PAPER DESIGN OF LOW-PROPPED EMBEDDED RETAINING WALLS IN STIFF CLAYS

Recent. papers have discussed the construction of embedded retaining walls which are propped at or close to excavationlevel. The advantages of such a method of providing restraint are significant, and the use of this structural form has beenmost commonly adoptedin transportation projects such as highway and railway underpasses. This paper considers theavailable methods of analysis and design and concludes that the common assumptions and earth pressures adopted arenot truly representative. While the walls already constructed are safe in terms of stability, they may be overlong. A

modified pressure distribution has been adopted and shown by numerical analysis to be conservative. Significantlyshorter depths of wall embedment are predicted for this structural form, associated with potential major cost savings.The use of numerical analyses, with careful modelling of construction sequences, is considered to be a fundamental andimportant aspect of design. The Observational Method is the most suitable for assessing and utilising the available shortterm properties of stiff overconsolidated clays.

',lesic no~'w-

~ro s )ecel'ec c ecreI:aininc! wa s insl,'ill'c aI!tsSteven Evetton, Senior Project Engineer, Sir AlexanderGibb L Ptns Ltd, Earley House, London Road, Reading,Berkshire RG61BL.

e use of retaining walls formed in situ allows a considerablereduction in the space required to be taken when compared withconstruction of traditionally backfilled walls, and reductions in

construction periods. With in situ walls excavation follows installation.Stability of the system is provided by embedment of the wall below for-mation level, or by propping.

The design of embedded walls, which are commonly of sheet pile,secant or contiguous pile, or diaphragm panel construction, includesthe determination of bending moments and shear forces imposed onthe structure due to earth and water pressures. These pressures varywith time as a result of the construction sequence, imposed sur-charges, magnitude of deformation, changes in soil properties andgroundwater conditions.

Historically, the assessment of earth pressures acting on structureshas been studied at least as far back as Coulomb's memoires. The devel-opment of active and passive pressures adjacent to unpropped can-tilever walls is well understood, where wall and ground displacementsare sufficient to develop these limiting states of stress.

Use of supportThe incorporation of support to a wall in either the temporary or per-manent case allows for greater earth and water pressures to be resist-ed, greater heights to be economically retained and serves to reducedisplacements of the wall. The effect of a support on the developmentof active and passive earth pressures is not necessarily straightfor-ward however. For double or multi-propped walls empirical or complexnumerical methods are relied upon for design.

If the wall is singly supported then the system becomes staticallydeterminate and the relatively simple conventional equilibrium meth-ods are used. Design standards and codes of practice, based on theexperiences of observations and back analyses, recommend the use ofthe limiting active and passive pressures on the retained and excavat-ed sides for design. CIRIA 104 gives a series of definitions for the appli-cation of a factor of safety which serve to increase the wall length,reduce the resisting earth pressures by an amount, or by limiting theavailable strength of the soil. Commonly the 'CP2'efinition is adopt-

O

Mott McDonaldReview '.

(low prop)L

Mott McDonald,Review

(high prop) A3 (Maiden)

~ Walthamstow

~ Rayleigh Weir

A406Great Cambridge

Walthamstow,'lackwater

Valley

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Ftgure f.As constructed ernbedments for several low-propped walls ln london Clay.

ed whereby the required embedment is determined by equating themoments of limiting active earth pressures to those of the limiting pas-sive earth pressures divided by a factor of safety. The reason for fac-toring passive pressures is to provide a gross factor on failure, and tolimit the relatively large movements that would be required to developthe limiting passive pressures. For heavily overconsolidated clays(Potts & Fourie, 1986) similar magnitudes of displacement are requiredto reach both of the limiting states.

Low level supportThere are situations where the use of high level support may not bepracticable, for example in deep road cuttings where lines of sightwould be impaired; ground anchors may not be acceptable where theyhave to be installed beneath property or because of concerns withdurability or maintenance. An option which has received attentionrecently is to use a propping system which is at or below the final exca-vation level. Examples of road schemes where this has been adoptedinclude several underpasses on the A406 in London, the BlackwaterValley road scheme, and the Batheaston Bypass. The propping action atlow level is provided by either discrete props below the finished car-riageway level, or as is more commonly the case, by the carriagewayslab itself. The merits of these forms are discussed later. By moving thepermanent prop close to the excavation level, and therefore closer tothe centre of the wall, the load transfer from the earth to the wall andthen to the prop is a more efficient process.

An examination of the geometry of several constructed walls sug-gests however, that no clear difference in wall lengths has been made bythe use of low level propping, even though it would be expected to bemore efficient. From the published literature the as-constructedembedment ratios (defined as the ratio of wall embedment to retainedheight) from eight schemes have been collated. These are shown onFigure 1, and are seen to range between 0.9 to 1.9. The embedmentratios adopted in numerical analyses in research projects also followsa trend of embedment ratios close to or greater than unity(Nyaoro,1989; Li, 1990; Miles, 1991; Richards & Powrie, 1994). Figure 1also shows the trends of a study performed by Mott McDonald (1992) ofretaining walls on roarl schemes in the London area. It can be seen thatthe embedment ratio increases with retained height for high proppedwalls, but decreases for low-propped walls.

GROIIND ENGINEERING OCTOBER 1996 37

PAPER DESIGN OF LOW-PROPPED EMBEDDED RETAINING WALLS IN STIFF CIAYS

Hyd

water pressure

< Hydr

KA (y(h+d) -uJ

y (d+ —Jyield+ —J KR(yd - uj KF (y (h+d) - uJ K~(yd - uj

Rgure 2.Conventtonal assumed earth and water pressure dlstrlbutlons for anembedded wall singly propped at high level.

Rgure 4.Shnln compatible earth pressures for an embedded wall singly propped atlow level, due to Nyaoro. G ross pore water pressure distribution as Figure 2.

High prop (1<FF<2)

Ka jyh - uj ster pressure

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Rgure 3.Fugy reversed earth pressures for an embedded wag singly propped at low Figure g. Embedment ratio for singly propped embedded wallslmrel. Gross pore water pressure distribution as Figure 2.

The codes and standards governing embedded wall design (BS8002,CIRIA 104) give little guidance for designing low-propped walls. CIRIA 104does however state that limit equilibrium techniques are only applicablein cases where the prop is close to the top of the wall. If the limit equilib-rium methods themselves are not really suitable, then the use of conven-tional factors of safety (which were in reality empirically determined)have no basis in design, unless they can be shown to be conservative.

Factors of safety and earth pressuresIn conventional calculations (CP2 method) a triangular active pressuredistribution is generally considered to act against the wall, beingresisted by a triangular passive distribution (Figure 2). A net waterpressure distribution with linear seepage around the toe of the wall iscommonly adopted. The assumed mode of deformation is of rotation ofthe rigid wall about the prop towards the excavation. The active earthpressures and the net water pressures are treated as overturningmoments, and the passive pressures as restoring moments. Note that inthis definition net water pressures are used in order to prevent factor-ing them, as they are usually more accurately and reliably determined.The conventional definition of factor of safety on moments (CP2 defin-ition) is thus:

Restoring Passive

Overturning Active + Net Water

In the case of a low propped wall however, the situation changes. For aperfectly rigid wall the mode of deformation is rotation about the lowprop, and it is assumed that the wall above the prop moves towards theexcavation. This deformation mode has been studied by Li (1990),Nyaoro (1989) and Miles (1991)using numerical techniques. They con-firmed this forwards type rotation for relatively small embedments,and a backwards rotation for relatively deep embedments. An interest-ing point was seen in Nyaoro's work where it was impossible to simu-late mass failure of the wall purely by placing large surcharges on theretained side. Failure was only inevitably caused by structural failureof the wall or prop.

The forward rotation mode is often noted by designers, but has leadto a rigid use of active and passive limits in the zones surrounding thewall (eg Symons, 1992). Figure 3 shows the resulting pressure distrib-ution from these assumptions. As a result of the assumed stress rever-sal a major stress discontinuity occurs in the retained soil immediate-ly behind the prop. For a plastic clay (say (l = 22') the ratio of this dis-continuity (KF/KA) is approximately 5, and for a dense granular soil(g = 40') the ratio is approximately 20. Bolton 5 Stewart (1994) considera horizontal frictionless discontinuity at this level to simplify theanalysis. In practice this is considered to be unreasonable, as at thislevel immediately behind the wall the soil will undergo little, if any,shear strain due to being restrained by the prop, and the soil will notapproach the strain softening region of its stress-strain behaviour.

3g ('RDDND FNGINEFRING 0(yroRRR 1996

PAPER DESIGN OF LOW-PROPPED EMBEDDED RETAINING WALLS IN STIFF CLAYS

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-: Bell Common —retained height 8m

Walthamstow —retained height 8.8m(Carswell et al)

Rayleigh Weir —retained height 8.5m

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This fundamental point can be relatively easily addressed in limit equi-librium calculations performed by hand. Extra care needs to be taken,however, when using computer software for the calculations. Somecodes may automatically assume active pressures on the retained side

Depending on the nature of the soil it is unlikely that the full passivelimit could be reached at this depth to provide resistance to overturn-ing. At the toe of the wall however the strains may be sufficient for thepassive pressures to develop. To account for this, Nyaoro proposed thepressure distribution shown in Figure 4.

With both of these pressure distributions the definition of factor ofsafety necessarily changes, as restoring moments are instead generat-ed in the retained earth, and overturning moments are generated onthe excavated side. Taking the essence of the CP2 definition to berestoring moments divided by overturning moments, a quite sizeablenet water pressure acts here to provide stability. In line with generalpractice, water pressures would not be factored. However, care must betaken where water conditions may not be fully understood or areunable to be relied upon (for example, leakage through the wall wouldin this case serve to reduce the stability of the wall) and it may be nec-essary for factored water pressures to be included.

The factor of safety equation now becomes:

over the whole length for example, thereby giving a false indication offactor of safety.

Effect of method of analysis on wall embedmentThe actual effect of the low prop on stability and embedment is shownon Figure 5. Embedments were calculated using effective stresses fora c'=0 material in saturated conditions. There are significant reduc-tions in wall length due to the use of the low prop, and embedment canbe seen to be relatively independent of both factor of safety and angleof friction.

The simple calculations were extended by considering a real designcase. A series of calculations for a wall retaining 8.5m of saturatedgranular material, embedded into London clay were performed todetermine the variation in calculated embedment for various assump-tions regarding propping, factors of safety and mobilised soilstrengths. This process is typical of that commonly performed for theactual design of a specific wall. For the low propped sections Nyaoro'sdistribution was used. The results are summarised on Figure 6. It canbe seen that, were common design practice to be followed rigidly (usingthe current recommendations of CIRIA 104 say) then the short termtemporary construction case (with a high prop) would govern walllength, and the benefits of the low-prop system would not be obtained.This may well be the reason for the deeply embedded low-propped wallsshown in Figure 1.

The reason why the temporary case is significant is due to the con-

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Figure 6.Calculated wall embedment for an 6.6m retained height using a variety of design assumptions.

GROUND ENGINEERING OCTOBER 1996 39

PAPER DESIGN OF LOW-PROPPED EMBEDDED RETAINING WALLS IN STIFF CLAYS

rary works design. The aim is to reduce cost and save time whilst assur-ing safety.

Powderham (1994) presents a case study of retaining walls for theChannel Tunnel. The Observational Method was used to reduce theamount of temporary support required for a multi-propped wall. Thedesign was based on CIRIA 104's 'mixed total/effective stress'pproachusing 'most probable'arameters. Even so, this approach was consid-ered to be conservative due to the use of plane strain analysis, and arelatively high factor on restoring moments. Construction/excavationwas monitored and kept within predefined tolerances by control ofphasing of works. Additional propping was available in the case of tar-get measurements being exceeded. Excavation phasing was controlledby limiting the maximum width open at any one time, thereby not rely-ing on any three-dimensional behaviour of the ground. Followingsmall measurements of wall movements this width was increased toease construction without compromising stability or serviceability.

This is a good example of how an observational approach could beadopted for low-propped walls in stiff clays, whereby contingency mea-sures (such as additional propping or the ability to partially backfill ifnecessary) are provided for the short term case in order to obtain thebenefits of reduced embedment for the permanent (long term) case.

ConclusionsThe rigid use of limiting earth pressures has been examined and foundto be not truly representative for the design of low-propped walls. Thesimplistic assumption of fully reversed pressures below the prop pro-vides a design which relies on passive pressures which are unlikely tobe developed in practice. Rigid and consistent definitions of factor ofsafety need to be adopted and care taken with the use of historicallydetermined factors.

Design in heavily overconsolidated clay using current recommenda-tions developed for other propping conditions provides a considerablyoverconservative solution. A modified earth pressure distribution pro-posed by Nyaoro which accounts for low strains close to the low prop,in conjunction with a mixed total/effective stress approach using con-ventional factors of safety, has been shown to produce an economic yetstill safe and conservative design.

Numerical methods are recommended as a fundamental part of thedesign of low-propped walls. This will enable the potential cost savingsto be realised in reducing wall lengths, and allow better determinationof bending moments, shear forces and displacements. Observationaltechniques should be adopted wherever possible in the construction oflow-propped walls.

Other aspects of wall designThe design of walls has been centred in this paper on determining asafe economic wall length. The numerical analyses are an importantpart of the general design process however because, for this structuralform, the use of lower bound soil strength parameters is not conserva-tive. This can be seen in the increase in bending moments in the wallbeneath the prop when soil strength increases. As interpretation ofsoil parameters is generally a highly conservative process, this doesnot provide a correct approach to design. Attempts must be made forthis case to determine the 'best estimate'r 'most probable'arametersand design accordingly.

The structural design work which needs to be considered is usuallydirected towards the form of the permanent prop and its connection tothe wall. Two options which have been considered are:~ discrete reinforced concrete props below carriageway level; and~ a continuous unreinforced slab forming the road carriageway.The choice of option is not straightforward, although it is interestingto note that recent designs performed by contractors tend towards theslab, suggesting cost, time and ease of construction benefits.

Designs with discrete props usually place the props sufficientlybelow the road carriageway to allow for installation and maintenanceof drains placed above them. This requires greater depths of excava-tion, and hence a deeper wall. There are also concerns with ride quali-ty over the props.

For the unreinforced slab, the system relies on the self-weight of theslab and the compressive forces from the walls to overcome any tensileactions caused by heave of the ground. Attention has to be given to theexpected long term heave, and to providing suitable drainage measuresto deal with this throughout the design life of the wall. Any build up ofwater pressure beneath the slab will reduce its effective self weight andreduce its stability. Other options are to precamber the slab/prop suchthat the heave forces serve to prestress it, or to provide a tension pile atthe centre of the excavation.

Whichever option is adopted, its connection to the wall must be con-sidered. 'Pinned'oints have been constructed using PTFE coated semi-circular seatings for both individual props or slabs. For partially orfully fixed connections the possibility of transfer of moments from thewall to the prop, or vice versa, should be addressed. Analyses show thatwith discrete props the wall transfers quite sizeable moments to theprops due to the large relative stiffness between the wall and the prop.With continuous slabs the stiffnesses are of similar order. It is also pos-sible for a sufficiently stiff slab to transfer moment to the wall duringlong term heave.

The measurements of prop forces and bending moments from theinstrumented schemes are important. Most, if not all, cases show thatthe major factor in prop loads is temperature, with large daily varia-tions being recorded. Experience seems to show that measured loadsare significantly less than those 'predicted'uring design. This is prob-ably due to overconservatism in design, 'poor'implistic methods ofanalysis, and the effects of construction sequence. It may also be linkedto the low bending moments which have been measured in the instru-mented walls. At Bell Common this was attributed to the low stressesmeasured in the Sm zone behind the wall. It is not clear howeverwhether the pressures will come back onto the wall in the intermediateto long term. It would not be unexpected for the prop forces to then alsoincrease. Without further long term measurements it would not be pru-dent to reduce safety factors in permanent prop design.

Bolton, MD & Steward, DI (1994). 'The effect on propped diaphragm walls of risinggroundwater in stiff clay'. Geotechnique 44 Nol.BS 8002 (1994). 'Earth Retaining Structures'. BSI.Carder, DR; Carswell, IG & Watson, GVR (1994). 'Behaviour during construction of apropped diaphragm wall in stiff clay at Walthamstow'. TRL Project Report 17.Carswell, I; Carder, DR & Gent, AJC (1993).'Behaviour during construction of a proppedcontiguous bored pile wall in stiff clay at Walthamstow'. TRL Project Report 10.Darley, P; Carder, DR & Alderman, GH (1994). 'Behaviour during construction of apropped contiguous bored pile wall in stiff clay at Rayleigh Weir'. TRL Project Report 23.Li, ESF (1990). 'On the analysis of singly-propped diaphragm walls'. PhD Thesis,University of London.Miles, SR (1991).'On the stability of retaining walls propped at carriageway level'. MScThesis, University of LondonMott McDonald (1992). 'LRO retaining wall review'. Proceedings of the Retaining WallSymposium, Department of Transport, London Regional Office.Naoro, DL (1989). 'Analysis of soil-structure interaction by finite elements'. PhD Thesis,University of London.Padffield, CJ & Mair, RJ (1984). 'Design of retaining walls embedded in stiff clay'. CIRIAReport 104.Peck, RB (1989). 'Advantages and limitations of the observational method in applied soilmechanics'. Geotechnique 19 No2.Potts, DM & Fourie, AB (1984). 'The behaviour of a propped retaining wall: results of anumerical experiment'. Geotechnique 34 No3.Powderham, AJ (1994). 'An overview of the observational method: development in cutand cover and bored tunnelling projects'. Geotechnique 44 No4.Richards, DJ & Powrie, W (1994). 'Finite element analysis of construction sequences forpropped retaining walls'. Geotechnical Engineering, Proc Institution of Civil Engineers107.Symons, IF (1992). 'Design of embedded retaining walls in stiff clays'. TRL ResearchReport 359.Tedd, P; Chard, BM; Charles, JA & Symons IF (1984). 'Behaviour of a pi'opped embeddedretaining wall in stiff clay at Bell Common Tunnel'. Geotechnique 34 No4.

GROUND ENGINE()RING OCTOBER 1996 41