Designing retaining wallseI—IIbedded in stiff clayby J.M. HEAD, MSc, CEng, MIMM, FGS, and C.P. WYNNE, MSc, DIC, FGSt

The design of retaining walls is complex and has always posed problems for geotechnical and structural engineers. Totest current standards of design practice, a "Design Challenge" was set in the May/June edition of CIRIA News (1984).The results were surprising. Analysis of the submissions to the challenge identified a number of recurrent designdifficulties and uncertainties, the most important of which are:~ The choice of an appropriate and rational design method and application of realistic safety factors.~ Application of earth pressure coefficients, particularly when dealing with wall friction and groundwater pressures.~ Understanding the implications of the "fixed earth" and "free earth" assumptions.~ Confidence; 45tye of the entrants stated that they did not believe their answers or aspects of their calculations.

The eleven respondents represented a wide cross-section of organisations involved with design. Six of the elevenwere aged over thirty. The difficulties that these engineers encountered are felt to be typical of the problemsexperienced by most practising engineers.

The last authoritative statement settingout good practice was that contained in theBritish Standard Institution's document CP2in 1951. In the last 33 years, methods ofdesign have changed and new procedureshave been adopted. At least five theoreticalmethods are available at the present time.

CIRIA Report 104: Design of retainingwalls embedded in stiff clays (Padfield andMair, 1984) is a handbook that covers all

aspects of design. The structural formconsidered is that of a thin wall installed in

undisturbed ground by diaphragm wallingtechniques (or equivalent process) or bydriving sheet piles. Various methods ofassessing the factor of safety against overallfailure are compared and four of these arerecommended.

The problems set by CIRIA and theirsolutions, are given on pages 32 and 33.Table I summarises the wide range ofanswers received. The method of calculationinvolves three distinctly differentassumptions about safety factors:F,—safety margin achieved by factoring

passive pressure (CP2 Method)F, —safety margin achieved by Burland-

Potts MethodF, —safety margin achieved by factoring soil

strength parametersThe available design methods do not

model the true ground behavioursatisfactorily, and there is no single approachthat is theoretically adequate. Solutions tothe problems set are therefore based on anaverage design depth from the above threemethods. For design purposes, solutionsshould be based on results from more thanone method. Having determined the basicpressure distribution for one method, it isrelatively simple to perform the additionalcalculations. The full solutions are not givenin this Paper . The factors of safety adoptedfor each method are indicated by arrows onthe summary plots of, F (safety factor againstfailure) against d (embedment depth) onpages 32 and 33. The values of F vary forboth short- and long-term conditions and area function of the method used and thematerial strength parameters. Full details are

tEarthworks and Foundations Research, CIRIA, 6 StoreysGate, London Swtp 3AU.Detailed solutions are obtainable from CIRIA on request

Typical applicationof a retaining wa/I;in this case boredcast-insitu pileswere used to form aretaining wa/I at TheSide, Newcastle-upon- Tyne

given in Table 5 of the CIRIA report.A fourth approach, F„,, is also included on

the summary plots. This method achieves asafety margin by factoring net pressures (asoutlined in the British Steel CorporationHandbook). Although a large proportion ofentrants used this method, the resultsproduced can be very misleading and it istherefore not recommended. For stiff clays,one of the dangers of the method isillustrated on the summary graphs —thedesigner is not aware that large increases in

the values of safety factor result in onlynegligible increases in depth of embedment.The method, as presently set out, does nothave a rational base and in reality, safetyfactors are always close to unity.

The following summarises the solutionsgiven below:

Problem one: Cantilever WallPermanent worksDesign Depth d, = 10.9x 1.2 = 13.1mEntrants'ange = 4.5m to 13.3m

Problem two: Propped WallPermanent worksDesign Depth d, = 11.9mEntrants'ange = 5.4m to 19.2mTemporary worksEffective Stress Design d, = 7.2mTotal Stress Design, d, = 5.6mEntrants'ange = 0.7 to 7.2m

TABLE I: SUMMARY OF ENTRANTS'ESULTS

PROBLEM 1:CANTILEVER WALL

Permanent Works

PROBLEM 2: PROPPED WALL

Temporary Works Permanent Works

Method Embedment F valued (ml used

Method Embedment F va/ued (ml used

Embedmentd (mj

F va/ueused

F„pFnp

Fnp

F„pFnp

F„pFopFu

FpFpF,

7.67.69.04.57.8

1 3.39.89.17.87.8

10.5

2.02.02.01.2 (2.0)2.01.0 (2.0)1.0 (2.0)1.0 (1.5)0.5 (1.75)1.3 (1.75)1.3 (1.5)

Fnp

Fnp

F„pFnp

Fnp

Fnp

Fop

Fnp

Fp

Fp

F,

1.52.54.06.47.2

0.90.73.03.0

2.02.01.2 (2.0)1.2 (2.0)2.0

2.01.0 (2.0)2.01.0 (2.0)

9.11 3.09.58.07.2

1 6.51 7.11 9.28.05.4

1 1.0

2.02.01.5 (2.0)1.5 (2.0)2.01.5 (2.0)1.0 (2.0)1.0 (2.0)1.0 (1.5)1.3 (1.5)1.3 (1.5)

Nore: Factors in parenthesis are those that should have been applied. See References 2 and 4.

30 Ground Engineering

The entrants'ange of design depths(Table I) clearly shows the problems that canoccur when applying a single designtechnique, even to a simple problem in whichthe parameters are specified. In most cases,establishing appropriate design parametersis itself a complex problem. Typicaldifficulties experienced by the entrantsinclude:Earth pressures: Coefficients are oftenincorrectly applied to total stresses. In somecases, water pressures were added to totalstresses —therefore allowing for the effectsof water twice. There is uncertainty about thedegree of wall friction that is developed and itis therefore often incorrectly assumed aszero. The values presently recommended forthe above effective stress analyses are; activezone 5 = /ar)', and passive zone 5 = '/>(a'.

Groundwater: In the long-term, seepagearound the wall increases water pressure togreater than hydrostatic on the passive sideand reduces the water pressure on the activeside. Allowance for seepage is relativelysimple and yet is often neglected.Design method: Most entrants used theF„, approach with various arbitrary orirrational amendments. Although the safetyfactors applied varied considerably, the largerange of embedment depths is dueprincipally to assumptions made in

calculating earth pressures.In Problem 2, the free-earth support

approach is recommended. However, manyentrants assumed fixed earth conditions.Since the wall is propped, rotational failure

requires some movement of the wall toe. Thefixed earth assumption requires that the walltoe is also fixed and, by implication, restrictsthe mode of failure, to one of wall fracture.Furthermore, the fixed earth approachrequires the unnecessary (for the purposes ofthe problem) calculation of the prop force.

ln Problem 1, many confused thesimplification using fixed earth assumptions(d~ ——d + 20()/o) with the factor of safety, orneglected to allow for it.

Current design practiceA series of seminars were recently held to

discuss CIRIA report recommendations andthe results of the design challenge. It alsogave delegates an opportunity to discussother problems associated with currentdesign practice. Inevitably, many of thequestions raised relate to the temporaryworks design, selection of design parametersand application of realistic safety factors.

To date, the majority of engineers havedetermined the depth of embedment fortemporary retaining walls using the BSC (F p)Method, and assumed undrained (totalstress) conditions. This approach appears towork adequately, and although there isconsiderable experience with it, theconceptual problems previously mentionedcan cause misunderstandings andsometimes unsafe designs. The F„,approachinvolves the elimination of all balancing loadsfrom the equilibrium equation, leaving onlythe out of balance pressures. This grosslydistorts the factor of safety derived from the

moment equilibrium relationship. Even if verylarge F„, values are chosen, the resultantdesign will not be consistent with the othermethods recommended. If the trueoperational shear strength is used, the actualmargin of safety is always very small, nomatter how large the implied F„,values.

There is a danger that less experiencedengineers will assume that a high factor(F = 2) is in operation, when in reality themargin of safety is close to unity. Subsequentoverloading of the rear wall area, or over-excavation in front of the toe area, can causefailures.

There is a reluctance within the industry toreport known failures. Usually, failures do notresult in ultimate collapse but cause service-ability problems, such as unacceptable wallmovements. Problems are rectified throughimmediate remedial action in the form ofadditional support or increased embedment.This approach is almost a trial and errortechnique. It can be very much more expen-sive than the recommended alternative ofadopting a rational design method, that canbe understood by both experienced andinexperienced engineers.

ln practice, experienced engineers tend toadopt over-conservative values of undrainedshear strength, c„, and it is through thisprocess that the real safety margin isintroduced. In effect, therefore, the Factor onShear Strength (F,) Method is being applied.The problem with this approach is that theengineer is not then aware of the real marginof safety incorporated in the design.

SOLUTION ONE

Problem 1:Cantilever wall

What is the value of d for permanent works?2.2-

2.1

Cantilever wall — permanent works

2.0 Fn

10 kN/m'urcharge

Medium dense(i RAVEL

) m y'= 20kN/m''=

lS'1.)I-

Stiff, fissured,overconsolidatedCLAY

y = 20 kN/m''

2S''0

1.6-

By carrying out an effective stress analysis, by takinggroundwater seepage into account, and by making assumptionsfor wall friction, an average value of 10.9m is produced from thesummary plot opposite. This is increased by 20% to take accountof simplifying fixed earth assumptions.

Solution = 13.1mEntrants'ange = 4.5 to 13.3m

i6

A)eragc ii) design range d 10.')

1)esign depth d,i 10.') x 1.2 I L I m

32 Ground Engineering

A separate but related issue is the questionof whether total stress methods areappropriate, even for temporary works—they are certainly not recommended forpermanent works design. Because of theproblems involved with the selection ofrealistic c„values, there are advantages in

applying effective stress calculations, evenfor temporary works. Three designapproaches are identified in the CIRIAReport: effective stress, (c'e'); total stress(c„,e„);and mixed total and effective stress(c„,e').The problems caused by softening oforiginally stiff clay are also identified andexplained.

If effective stress methods are used fortemporary works design, values of F closer to1.0 are appropriate and the resultingdifference obtained by the F„, approach isnot so great. However, for permanent works,use of F„, with low angles of effectiveshearing resistance, e', would certainly leadto potentially dangerous depths ofembedment. It may well be safe for higher

e'alues(greater than 35'), but these are notrelevant to stiff clays.

One of the main concerns expressed is thatdesigners are not given sufficient laboratory

test results to confidently select the soilparameters needed. Usually details of thegroundwater regime are also absent.Laboratory test results are often dominatedby undrained soil strengths instead of therecommended effective stress parameters. Ifthe engineer does not have sufficient data tomake a realistic assessment of designparameters, then higher safety margins arenecessary. The additional cost ofdetermining effective stress parameters canbe far outweighted by the potentialimprovements in design confidence. There isstill a pressing need to persuade thosecontrolling ground investigations to ensurethat more comprehensive data is produced,otherwise advances in design approacheswill be to no avail.

A majority of retaining walls constructed atthe present time (up to 75%) are of thecantilever type. This is because walls oftensupport adjacent structures and there isusually no permission for access to anchorbeneath them. Prediction of movement forcantilever walls is difficult, but magnitudesmust often be assessed and restricted tosafeguard adjacent properties and services.The design of propped walls is affected by

the position, angle and properties of the propsystem.

Design uncertaintiesThere are still many aspects of design that

require clarification or supporting data.These uncertainties all point to an urgentrequirement for well-documented fieldobservations of the behaviour of simplestructures in reasonably uniform soilconditions. Numerical analyses, linked withcareful laboratory studies, are also requiredto complement the field measurements, sothat observed behaviour can be understoodand design methods can be improved.

References1. British Standards Institution: Earth retainingstructures CP2: 1951 (currently being revised)

2. PadBald, C.J. 8 Mair, R.J.:Design of retainingwalls embedded in stiff clay, CIRIA Report 104:19843. Burland, J.B.,Potts, D.M. & Walsh, iy.M.: Theoverall stability of free and propped embeddedcantilever walls. Ground Engineering, July 198114(5), 28 to 384. British Steel Corporation: Piling Handbook,1981

SOLUTION TWO

Problem 2: Propped wall

What is the value of d: a) for short term temporary works?b) for permanent works? 2.2

Propped wall — temporary and permanent works

2.1

2.0cu c„,

d3'tiff.

fissured,overconsolidatedCLAY

y = 20kN/m'y'

20c'=0c„=75 kN/m't ground

surface. increasinglinearly to 180

kN/m't

a depth of 18 m.

F18

1.7

1.6-

1.4

1.3-

1.2-Temporary works Ic'. 4G')

choose d< = 7.2m at F = 1

For permanent works, by carrying out an effective stressanalysis and adopting the same assumptions as for the cantileverwall, an average depth as follows is calculated:

Solution = 11.9mEntrants'ange = 5.4 to 19.2m

For temporary works, assuming that the effective stressparameters given are worst credible values, the depth is asfollows:

Effective Stress Solution = 7.2m

Using the c„profile given, the alternative total stressapproaches give:

Total Stress Solution = 2.3mMixed Total and Effective Stress Solution = 4.9mEntrants'ange = 0.7 to 7.2m

0 2 4 6 8 10Permanent works

Average o( design range d = 11.9m

I)esign depth d4 = I I.gm

~ [ ~ ~ ~

12 14 16

Modified c„Profile Solution = 5.6m

The discrepancy between total and effective stress methodsillustrates the problems and dangers of using c„which isdependent on too many external factors. The values givenrepresent 38mm diameter test samples. A reduction in strengthto account for the higher strengths obtained from conventionaltriaxial tests due to sample disturbance and a 30% reduction tocorrect for softening on the passive side increases the value ofthe design depth as follows:

April 1985 33