared to the nails, laggings are subject to less aggressive conditions, especially mechanical stresses. Therefore, it could be possible to select materials having more common

238

Chapter 6: Durability of Structures

properties (plastic materials, wood, stone, etc.). However, the strength and durability of these materials must be justified with regard to the service life of the structure.

7.

PROTECTING THE NAIL HEADS AND AREAS THAT INTERFACE WITH THE FACING

7.1. Generalities The nail head is generally connected to the facing with a bearing plate. This area of the head, particularly the part located at the ground interface, is the area most exposed to corrosion. • In addition to the various stresses that have developed as the work progresses, numerous other stresses of varying intensity might also exist, particularly: Stresses caused by variations in temperature and that, depending on the circumstances, can occur in cycles that last from several hours to several months. Stresses due to variations in the hydrometric conditions in the ground, assisted by thermic variations, and that cause "breathing" in the soil. Stresses on the facing caused by temporary hydrostatic pressure following heavy rainfall. Stresses due to the water in the soil expanding in icy conditions. • These stresses can cause the concrete around the nail to crack unless it is adequately reinforced and may break the bond of the anchoring grout in the vicinity of the head. If this cracking becomes worse, the steel will be subjected to direct attacks of corrosion because it is being alternately exposed to air and water. This water might even be loaded with some sort of de-icing salts, fertilizers, industrial waste, etc.

• However, beyond a certain distance from the face, the impact of these forces is reduced, and standard protective measures of cement grout and/or additional thickness of steel will allow normal functioning to continue. This distance varies depending on the type of ground; it can be estimated as 0.30 m to 0.50 m beyond the point where facing and ground interface. On the other side of this interface, the head of the nail is usually embedded in either shotcrete or cast-in-place concrete. It is therefore safe from corrosion as long as the thickness of the covering conforms with the recommended standards. However, in the case where the head is not embedded (i.e., when using prefabricated facing) it must be protected using a cap filled with cement grout (C/E = 2.4 to 2.6) or a mortar containing 500 kg/m3 of cement, or one of the products given in the P2 protection chart contained in the Recommandations TA 86 (figure 1).

239

Soil Nailing Recommendations-1991

7.2. Types of protection • Protection involves reinforcing the area around the nail heads to prevent cracking that will otherwise occur from loading conditions previously mentioned. It also involves preventing the cracks from being in contact with the outside atmosphere. The use of a plastic or metal "trumpet" around the nail's structural element allows for this protection. The precautions to be taken for the installation of both the "trumpet" and the support plate are described in the diagram given in figure 1. •

For nail heads embedded in concrete, the following important points must be followed: The head must be structurally designed and arranged in the facing so that it transfers stresses to the concrete but does not cause it to crack either on the soil interface or the exterior face. The "trumpet" (0.30 m minimum in length) must fit solidly with the head. The whole layout - head, "trumpet" and reinforcing of the facing - should be such that the concreting can be properly carried out (shotcrete or cast-in-place concrete). Provisions must be made for filler and vent tubes for filling between the "trumpet" and the nail, as well as between the "trumpet" and the ground; particular attention should be paid to the area just beyond the "trumpet," which must be fully filled.

240

Chapter 6: Durability of Structures

A B

C

o

1

2 3 4 5 6 7

Concrete failure must be calculated according to the BAEL 83 code. The connecting zone between the nail head and the trumpet must be perfectly sealed. A structural connection between the trumpet and the nail head is a good method. This area becomes a dangerous zone if it is poorly sealed. It is necessary to make sure the seal is formed by filling with grout. This zone must be filled because it is the assurance that the area C has been filled.

Bearing plate set so that the dimensions hand e allow for the proper distribution of the loads without spalling. Nail: Generally a ribbed bar or other steel section with high adherence to grout. Structural concrete facing : shotcrete or eastin-place. Steel or plastic trumpet. Grout or mortar filling the trumpet. Grout or mortar filling the boring. Reinforcing steel (mesh). Figure 1. Important details in the "head-nail-facing" layout.

241

Soil Nailing Recommendations-1991

Venting --=tube

PERMEABLE

GROUND

Injection tube ep 16 x 12, split 20cm at its base

a. Protection provided by additional thickness.

Venting tube IMPERMEABLE

GROUND

in corrugated

%/ 'W//.'

; Device for connecting head with corrugated I ~sheath, welded at the head and filled with mortar after installation

b. Protection using a plastic sheath.

242

Chapter 6: Durability of Structures

:~!"Sealant (polyurethane foam)mortar

Paper plug

ep 75/80

PVC tube

Cap---;

~3~~~==:~~~~~~~~~~~

Structural plate ----,---r (load distribution)

Cement grout ~~

",'-.•. j

Stuck in place using epoxy resine

/

/

/

1----

'I

//--_ 1-/ ·~~~~i 1/-

Protective paint

\/0\\.

•

/'. 0

I

Anchoring grout Injector tube (10/14)

/ /';(L

4~~-·--·J:~:I

Prefabricated plate

c. Prefabricated panels. Figure 2. Examples of nail head corrosion protection.

If there are any drainage features near the nail heads, the "trumpet" will be sealed into the ground using a non penetrating product or an insulating system of the injectable separator type. The figures shown in 2a, 2b, and 2c, are examples of the various types of protection the nail heads can be given (embedded or not, shotcrete or cast-in-place, prefabricated panels, etc.). •

Where nail heads are not embedded, the Recommandations TA 86 (P2: Protection) will be followed with the possibility of filling using cement grout.

243

Soil Nailing Recommendations-1991

8. MONITORING OF THE CORROSION PROCESS

The recommendations given in this chapter, have been developed with the clear understanding that more long-term monitoring is needed. The recommendations apply to any structure whose service life is anticipated in terms of several tens of years, and sometimes deal with phenomena whose progress is not always fully understood. This monitoring entails a periodic inspection of the reinforcing bar samples for corrosion control at regular intervals. This will be done by sample reinforcing elements, installed during construction, being extracted at regular intervals to ensure that any subsequent corrosion does not call into doubt the hypotheses on which its structural design was based. The practical considerations involved (installation procedures, number, timetable, and tests to be carried out), concerning these bar samples are described in paragraph 4.2.3.4. of chapter 7.

244

Chapter 6: Durability of Structures

BIBLIOGRAPHY AFNOR (1990): Corrosion pour les sols. Evaluation de la corrosivite: Canalisations enterrees en materiaux ferreux non et peu allies. Norme A05250. Corrosion for soils. Evaluation of corrosivity: buried metal pipes - steel and some alloys. Evaluation de la corrosivite: Ouvrages en acier enterres (palplanches et pieux). Norme A05251. Evaluation of corrosivity: buried steel structures (sheet piles and other pipes). Acier galvanise ou non mis au contact de materiaux naturels de remblais (sols). Norme A05252. Bare or galvanized steel in contact with natural fill materials (soils). BUREAU SECURITAS (1986). Recommandations concernant la conception, Ie calcul, l'execution et Ie contrale des tirants d'ancrage (TA 86), Ed. Eyrolles, 1986. Recommendations for design, construction, and control of ground anchors. DARBIN, M., JAILLOUX, J.M., and MONTUELLE, J. (1979). Experiences et recherches concernant la durabilite des armatures de Terre Armee. Bulletin de Liaison des Laboratoires des Ponts et Chaussees No.99, Janvier/Fevrier 1979. Experiences and research on the durability of Reinforced Earth reinforcements. LCPC - SETRA (1979). Ouvrages en Terre Armee - Recommandations et Regles de l'art, Septembre. Reinforced earth structures, recommendations and state of the art. NEVEUX, M. (1968). La corrosion des conduites d'eau et de gaz, Ed. Eyrolles. Corrosion of water and gas pipes. RAHARINAIVO, A. (1985). La durabilite des materiaux pour Ie renforcement des sols. Congres europeen de corrosion, Nice, Novembre 1985. Durability of materials used for soil reinforcement ROMANOFF, N. (1957). Underground corrosion. National Bureau of Standards, 579 USA/1957.

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Soil Nailing Recommendations-1991

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Chapter 6: Durability of Structures

APPENDIX

SPECIFIC TEST METHODS DETERMINING THE pH VALUE OF A SOIL SAMPLE Mix the soil sample with distilled water in the ratio of one part soil to two parts water (by weight). Soils in which the water content is high (> 40%), muds or sludge for example, are analyzed in the condition in which they are found. All the soils must have a fluid consistency. Standard equipment is used to determine the pH level - see the following standards for details of sample taking and measuring: NF T 01-012: pH metrie - Solutions etalon pour l'etalonnage d'un pH metre. Measuring pH levels - standard solutions to be used when calibrating a pH. NF T 01-013: pH metrie - Mesure electrochimique au moyen d'une electrode de verre. Vocabulaire et methode de mesure. Measuring pH levels - electrochemical measurements using a glass electrode. Glossary and measuring methodology.

DETERMINING THE MOISTURE CONTENT OF A SOIL SAMPLE •

In a laboratory

Weigh out about five grams of the soil sample in a porcelain crucible or refractory dish. Heat in an oven for two hours at 105°C. Leave to cool in a desiccator, then weigh and reheat to 105°C for an hour.

247

Soil Nailing Recommendations-1991

Continue the drying process until a constant weight is achieved. The moisture content is calculated as follows: p -p

Moisture content

Po

= weight of sample before drying (g)

P

= constant weight after drying (g)

•

On site

=

100

_ 0_

p

Weigh out about 100 g of the soil sample, which should be crumbled into a metal receptacle. Sprinkle the sample with alcohol (about 20 cm3) and ignite the mixture. Reweigh the sample after the alcohol has burned. The approximate moisture content is calculated in the same way as in the laboratory test:

P - P Moisture content = 100 _o-=-_ P Po

= weight of sample before drying (g)

P

= constant weight after drying (g)

ON-SITE EVALUATION OF THE RESISTIVITY OF A SOIL •

The Wenner method with four probes (see figure 1)

This measurement is taken from the surface of the soil. Four metal electrodes are arranged in a straight line at equidistant intervals (length 50 cm, diameter 1 cm). These are then connected by conductors to a quadripolar resistance bridge (alternating current is used).

-

I

a

a

a

Figure 1. Wenner method of measuring resistivity of the soil.

248

Chapter 6: Durability of Structures

The depth of soil encompassed in the measurement roughly corresponds to the distances between the electrodes "a". The apparent specific resistance of the soil can be calculated using the formula:

p =21taR

where R is the resistance measured between the two middle electrodes.

•

Cane probe method (see figure 2)

This method involves pushing into the ground a probe fitted with either two or four electrodes and linked to an alternating current resistance bridge (Kohlrausch, etc.). The method tests only a small volume of soil in the vicinity of the probe tip. For more details, see Neveux, 1968.

Sounding probe

Koh Irausch bridge Ewphone

o

Figure 2. Cane probe for measuring soil resistivity.

DETERMINING THE RESISTIVITY OF A SOIL SAMPLE -

ON-SITE TEST

The retrieved soil sample is cleaned of any gravel or stones it might contain. Resistivity is measured in a specific cell (with two or four electrodes). The naturally moist soil is densely compacted into the cell to duplicate as closely as possible the in situ density of the soil. To determine the minimum potential ground resistivity, water without minerals is added in volumes of 10, 20, and 30 percent - and mixed with the soil sample. Make sure that proper homogeneity is achieved and that any soluble salts are put into the solution before resistivity level is measured. The minimum resistivity value found after the water has been added will be used when evaluating absolute corrosiveness levels from tables.

249

Soil Nailing Recommendations-1991

This resistivity value will have been corrected to reflect the effects of temperature as shown in the following ratio:

P (t)

where: t taken.

[1

+

x (to -

= the temperature of the soil (in degrees Celsius) to

x x

250

p (t) /

= = =

18°C 0.03 where t < 18°C 0.02 where t > 18°C

t) ]

at the time the measurement was

C HAP T E R

7

_

SPECIFICATIONS AND INSPECTIONS

1.

SPECIFICATIONS AND INSPECTIONS OF MATERIALS

INTRODUCTORY REMARKS Passive soil nailing is often associated with the use of prestressed ground anchors, particularly on urban sites, so that the magnitude of displacements in the structures can be limited. It is important to remember that the difference between passive nails and prestressed ground

anchors is not limited simply to the fact that the latter are put into tension. This is of particular importance because it is not uncommon, for a variety of reasons, for so-called passive nails to be tightened against the facing after they have been put into partial tension (or traction). The difference between passive nails and prestressed ground anchors extends well beyond this particular aspect of their installation. The technology involved, the nature and qualities of the steels used, the durability of the steels and the way corrosion protection is addressed, the field inspections during construction, and even justification for their use during design are all differences worthy of discussion. For prestressed ground anchors, these aspects are dealt with in the Recommandations TA 86 developed by SECURITAS, which should be consulted. Only the recommendations concerning passive soil nail inclusions are dealt with here.

1.1. Materials used for nail reinforcements 1.1.1. Metal reinforcements The metal reinforcements must be manufactured from the following types of steel: High adherence bars made from a steel included on the list of accepted or authorized bars compiled by the Acceptance and Inspection Committee for Reinforced Concrete

251

Soil Nailing Recommendations-1991

(Commission d'agrement et de contrale des armatures pour beton arme); the specified elastic limit must be lower than or equal to 500 MPa. Nonalloyed steels that are not the subject of an acceptance procedure (profiles or rolled merchant bars, rods, extruded bars, etc.) but that conform to the relevant French standards: "Shades and Qualities," "Dimensions and Tolerances" ("Nuances et Qualites," "Dimensions et Tolerances").· The specified elasticity limit must be lower than or equal to 400 MPa. Nonalloyed "soft" steels used in the oil industry. The use of high-strength steels (elastic limit higher than 500 MPa) for passive inclusions is not recommended mainly because of their low resistance to bending. It must also be remembered that because they are highly prone to stress corrosion, the use of

high strength steels, without exception, requires the same protection as prestressed ground anchors given in the Recommandations TA 86. The heads of the anchoring devices must be protected the same as prestressed bars, and the anchoring devices themselves (nuts, couplings, support plates, etc.) must be made of nonalloyed steel so that no galvanic corrosion cell is formed. In all cases, prestressing steel must come from the approved list prepared by the Inter-Ministerial Commission for Prestressing (Commission Interministerielle de la Precontrainte). NOTE: For the reasons explained in chapter 6, passive and alloy steels must not be used in

soil nailing. 1.1.2. Nonmetal bars

As a general rule, bars are made of metal; however, in certain cases where special requirements need to be met, other materials have sometimes been used. Fiberglass is a case in point when it was used as a short-term measure to facilitate the subsequent destruction of the reinforcements. Materials other than steel are not used except in short-term soil nailed structures or as part of an experiment. This is because our current level of understanding into the way these materials work is still not sufficiently advanced, particularly their long-term behavior and durability. For the reasons given above, and until such time as we have more sophisticated understanding of these materials, only metal inclusions (Le., steel) will figure in the specifications.

252

Chapter 7: Specification and Inspections

1.2. Procedures and materials for protecting nail bars against corrosion 1.2.1. Protection for standard steels

See recommendations contained in chapter 6. 1.2.2. Protection for prestressing steels

Types of protection and the conditions for implementing them must conform with the Recommandations TA 86 developed by SECURITAS (January 1986).

1.3. Grout used for securing the nails This paragraph deals only with cement-based grouts, which are the types used in the majority of cases. 1.3.1. Specifications for grout components 1.3.1.1.

Cements

Cements must conform to the stipulations of NFP 15-301 and the NF-VP lists, as issued by AFNOR. The type of cement used must be chosen in keeping with the aggressivity of the local environment (ground water), the type of structure (long-term or short-term) and the duration of the excavation phases. In aggressive settings (see chapter 6), the type of cement used must be chosen from the relevant COPLA lists. Other criteria might have a bearing on the type of cement chosen, such as its short-term resistance or how long it takes to set. If there are plans to use prestressing steel, the specifications contained in the Recommandations TA 86 relating to chlorine and sulphur levels must be respected:

Total chlorine Total sulphurs/sulphides 1.3.1.2.

~ ~

0.05% of the weight of the cement 0.15% of the weight of the cement

Water

Water in the grout must conform to standard NFP 18-303.

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Soil Nailing Recommendations-1991

1.3.1.3.

Specific ingredients and additives

Any additives used must conform to NFP 18-103 and NFP 18-331 to 338 standards, and be stamped with a mark signifying that they comply with the relevant French standards, otherwise they can be chosen from the CaPLA list of approved additives.

In the case of medium-term or long-term soil nailed walls, additives must not contain any substance that would prove harmful to steel. A compatibility study must be made if more than one additive is used. 1.3.2. Specifications relating to grout mix design

Grouts are mixtures of cement and water, possibly stabilized by a small quantity of bentonite. The cement/water ratio is generally between 1.5 and 2.2. The bentonite dosage should not exceed 10 to 15 kg/m3 • Special grouts can be used in fissured, karstic, or very porous ground in order to limit the quantities needed. These can be: Grouts with fillers (fine sands, fly ashes, etc.). Stiff grouts in which the set time is speeded up by the use of additives (sodium silicate is the most commonly used). Special grouts, etc. 1.3.3. Checking the quality of the grout 1.3.3.1.

Checking that the grout components conform

Care should be taken to ensure that the grout components comply with the stipulations of paragraph 1.3.1. and to the contract documents. 1.3.3.2.

Conformity and control tests

The usual tests check such things as the composition of the grout, uniformity of production, the conformity to the design, the mechanical resistance of the grout, etc. Some of these tests are carried out either before work starts on the building site (conformity tests and preliminary pull-out tests) or during the course of construction. The following factors are measured: Density. Viscosity.

254

Chapter 7: Specification and Inspections

-

Resistance (simple compression).

In most cases these tests will be the only ones required. However, in special circumstances, additional tests can be carried out (measuring the time needed for the grout to set, temperature measurement, test tube settling measurements and spin-drying using a filter press). It is not anticipated, however, that these additional tests will be systematically carried out. • Density Measurements Measurements are taken using either: BAROID scales with liquid grout, or Hydrostatic (submerged) weighing using hardened grout. These measurements can then be used to check the composition of the grout. Indeed, by looking at the volumic weights of: the cement the bentonite the water

Yc = 29.5kN/m3 (CLK) to 31 kN/m3 (CPA)

= 25.5 to 26.5 kN/m3 Yw = 10 kN/m3 Yb

and the respective proportional weights of the cement (C), the bentonite (B) and the water (E), the density of the grout, using the weight ratios C/E and B/C, are expressed using the formula:

1

C

+-

E

d 1

C

+-

E

B

+-

C

Yw

Yw +-

Yc

Yb

"-

1

B C /

• Viscosity Measurements Viscosity is traditionally measured using a Marsh flow cone with a 5 mm-diameter nozzle. The viscosity v is expressed in seconds and corresponds to the flow of a grout volume of 946 cm3 (standard API 13B). The initial viscosity of the grouting is generally regulated, whatever its composition, to ensure it is stable (only low sweating rate or none at all). Experience demonstrates that this result is obtained for values of around 50 seconds. In this range of values, the simple-to-use Marsh cone test is perfectly suitable.

255

Soil Nailing Recommendations-1991

However, with high viscosity levels, the values rapidly become meaningless (v > 80 seconds). Here, the modified Marsh cone - also called the LCPC cone with adjustable nozzle should be used.

• Simple compression resistance At the moment, there is no standardized method for this type of test; the sample sizes are not standardized. The method recommended involves putting the grouting sample into hermetically sealed cylindrical moulds with a cross sectional area of about 12 m 2• The samples should have a height to width ratio of 2. However, other sampling modes are also acceptable, particularly cube-shaped samples. The shape of the samples should then be specified. Simple compression resistance is measured in grouts that are 7 days and 28 days old, and wherever possible after 24 or 48 hours. The need to know the short-term resistance of the grout stems from the fact that the nails are subjected to tensile stresses from the time of the excavation layer following the one in which they were installed (often involving a period of less than 24 hours). Since it is not always possible to carry out short-term compression tests on site, the following diagrams (figure 1) can be consulted to help with the choice of grouts. These give data on the way the grouts' resistances develop over time depending on: The classification and type of cement. The C/E ratio. These diagrams reveal the following: The dispersion of the test results is relatively high for class 55 CPA grouts where the C/E ratio is equal to 1.5. This high performance cement is not especially recommended in situations where the grout can become diluted. Temperature also influences the way its resistance develops. The dispersion noted when testing CLK cements is explained primarily by differences in the origin of the cements (their compositions are therefore different). Where the nails are subjected to tensile stresses within 24 hours, a minimum indicative resistance of 5 MPa after 24 hours of the mix design is required. A higher resistance can be achieved using special grouts. Attention is drawn to the fact that simple compression resistance can show strong dispersions over fairly short periods of time.

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Chapter 7: Specification and Inspections

Portland cement

Portland cement

CLASS 45

CLASS 55

(MPo)

Slag cement (elK 45) CL ASS 45

(MPo)

C "[=2

0

a.. ~

w

40

40

u z e:t

~=2 E

l~

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30

30

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z

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0

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20 TIME

:y) (d)

0

0

10

;n

30 (d)

TIME

Figure 1. Simple compression resistance of grout versus time (Document Soletanche).

If the need exists, traction tests on young grouts can be used to verify proper adhesion between the grout and the reinforcement. 1.3.3.3. Test frequency

Density and viscosity measurements must be taken during conformity tests and inspection tests, as well as once per work shift. Simple compression resistance measurements can be taken at the same time as the conformity tests. However, inspection tests need not be taken systematically since the time-scales needed for the test results to become available are often incompatible with work schedules.

1.4. Concrete bars for the facing Facing bars can be made from high-adherence reinforcing bars or welded mesh. The types of steel used must meet the specifications of NFA 35-015 to NFA 35-022. Furthermore, the high-adherence reinforcing bars and welded mesh chosen must have been approved by the Inter-Ministerial Commission on the Approval and Inspection of Reinforced Bars for Reinforced Concrete (Commission interministerielle d'homologation et de contrale des armatures pour beton armee). Certification documents must be supplied before work starts.

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Soil Nailing Recommendations-1991

1.5. Concrete to be used for the facing (shotcrete or cast-in-place) 1.5.1. Shotcrete

At the moment there are two methods for placing shotcrete:

• Dry method A mixture of small aggregates and cement is transported in a flow of compressed air to the placement nozzle. Water is added as the mix exits the nozzle and is being placed.

• Wet method The fresh concrete, having first been blended with the mixing water, is pumped to the placement nozzle. This can be done either pneumatically (diluted flow), or using a concrete pump (dense flow).

1.5.1.1. Specifications relating to the concrete mix design

• Small aggregates The aggregates used for either dry or wet methods must conform with current French standards NFP 18-301 or NFP 18-302. The granulometric curves must be well-graded so that rates on the shooting will be achieved. For guidance the AFTES or AFB Recommendations are reproduced (see figure 2).

a ) Ma x i murn grain size

ep 8 mm

.. w

(%) 100

Passing

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00 6

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4

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2

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PMODULES FIEVE SIZE

ep(mm)

MAXIMUM GRAIN SIZE I ep 8mm

-

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Figure 2. Granulometric curves for shotcrete. Aggregates only (in accordance with AFTES Recommendations).

258

Chapter 7: Specification and Inspections

b) Maximum grain size

¢ 12,5mm

(%) 100 r------.---r---r-----,--..---,,----,---r---r---r---,-~..,._~-.~---=----,------,

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c) Maximum grain size

140 141

142143144

1

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MODULES

SIEVE SIZE ep (mm)

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MAXIMUM GRAIN SIZE ep 16mm I

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p.OO

0.16 10.315 10063~ISI~ ~121 ~I ~I ~ I~

140 141 I ~ IS 110

-

142143144 1 le.5 J 16120

1

Figure 2. Granulometric curves for shotcrete. Aggregates only (in accordance with AFTES Recommendations).

The tolerance allowed for the building contractor in relation to the optimum granulometric curve used by the owner or the quality control engineer following the preliminary or conformity tests, must be less than ± 10 percent. It is recommended that sand with only a low percentage of flat grains be used. The gravel should have a flatness ratio lower than 0.30. For preference, rounded aggregates should be used rather than crushed ones.

259

Soil Nailing Recommendations-1991

When using the dry method, the water content of the aggregates should be homogeneous and remain low (2 to 4 percent). It is recommended that the aggregates be stored under cover.

• Cements All cements should conform with the specifications of standard NFP 15-301 and be included on the NF-VP lists issued by AFNOR. If the cement is to be exposed to sea water or to waters with a high sulphate content, the suitability lists for cements compiled by COPLA and published annually by the relevant Ministry should be consulted. If either acidic or very pure water is present, the choice of cement will have to be justified. CLK and CHF cements can be used.

• Water The water used must comply with standard NFP 18-303.

• Specific additives and special ingredients The placing of the shotcrete can be made easier and its in situ quality improved by using the following products: Traditional concrete additives, for example, to speed up its set time, plasticizers, etc., must meet the specifications contained in standards NFP 18-103 and NFP 18-331 to 338. They must be stamped with the relevant NF mark authorizing their use or be chosen from the COPLA list of approved additives. Stiffening additives complying with NFP 18-103 are defined as being "additives whose main function is to encourage adherence of the concrete and its being held in place without slumping after it has been sprayed on to the surface, irrespective of the slope of that surface." It is recommended that preliminary or conformity tests be carried out on the building site in

order to assess the amount of additives required, bearing in mind the type of cement used and the conditions under which it is to be placed.

If more than one additive is to be used, a compatibility study should be carried out before the conformity test. The use of calcium chloride and chlorinated adjuvants will only be authorized within the limits set down in DTU No. 21-4, "Technical prescriptions regarding the use of calcium chloride and chlorinated additives in the manufacture of grouts, mortars and concretes." (Prescriptions techniques concernant l'utilisation du chlorure de calcium et des adjuvants contenant des chlorures dans la confection des coulis, mortiers et betons.)

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Chapter 7: Specification and Inspections

NOTE: The word "additive" is used when these products represent less than 5 percent of the weight of the cement, and the words "special ingredients" are used when this percentage is more than 5 percent. 0) Shotcrete (dry method)

(%l 100 Pass ing

80

60

40

....<"(K\.''',I).[>.-P~[;»~

MAXIMUM GRAIN

SIZE

ep

16mm

20

Ol---..,--..L....,-'--r---''----,r----'-r-.......'-r-'-r-'-.......,.-'-..,.....'-r--'-r---'-r--'-;r-'-r--'--r~...__'_r--'-_,

MODULES 20 23 26 SIEVE SIZE 0.08 0.16 0.315