Design guidance on the use of a row ofspaced piles to stabilise clay highway slopes

Prepared for Geotechnics Operations Support Division,

Highways Agency

D R Carder

TRL Report TRL632

ii

First Published 2005ISSN 0968-4107ISBN 1-84608-631-0Copyright TRL Limited 2005.

This report has been produced by TRL Limited, under/as partof a contract placed by the Highways Agency. Any viewsexpressed in it are not necessarily those of the Agency.

TRL is committed to optimising energy efficiency, reducingwaste and promoting recycling and re-use. In support of theseenvironmental goals, this report has been printed on recycledpaper, comprising 100% post-consumer waste, manufacturedusing a TCF (totally chlorine free) process.

iii

CONTENTS

Page

Executive Summary 1

1 Introduction 3

2 Objective of strengthening 3

2.1 Remediation of deep-seated slope failures 32.2 Steepening of slopes for highway widening purposes 3

3 Site investigation 3

3.1 Desk study 33.2 Topographical survey 43.3 Soil profiling 43.4 Ground water and drainage 43.5 Laboratory testing 43.6 In situ testing 4

3.7 Instrumentation and monitoring 53.8 Site specific features 53.9 Environmental issues 5

4 Factors for consideration in the design 5

4.1 Spacing and diameter of piles 54.2 Lateral resistance supplied by piles 6

4.3 Penetration and location of the piles 64.4 Overall stability 64.5 Differences between cutting and embankment situations 64.6 Influence of construction sequence 74.7 Other issues 7

5 Design procedures 7

5.1 Methods based on rigid-plastic behaviour 75.2 Methods based on plastic deformation 95.3 Numerical analyses 105.4 Overall stability analyses 115.5 Retaining wall methods 12

6 Site operations 12

7 Summary 15

8 Acknowledgements 15

9 References 15

Abstract 18

Related publications 18

iv

1

Executive Summary

A single row of bored piles spaced at intervals along a clayslope provides an effective remedial measure to improvethe stability of slopes that have either failed or are showingincipient signs of failure. The technique can also be used tosteepen slopes so that highway corridors can be widenedwithin existing boundaries to accommodate extra vehicularlanes or other modes of transport.

Determination of the optimum design for the piled slopeis a complex soil-structure interaction problem and thisreport is intended to provide advice on relevant issues. Thisdesign advice is primarily based upon the findings from:

! a literature review of existing case histories and currentdesign methods;

! centrifuge modelling of a cutting slope;

! three dimensional finite element modelling;

! instrumented trials of remediation of a Gault Claycutting slope on the M25 and a London Clay cuttingslope on the A12.

Guidance on the nature of the site investigation(including an assessment of environmental impacts andsite specific details relevant to plant operations) areincluded in the report. For the geotechnical design it isessential to identify the potential or active failure plane orzone, any water-bearing or weak strata, and a stifffounding stratum for the piles.

The main factors for consideration in the design includethe spacing and diameter of piles, their lateral resistance,the optimum penetration and location of the piles, andoverall stability. The depth of penetration and requiredlateral resistance of stabilising piles in a cutting are likelyto be greater than in an embankment situation.

Current design approaches are aimed at evaluating therestoring force required to stabilise the slope, therequired shear and bending moment capacities of thepiles, and pile dimensions and layout. The availabledesign methods are discussed in the report.Approximate methods of using retaining wall theoryalso proved a useful guide to performance.

The recommendations within the report are generallyapplicable for clay highway slopes which are typically upto about 10m in height. For higher slopes more than onerow of piles or anchoring of the pile heads may beconsidered necessary for stability. It should be noted thatthis report concentrates on giving design guidance whenonly a single row of cantilevered piles is employed,although it is anticipated that some of the philosophies canbe used more widely.

2

3

1 Introduction

One of the options for improving the stability of clayhighway slopes is to use a single row of spaced piles toprovide support. This piling technique for stabilisingslopes has been used widely and with success for someconsiderable time primarily elsewhere in Europe and inJapan (Carder and Temporal, 2000), although there is apaucity of well documented case histories. Stabilisation oflandslides and natural slopes appears to have been moreusual than attempts to stabilise embankments. Thetechnique is however now becoming more frequently usedin the UK both on the highway and rail networks forstabilisation of soil slopes.

Determination of pile diameter, spacing, penetrationdepth and location on the slope is a complex soil-structureinteraction problem on which further design guidance isneeded. This report attempts to address this problem andprovides design guidance, which is primarily based uponthe findings of the following:

! a literature review of existing case histories and currentdesign methods undertaken by Carder and Temporal(2000);

! centrifuge modelling of a cutting slope reported byHayward et al. (2000);

! three dimensional finite element modelling discussed byCarder and Easton (2001);

! an instrumented trial of remediation of a Gault Claycutting slope on the M25 reported by Carder and Barker(2005a);

! an instrumented trial of remediation of a London Claycutting slope on the A12 reported by Carder and Barker(2005b).

Recommendations within the report are generallyapplicable for clay highway slopes which are typically upto about 10m in height. For higher slopes more than onerow of piles or anchoring of the pile heads may beconsidered necessary for stability. It should be noted thatthis report concentrates on giving design guidance whenonly a single row of cantilevered piles is employed,although it is anticipated that the some of the philosophiescan be used more widely.

The piling technique can be used as a remedial measureto improve the stability of slopes that have either failed orare showing incipient signs of failure. Also, with anintegrated transport policy there is likely to be a need towiden highway corridors by steepening slopes withinexisting boundaries to provide extra space for bus/cyclelanes and other modes of transport. The application of thistechnique in meeting these objectives is now discussed inmore detail.

2 Objective of strengthening

2.1 Remediation of deep-seated slope failures

Many clay slopes which are prone to shallow failures (Perry,1989) are now reaching a critical age for deep-seated failure

to occur. In addition, prolonged delays in dealing withshallow failures may lead to the formation of deeper slips(Geotechnical Consulting Group, 1993). This situation isalso exacerbated by the increasing incidence of heavyrainfall and flooding, which will add to the risk ofsoftening of clay, that has occurred in recent years due tothe effects of climate change.

The option of using a row of spaced piles is attractive inproviding a near permanent solution to deep-seated slopeinstability. The longevity of the repair is expected to besignificantly better than that provided by more traditionalmethods such as digging out and replacing with higherquality fill.

2.2 Steepening of slopes for highway widening purposes

In highway widening situations there is an increasingdemand for embankment and cutting slopes to besteepened, so reducing the landtake and associated costsand delays to construction. However, depending upon thegeological strata, Perry (1989) generally recommendedthat cutting and embankment slopes on highways areconstructed at slope angles of between 1v:5h and 1v:2h toavoid instabilities developing. Therefore, when steepeningslopes, additional slope support is often required.

In many cases, improvements in deep-seated slopestability can be provided by installation of a single row ofpiles spaced at intervals along the slope to accommodatethe additional lateral loading developed near the toe of theslope. These measures may need to be used in conjunctionwith techniques such as that of increased vegetation(MacNeil et al., 2001) to improve the stability againstshallow slip failures.

3 Site investigation

When using bored piles to stabilise or strengthen a slopeagainst a potential or active deep-seated failure, there arethree fundamental factors to ascertain during the siteinvestigation in order to achieve a safe and economicdesign. These are:

! to identify the potential or active failure plane or zone;

! to identify any water-bearing or weak strata;

! to identify a stiff founding stratum for the piles.

This information can be obtained from a combination ofthe following activities.

3.1 Desk study

Past records may be invaluable in assisting with the design ofa bored pile system to stabilise or steepen a highway slope.These records may include the site investigation report forconstruction of the embankment or cutting slope, informationfrom other borehole logs in the area, and any other relevantdata on the geological history of the site and the neighbouringarea. Sources of information may include geological maps,aerial photographs, local knowledge, the Highways AgencyGDMS database and the British Geological SocietyGeoscience Data Index (www.bgs.ac.uk).

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During this desk study any evidence of previous slopeinstability in the region should be sought so that any relicshear planes can be identified.

3.2 Topographical survey

Walkover and topographical surveys are essential to establishparameters such as the slope height, slope angle, the extent ofany vegetation, proximity to the highway and any structures,and the drainage situation (see Section 3.4). Any signs ofeither incipient or actual failure should be sought. These mayinclude the location of tension cracks, the extent of anybackscarp, and any areas where bulging or slumping hasoccurred. Deformed lines in any fencing or safety barrier, treemovements, or loss of verticality in highway furniture such aslampposts may provide useful indicators of slopeperformance. Changes in vegetation cover may indicate eitherdifferent geologies or past remedial works.

3.3 Soil profiling

When using a row of bored piles to stabilise or steepen aslope, the potential for failure is likely to be one of a deep-seated failure. For this reason, mobilisation of a cable toolpercussion or small tracked rotary drilling rig is normallyrequired as the soil profile will need to be known to at leastthe full depth of the piles. During this process, as well aslogging the strata, soil samples can conveniently beobtained for classification, moisture content determinationand laboratory strength testing following the groundinvestigation procedures given in BS5930 (1999).Identification of weathered and unweathered zonestogether with any water-bearing strata is an essential pre-requisite during this operation.

In some cases, access to the slope may be difficult andrecourse may need to be made to profiling by trial pit andwindow sampler. Trial pits may be necessary in any eventfor detecting rupture surfaces where they emerge withinthe slope in a failure situation.

The information obtained from the soil profiles isusefully plotted in a section through the slope so that anypotential or active shear planes can be related to thegeology. The optimum location for the row of bored pilescan then be assessed (see Section 4.3).

3.4 Ground water and drainage

An accurate knowledge of the ground water conditions andthe effectiveness of any existing drainage systems is ofparamount importance in assessing the current stability ofthe slope and also in the design of the piling. Ground waterlevels are best determined by direct measurement of porewater pressures using piezometers (see Section 3.7) andsufficient data need to be obtained to identify if anyperched water table conditions exist. The seasonalfluctuation in pore water pressures may also be significantwith suctions developing in the slope surface during thesummer particularly if there is vegetation on the slope. Inthe design of the slope strengthening system, the mostsevere pore water pressures that are likely to exist in wintershould be taken into account.

The effectiveness of any existing or proposed drainagesystem is important in ensuring stability after installationof a spaced pile system. A failure in the existing systemmay have been the root cause of any slope instability. Anew system including drainage at the toe or crest of theslope, and possibly slope or counterfort drains may need tobe considered. Control of ground water is important inrestricting wetting up of the soil retained by the spacedpiles, which may lead to plastic flow or washout of the soilbetween them.

3.5 Laboratory testing

Guidance on the methods of laboratory testing of soils isgiven in BS1377 (1990). Preliminary classification of thedifferent soil strata can be carried out on the soil samplesretrieved from the site investigation by determination ofthe changes in plastic limit, liquid limit, and moisturecontent with depth. This provides an early indication ofclay plasticity and the extent of any weathering.

Strength testing is essential to establish parameters forlimit equilibrium (slope stability) type analyses and formore sophisticated numerical methods employing finite ordiscrete element, or finite difference approaches. Effectivestress parameters are normally employed for these analysesto establish long term behaviour. Peak and residualstrengths can be established by triaxial testing ofundisturbed samples. Residual strengths should bepreferred to model behaviour in zones where shearing hasalready occurred. Back analyses of slope failures by Crabband Atkinson (1991) have also shown that critical statevalues, which lie between peak and residual strength,provide a good prediction in overconsolidated clay slopes.

For more sophisticated numerical modelling, themodulus of each geological stratum is required. This canbe approximately determined from triaxial test results onundisturbed soil samples, or from unload-reload loopscarried out using the self-boring pressuremeter. These dataare however hard to reliably obtain and in many instancesit may be acceptable to use historical data and carry out asensitivity analysis of performance.

3.6 In situ testing

Continuous profiling of changes in strength or stiffness canbe undertaken by penetration testing (Meigh, 1987) or byone of the range of portable dynamic probes which arenow available. In general this approach can only be reliedon to detect relative changes with depth rather thanabsolute values. Their use can however be informative insome situations where only limited data can be obtainedfrom other sources.

Potts et al. (1997) found that the lateral stress in the insitu clay has an influence on the stability of unreinforcedcutting slopes and the risk of a delayed collapse. Thisfactor needs to be taken into account when designing piledsystems to support steepened cutting slopes in over-consolidated clay. Where a stable slope is being steepenedfor highway widening purposes, in situ testing using theload cell pressuremeter (Carder and Bush, 2001) wouldprovide a measure of lateral stress levels.

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3.7 Instrumentation and monitoring

Measurement of the lateral movement of a slope isessential in terms of establishing whether it is stable or not.The design of a piling system will depend on themagnitude and extent of the movement zone whereshearing has occurred or is likely. In the case of piles beingused to steepen a slope for highway widening purposes,post-construction monitoring may also be advisable.

At simplest level, slope surface movements can bemonitored using a system of pegs or marker posts. However,if stabilising piles are being considered, it usually impliesthat there is a potential for deep failure and subsurfacemovements need to be measured to identify the planes ofshearing. The normal way of measuring lateral subsurfacemovement involves installing inclinometer access tubes atcritical locations within the slope. Indications of subsurfacemovement can also be obtained using slip indicators or byinstalling electrolevel strings within inclinometer tubes. Theformer detects the depth to movement but gives little feel forits magnitude, the latter technique requires specialistinstallation but has the benefit of providing real timemonitoring in highly critical situations.

In carrying out an inclinometer (or electrolevel) survey inan access tube, the movements are normally determinedassuming fixity of the base of the tube and for this reason itis essential to found the tube in a firm stratum. Alternativelymovements can be determined relative to the top of the tubeif its exact position can be established by electronic distancemeasurement from a stable reference point. Generallyinclinometer tubes are effective in determining lateralmovements to an accuracy of better than a few millimetres,although in extreme cases of instability the access tubes maybe damaged by excessive movement.

The other important parameter to be measured is that ofpore water pressure. Pore water pressures can be measured byinstalling profiles of piezometers in boreholes at differentdepths. If measurements of pore pressure are required near tothe slope surface and/or at shallow depths below vegetation,pore suctions may be measured during the summer and theappropriate piezometer type needs to be selected for thispurpose (Ridley, Brady and Vaughan, 2003).

3.8 Site specific features

Site specific features which may have either affected thestability of the slope or have an influence on the design ofthe slope strengthening system need to be identified duringthe site investigation. For example, a change in the drainagesystem or a change in the loading conditions may haveoccurred. The latter might be due to the excavation of adeep trench at the toe or crest of the slope for utility works.

If the site is prone to flooding this should be identifiedas it can cause softening of clay within a slope. Equallywell, vegetation can act as a stabilising influence althoughits presence may well impede construction operations.

Any problems with site access and boundaries, whichwill influence the operational zone of a piling rig and thelocation of the row of spaced piles, also need investigatingat this stage.

3.9 Environmental issues

An environmental assessment should be included in thecondition appraisal of the slope prior to implementing anypiling works. Where the slope has failed, it can be arguedthat the slope has already been disrupted in terms of itsecology. However, environmental pressures are likely torequire that the slope condition be restored to its originalstate or better using a bio-engineered approach.

4 Factors for consideration in the design

The main factors that need to be considered when carryingout the design of the piling system to stabilise the slope arenow discussed.

4.1 Spacing and diameter of piles

The spacing between and diameter of the piles must bedesigned to maximise the arching of the soil between thepiles whilst minimising the flow of soil between them.Various theories have been developed for design purposesbased on a combination of elastic and plastic soilbehaviour and these are described later in Section 5.

Carder and Temporal (2000) drew analogies with otherconstruction techniques, where soil flow and archingoccur. The principal findings from a review of theperformance of piled foundations embedded in clays is thata pile spacing of about 5 times the pile diameter issufficiently large to minimise the interaction betweenadjoining piles, no interaction is expected at a spacing ofmore than 8 diameters. On this basis it can be argued thatin a slope situation, flow of soil between piles is fairlycertain at spacings of more than 8 diameters and possibleat spacings of more than 5 diameters. A review of designrecommendations for spill-through abutments suggested asimilar result with an upper limit for spacing of 8 pierdiameters (or widths) for arching to be effective ingranular soils. It must be noted that some authors howeverrecommended a lower ratio of 5 diameters.

Centrifuge tests carried out by Hayward et al. (2000)concluded that a clay slope with piles installed at a spacingof 6.3 pile diameters failed, while the slopes with pilesinstalled at 3.2 and 4.2 diameters did not. Threedimensional finite element modelling of grounddeformations under working loads (ie. pre-failure)investigated the differential movement between the pilesand ground midway between them (Carder and Easton,2001). For both embankment and cutting slopes, littledifferential movement was predicted when piles werespaced at 3 diameter centres whereas significant movementoccurred when the spacing was increased to 6 diameters.

On the basis of the above discussion it is anticipated thatthe spacing between pile centres in a typical stabilisationscheme for a highway slope is likely to be in the range of 3to 5 pile diameters. Pile diameters will to some extent bedictated by site access and the type of rig which can bemobilised, but are generally expected to be between600mm and 1200mm.

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4.2 Lateral resistance supplied by piles

Although most of the existing design approaches enablethe lateral force acting on each pile to be determined, thelateral resistance provided by the piles will be limited bythe yield pressure of the soil both above and below thepotential slip surface. Information on yield pressures isavailable both for the case of a laterally loaded single pilepushing through level ground and creating pressures infront of the pile, and for the movement of soil past existingpiles. Both cases have similarities with smaller yieldpressures near the surface than at depth.

For non-cohesive soils, Broms (1964) used a limitingsoil reaction per unit length of pile of 3K

pσ

v'd where d is

the diameter of the pile. Fleming et al. (1994) suggestedfrom the data of Barton (1982) that at depths of up to 1.5diameters, the limiting force per unit length was bettergiven by K

pσ

v'd. At depths beyond this, K

p2σ

v'd was

considered a better approximation. Although these formulawere derived for a pile installed in level ground, accountcan then be tentatively taken of the slope angle bymodifying the value of K

p according to whether the

resistance of the soil to the top of the pile deflecting or thetoe fixity of the pile is being determined.

For piles in cohesive soil, Fleming et al. (1994) reportsthat the limiting pressure developed on the front of the pilefrom the wedge of the soil near the ground surface isapproximately 2c

ud. It can be conservatively assumed that

this value increases linearly to about 9cud at a depth of 3

pile diameters and then remains constant. Morecomplicated expressions from wedge theory have beenderived by Ashour et al. (1998) and for piles in slopingground by Gabr and Borden (1990).

The ultimate lateral resistance of the lower part of thepile below the potential rupture zone is important. If thepile is short because it is founded in an underlying stiffstratum, its lateral resistance will be high and largebending moments will develop on reversal of curvature asthe pile passes into the founding layer (Carder and Easton,2001). With long piles in more homogeneous ground, thelateral resistance will develop more uniformly along itslength, and the pile bending moments will generally belower. The maximum shear resistance of the pile itself alsoneeds consideration to ensure it has sufficient capacity.

The ultimate load which can be developed on the part ofthe pile embedded in the potential failure or failed zonewill depend on the amount of soil arching or flow betweenpiles as has been discussed in Section 4.1.

4.3 Penetration and location of the piles

The decision on the penetration of the piles is primarilydependent upon the findings of the ground investigation.When using a row of piles to stabilise a cutting orembankment slope or to steepen a slope for highwaywidening purposes, the piles must extend well below theexpected critical failure surface. This ensures that thefailure zone does not increase to encompass the toe of thepiles so obviating the potential support gained from them.The fixity of the toes of the piles is particularly importantas the larger the restoring force they can sustain, thesmaller will be the slope movements that occur.

Little guidance is available in the literature on theoptimum location of the piles in the slope. However, if therow of piles is placed near the top of the slope, asignificant failure may simply occur in front of the piles.Likewise, if the row of piles is installed near the bottom ofthe slope, although heave of the carriageway may beprevented a significant failure may occur behind the piles.Engineering judgement would suggest that the row of pilesis probably better placed about one third to one half of theway up the slope, although this can be validated for aparticular site situation using finite element or othernumerical techniques.

4.4 Overall stability

Methods for determining overall stability are separatelydiscussed in Section 5.4. The normal factors crucial to arealistic stability analysis, as with an unreinforced slope,are the pore water pressure distribution in the slope and theappropriateness of the soil strength parameters. Therelevance of peak, residual and critical state shear strengthparameters has already been discussed in Section 3.5.

It must be noted that the location of the critical failuresurface will change and is likely to become deeper when thepiles are installed and for this reason an iterative procedureneeds to be used to identify the new critical surface.

4.5 Differences between cutting and embankmentsituations

There are significant differences in the behaviour ofunreinforced cutting and embankment slopes, which haveimplications for the design of a strengthening system usingpiles. Firstly, because cuttings are constructed from in situclay the permeability of the clay will generally be lowerthan that of clay fill used for an embankment, and softeningwill therefore proceed at a slower rate over many decadesbefore a delayed collapse may occur (Skempton, 1964;Vaughan, 1994). If clay fill for an embankment has beenexcavated from a deep borrow pit, suctions will exist in theclay lumps and their dissipation may lead to significantsoftening and swelling over a shorter period of time.

The development of the failure zone will also be dependenton the lateral stress within the clay and Potts et al. (1997)reported that for cutting slopes the depth and extent ofprogressive failures tended to increase with over-consolidation ratio. Generally failures with cuttings,although often slower in developing, will be deeper thanthose with embankments. This is because the long termdissipation of suctions (caused by the unloading due toexcavation of the cutting) will result in significantsubsurface heave of the ground within the cutting. For thisreason the depth of penetration and the lateral resistance ofthe stabilising piles generally need to be much larger in acutting situation. Measurements of pile lateral movementsand bending moments showed increases in response toprolonged periods of heavy rainfall as the piles were calledon to provide more support to a cutting slope in Gault Clay(Carder and Barker, 2005a). Emphasis in design thereforeneeds to be placed on the long term condition.

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Carder and Easton (2001) found that for embankmentslopes, the influence of the drainage blanket was significantin improving stability not only because pore pressures arecontrolled but also because the potential rupture is confinedto above the stiff granular layer forming the blanket. Thiswas the case with the analyses of performance of both theunreinforced and pile reinforced slopes.

Finite element analyses carried out by Carder and Easton(2001) also indicated that in a cutting slope, a build-up insoil stress was observed as the top of the pile pusheddownslope although its magnitude will ultimately be limitedby the yield pressure of the soil. The limit pressure methodsof Broms (1964) and Fleming et al. (1994), which arediscussed in Section 4.2, may therefore be appropriate todetermine loads near the top of the pile in this case. In anembankment situation, a build-up in lateral stress waspredicted upslope of the top of the pile and only smallstresses downslope of the pile. This mechanism of theground pushing onto the piles means that methods such asthose of Ito and Matsui (1975) and Wang and Yen (1974),which consider plastic deformation and soil arching betweenpiles respectively, may be more appropriate than limitpressure methods near the top of embankment piles.

4.6 Influence of construction sequence

As discussed in Section 6, the installation of large diameterbored piles involves access for a piling rig which generallyneeds to operate from a firm platform, ie. a bench cut intothe slope. After installation of the row of piles is complete,placement and compaction of fill to reinstate the bench andrestore the slope to its intended shape may act to pre-loadthe piles. This effect was reported by Carder and Barker(2005b) would found that the tops of the piles cantilevereddown-slope with an associated increase in pile bendingmoments during the reinstatement of significant benchingat one particular slope. Subsequent to this pre-loading,little further change in pile movement and moment wasthen recorded over the following two years in service.

Provided that the moment capacity of the pile is notexceeded, this effect is not viewed as a design problem.However, reinstatements using a reduced thickness for filllayers and light compaction plant may be a sensibleprecaution in some critical cases.

Where the piles are not pre-loaded in this way, someslope movement is anticipated before pile resistance can bemobilised. The development of further pile lateralmovements and bending moments may then be in responseto prolonged periods of heavy rainfall as the piles arecalled on to provide more support to the slope (Carder andBarker, 2005a).

4.7 Other issues

In some situations, it may be appropriate to consider theuse of a structural capping beam between the adjacent pileheads. This capping beam could be used either as afoundation for environmental barriers or possibly have theadded benefit of preventing the development of shallowfailures of the slope.

A further option is the installation of ground anchors tosupport the capping beam which would be expected toreduce pile bending moments and also provide moresupport to the upper part of the piles. However, if usinganchors, it should be noted that (a) fixity of the toe of thepile is still required to prevent rotational failure about theanchor head (b) the fixed anchor length must be in firmground beyond the potential zone of any deep-seatedfailure.

These optional construction features were not thesubject of the TRL study, and recourse should be made toBS8002 (1994) and Eurocode 7 (1994) for information ontheir design.

5 Design procedures

In general the current design approaches involve threemain stages, these are:

! evaluating the restoring shear force needed to achievethe required factor of safety for stability of the slope;

! evaluating the shear force that each pile can provide toresist sliding;

! selecting the diameter, centre to centre spacing,penetration and most suitable location on the slope forthe piles.

It does not necessarily follow that these stages need tobe considered in the above order, as they are inter-related.In many cases, for site specific reasons, details of the piledimensions and layout may well be established first.

The alternative fundamental methods of design are nowconsidered in turn, although a hybrid of these methodsmay prove convenient in many cases.

5.1 Methods based on rigid-plastic behaviour

Various authors have developed methods, which are basedboth on a yielding layer that is parallel to the slope and onwedge type failures. Some of the better known methodsare now considered:

a The method reported by Wang and Yen (1974) is fairlyclassical for an infinitely long slope and uses rigid-plastic theory to consider the forces on an element ofsoil in a yielding layer which is parallel to the slope asshown in Figure 1. As would be anticipated they foundmore arching occurred as the strength of the soilincreased, ie. Ø' and c' increased.

For a sandy soil (c'=0) and using the nomenclature asdefined in Figure 1, Wang and Yen found a criticalspacing (m

cr = B/h) as follows:

( )( )1

1 tan

cos tan tancrK K

mi i

+ ∅=

− ∅

where K is the coefficient of earth pressure at rest andassumed to be equal to (1-sin Ø'). At spacings greaterthan m

cr the piles are not likely to provide any

stabilisation. An optimum spacing (mm) was also derived

at which arching is likely to be most effective and thevariation of critical and optimum spacings with Ø' isillustrated in Figure 2 for a 1 in 2 slope.

8

Dow

nslo

pe d

irect

ion

A’

Pilea’

P+dp

b’

b

(a)

(b)

(c)

d

Element

a

P

B

A

h

d

i

Firm layera

dx

xy

dx

P

R1

R2P+dP

W

2.0

Mm

Mcr

10�

Mcr

and

Mm

1.5

1.0

0.5

020� 20� 40�2:1

Figure 2 Relationship between pile spacing and soil properties for 2h:1v sandy slope (After Wang and Yen, 1974)(By courtesy of ASCE)

Figure 1 Plan view of series of piles: (a) on slope; (b) cross-section; (c) generic element (After Wang and Yen, 1974)(By courtesy of ASCE)

9

A similar relationship was developed for cohesive soil(Ø'=0) and this is as follows:

( )( )

2 / cos

cos sin /crc h i

mi i c h

γγ

=−

This equation is plotted in Figure 3 for c equal to c1

and a number of slope angles.

It must be noted that the diameter of the piles is notdirectly involved in the above formula as the arching isassumed to be related to the opening between piles andWang and Yen assumed that pile size would be sufficientlylarge to enable the arching mechanism to develop.

b Typical of the wedge type methods is the approachreported by Day (1999) for drilled piers used to stabiliseslopes. Figure 4 shows a planar rupture surface inclinedat an angle (a) to the horizontal. The factor of safety ofthe slope can then be defined by considering restoringand perturbing forces parallel to the rupture surfaceusing the equation:

i

Factor of safety =

{c'L + (Wcos - uL) tan ' + P }/ sinWα α∅

where L is the length of the slip surface, W is the weightof the failure wedge material, and u is the average porewater pressure along the slip surface. For a particularfactor of safety, the inclined force on the pier (P

i) can

then be calculated and the lateral design force (PL) for

each pier determined using:

cosL iP S P α=

where S is the spacing between pier centres.

5.2 Methods based on plastic deformation

Ito and Matsui (1975) and Ito et al. (1981) considered thestate of plastic deformation in the ground just around thepiles (Figure 5) assuming it satisfied the Mohr-Coulombyield criterion. When piles are placed at intervals along theslope they have a preventive effect against plasticdeformation. A number of equations and design chartswere developed for different soil strengths which enabled

α Proposed pier wall

PiH

Unstable wedge

Slip surface

Figure 4 Design of pier wall for wedge slope failure (After Day, 1999)(By courtesy of ASCE)

20

1:1

4:1

3:1

0

Mcr 10

00.2 0.4

2:1

cγh

cosi sini -

2 . cosi cγh

cγh

Mcr =

Figure 3 Critical spacing of piles in clayey slopes (c1=c) (After Wang and Yen, 1974)

(By courtesy of ASCE)

10

the force acting on the pile to be determined. For examplethe equation for the lateral force (p) acting on a pile perunit thickness of layer is as follows:

1 2

2

1exp tan tan

tan 8 4

D Dp cA N

N D φφ

π φφφ

⎛ ⎧ ⎡ ⎤−⎪ ⎛ ⎞= +⎜ ⎨ ⎢ ⎥⎜ ⎟⎜ ⎝ ⎠⎪ ⎣ ⎦⎩⎝

( )( ) ( )1/ 2 1/ 2

1/ 2

1/ 2

2 tan 22 tan 1

tan 1

N NN

N N

φφφ

φ φ

φφ

φ

−+ + ⎞⎫− − + ⎟⎬ ⎟+ −⎭ ⎠

−

( ) ( )

( )( )

1/ 2 1/ 21/ 2

1 21/ 2

2 tan 22

tan 1

N Nc D D N

N N

φ φφ

φ

φ

φ φ

−−

⎛ ⎞+ +⎜ ⎟− −⎜ ⎟⎜ ⎟+ −⎝ ⎠

1 22

2exp tan tan

8 4

D DzA N D

N D φφ

γ π φφ⎧ ⎫⎡ ⎤−⎪ ⎪⎛ ⎞+ + −⎨ ⎬⎢ ⎥⎜ ⎟

⎝ ⎠⎪ ⎪⎣ ⎦⎩ ⎭

where the constants are A which is equal to D1(D1/D2)b

with b=(NØ

½ tan Ø + NØ -1) and N

Ø =tan2[π/4+Ø/2]. The

soil strength parameters are c and Ø, γ is the unit weightof soil, z is the depth, and D1 and D2 are as defined inFigure 5.

An example chart is shown in Figure 6. In general, for aconstant diameter of pile, the lateral force increases as theinterval between piles becomes progressively narrower.

This is because the soil just around the piles finds it harderto pass through the gap between them and more load istherefore transferred to the piles. On this basis the factor ofsafety of the slope would increase as the spacing betweenpiles decreases and more load is taken by the piles. For thesame reason, the lateral force on the piles increases as thesoil strength parameters c', Ø' increase. Popescu (1991),Cantoni et al. (1989) and others have used the designapproach of Ito and Matsui (1975) with apparent successand more recently the method has been further rationalisedby Hassiotis (1997).

5.3 Numerical analyses

Theories representing a Winkler beam on an elasticfoundation have been widely used in the design of laterallyloaded piles with empirically derived non-linear springs torepresent the soil (p-y curves) or a model of the soil as alinear elastic continuum (Matlock and Reese, 1960;Broms, 1964; Poulos and Davis, 1980). However theaccuracy of such solutions depends upon thecharacterisation of the interaction between the pile and thesurrounding ground. A particularly good representation ofthe soil-pile interaction yields a more realistic solution.Generally the analysis of a row of piles is based on eithersuperposing the behaviour of a number of single piles oron extrapolation of the solution to cover a row of piles

π4

FB

D

Pile

b

Direction ofdeformation

D1D2

F’B’

D’

C’

E’A’

O

A E

Pile

C α = ( + )φ2

( + )

π4 ( − )φ

2

π8

φ4

X

Figure 5 Plastically deforming ground around stabilising pile (After Ito and Matsui, 1975)(By courtesy of the authors)

11

using semi-empirical interaction factors. Most of theseanalyses also consider the soil to have a horizontal surfaceand make no provision for the situation where a row ofpiles is in a slope.

More sophisticated procedures in some cases involvingfinite and boundary element analysis and more complexsoil models have been developed (Rowe and Poulos, 1979;Chen and Poulos, 1993). These methods tend toconcentrate on the ultimate lateral resistance of the pilesrather than approaching spacing design via arching theory.Chen and Poulos (1993) and Yegian and Wright (1973)both concluded that for a single row of piles in a directionperpendicular to the loading, the spacing is generallygreater than 2.5 pile diameters and therefore pileinteraction has only a small influence on their ultimatelateral resistance.

More complex finite element analysis involving threedimensions and a plasticity model for the soil which allowsgap formation have been undertaken by Brown and Shie(1990 & 1991) and in addition to providing a better modelof arching between piles, these can give a better overallview of likely performance. It is not practical to givesimple rules for the evaluation of the optimum spacingbetween piles based on finite element analyses because ofthe large number of variables, although three dimensionalmodels of this nature are expected to provide an effectivedesign method.

5.4 Overall stability analyses

The stability of the slope can be investigated by takingaccount of the extra restoring force provided by the pilesduring calculations which may be based on limitequilibrium methods such as the friction circle method andthe method of slices (Bishop, 1955; Bishop andMorgenstern, 1960; Morgenstern and Price, 1967). This

force can be calculated from the theory of ultimateresistance described by Poulos and Davis (1980).

Figure 7 shows an idealised section through a slopereinforced by a single row of piles where a circular failureis being assumed. Poulos and Davis give the alternativeways in which the maximum value of the restoring forcecan be ascertained, these are:

i from the ultimate lateral resistance of a ‘short’ pile,where full mobilisation of soil strength occurs over thelower part of the pile (l

2);

ii from the ultimate lateral resistance of a ‘long’ pile,where lateral capacity is mainly dependent on the yieldmoment of the pile;

iii from the ultimate load that can be developed if the soilflows between the piles over the upper part (l

1);

iv from the shear resistance of the pile section itself.

For stability the least of the above four forces isappropriate. A sensitivity analysis of the stability of theslope, taking account of the restoring force from the piles,then enables decisions to be made on the pile penetration,its location on the slope, and the pile diameter. It must benoted that the critical failure surface changes when thepiles are installed in the slope and for this reason aniterative procedure needs to be followed.

Viggiani (1981) studied the different failure modes thatcan occur and derived expressions for the maximum shearforce exerted by the pile on the slip surface and of thebending moments acting on the pile. These solutions werefor the ultimate limit state of purely cohesive soils andtheir cohesion was assumed to be constant with depth inboth the unstable and stable zones. Poulos (1973)developed a more rigorous approach with the restoringforce from the piles being calculated from numericalanalysis using boundary elements.

150

30

30

90

120

D2 / D1

Forc

e ac

ting

on p

ile (t

/m)

20

10

0 0.5 1.0

D1 - D

2 = 60cm

γzφc

2 t/m3

5m10�0.1kg/cm2

Figure 6 Effect of pile diameter (D1-D

2) on plastic deformation (After Ito and Matsui, 1975)

(By courtesy of the authors)

12

Hassiotis et al. (1997), following the approach of Ito andMatsui (1975), using integration of the formula for plasticdeformation of the soil between piles (see Section 5.2) todetermine the ultimate load over the upper part of the piles.Their slope analyses based on determination of this forceshowed that, with the piles in place in both cases, theimprovement in the factor of safety of the slope needed totake account of the change in location of the critical failuresurface from its original position (see Figure 8). Althoughthe factors of safety generally increased with the horizontaldistance of the piles from the toe of the slope (S), it shouldbe noted that the optimum location to minimise risk offailures above or below the pile line is likely to be if they areinstalled about one third to one half of the way up the slope.

5.5 Retaining wall methods

An approximate check of the order of magnitude of thebending moments derived using the other methods can becarried out using retaining wall theory. The extraperturbing force on the piles due to the slope can beestimated using two alternative approaches as follows.

i The extra load from the clay slope is accounted for byincreasing the active pressure coefficient (K

a) on the

retained side of the wall to a value of 1. This is themaximum tabulated value for β/ϕ' of 1, where ϕ' is theangle of internal friction of the soil and β is the slopeangle to the horizontal, beyond this the value for β/ϕ' ofgreater than 1 is indeterminate (Eurocode 7, 1994).

ii The height of the clay slope above the tops of the pilesis represented by a uniform vertical surcharge equivalentto 10kN/m2 per metre height. This is about half of thatwhich would exist behind the crest of the slope.

L1

L2

Figure 7 Idealised section through a slope reinforced by a single row of piles

The geometry and parameters adopted when using thetwo approaches are shown in Figure 9. In both cases, thebenefit of the support of the soil wedge in front of the pileis ignored. The water table is assumed at the top of the pilein the retained ground and at cutting level in front of thewall. The conservative assumptions of no wall friction andadhesion can also employed, so that the design looks at theworst case scenario.

On this basis of a continuous wall, the factors of safetyon mobilised soil strength can then be calculated in thenormal way following the recommendations of BS8002(1994). The results will be conservative because therestoring force from the wedge of soil in front of the piledwall has been ignored. If wished some allowance for thestabilising effect of the berm (ie. the wedge in front of thepiled wall) can be made by applying a uniform verticalsurcharge equivalent to about one third of the berm height(Fleming et al., 1991).

Wall bending moments can also be calculated using thisapproach, although the results need to be factored up to takeaccount of the spacing between pile centres. This crudeevaluation generally gives bending moments of the sameorder of magnitude as those predicted by other methods.

6 Site operations

Careful consideration needs to be given in the design to theaccess availability for a piling rig and to the trafficmanagement aspects if appropriate. Installation of largediameter bored piles generally requires a conventionalrotary piling rig, which needs to operate from a firm pilingplatform. If, for example, a row of piles are required atabout three-eighths of the way up a 1v:2h slope, a piling

13

Figure 8 Effect of pile location on factor of safety (After Hassiotis et al., 1997)(By courtesy of ASCE)

987

S

Critical surface after theplacement of the piles

(a) Shallow slope

Original critical surface thatdoes not change with thepresence of the piles

1.9

2.1

1.7

1.8

1.7

1.8

20 2550 10 151.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

2.0FS

S (m)

Slope = 30�

φ = 10�

c = 23.94 kPa

γ = 19.63

H = 13.7m

= 0.6

kNm3

D2D1

S

(b) Steep slope

3.5

4.0

3.0

63 5210 1041.0

1.5

2.5

2.0

FS

S (m)

Slope = 49�

φ = 10�

c = 23.94 kPa

γ = 19.63

H = 9.14m

= 0.6

kNm3

D2D1

14

Ignore benefit of this wedge

Load from soil wedge accounted forby increasing K in underlying layer

(a) Increasing K to account for slope

Ignore benefit of this wedge

Load from soil wedge accounted forby applying equivalent surcharge

(b) Adding in equivalent surcharge

Surcharge

Figure 9 Using retaining wall design approaches

15

platform will be needed between the pile line and the toeof the slope. Generally, as a rule of thumb, piling rigs ofthis type have a reach of about 4m and can drill intoground that is no more than about 1m above platformlevel. In cases where a carriageway is near the toe of theslope, traffic management measures may therefore benecessary to provide a safe working zone.

At sites where access is not available for a conventionalpiling rig either because of lack of space or because thelower section of the slope is not stable enough to supportits weight, consideration can be given to using a morelightweight tripod mounted rotary drilling rig. Tripod rigsare available which will enable bored piles of up to about700mm diameter to be constructed. Although progress willbe slower than with larger machines, the construction workis then more confined to the slope area. Trafficmanagement measurements, which can be a significantcost implication, may then not need to be so extensivealthough consideration may need to be given to screeningto prevent driver distraction.

7 Summary

A programme of research has been carried out on thetechnique of installing a single row of spaced bored pilesto provide additional support to clay highway slopes. Themain findings and factors studied were as follows:

i Piles spaced at intervals along a clay slope provide aneffective remedial measure and improve the stability ofslopes that have either failed or are showing incipientsigns of failure.

ii The technique can also be used to steepen slopes so thathighway corridors can be widened within existingboundaries to accommodate extra vehicular lanes orother modes of transport. In this instance the techniquemay need to be used in conjunction with othertechniques to prevent shallow instabilities developing.

iii Guidance on the nature of the site investigation(including an assessment of environmental impacts andsite specific details relevant to plant operations) aregiven in the report. For the geotechnical design it isessential to identify the potential or active failure planeor zone, any water-bearing or weak strata, and a stifffounding stratum for the piles.

iv The main factors for consideration in the design includethe spacing and diameter of piles, their lateralresistance, the optimum penetration and location of thepiles, and overall stability. The depth of penetration andrequired lateral resistance of stabilising piles in acutting are likely to be greater than in an embankmentsituation.

v Current design approaches are aimed at evaluating therestoring force required to stabilise the slope, therequired shear and bending moment capacities of thepiles, and pile dimensions and layout. The currentlyavailable design methods are discussed in the report.Approximate methods of using retaining wall theoryalso proved a useful guide to performance.

vi The recommendations within the report are generallyapplicable for clay highway slopes which are typicallyup to about 10m in height. For higher slopes more thanone row of piles or anchoring of the pile heads may beconsidered necessary for stability.

8 Acknowledgements

The work described in this report forms part of theresearch programme of the Infrastructure Division of TRLand was funded by SSR Geotechnics Operations SupportDivision of the Highways Agency. The HA ProjectSponsor for the study was Mr R K W Lung.

9 References

Ashour M, Norris G and Pilling P (1998). Lateralloading of a pile in layered soil using the strain wedgemodel. J Geotechnical and GeoenvironmentalEngineering, ASCE, vol 124, no 4, pp. 303-315.

Barton Y O (1982). Laterally loaded model piles in sand.PhD Thesis, University of Cambridge.

Bishop A W (1955). The use of the slip circle in the stabilityanalysis of slopes. Geotechnique, vol. 5, no. 1, pp. 7-17.

Bishop A W and Morgenstern N R (1960). Stabilitycoefficients for earth slopes. Geotechnique, vol. 10, no. 4,pp. 129-147.

British Standards Institution (1990). BS1377: Methodsof test for soils for civil engineering purposes. BritishStandards Institution, London.

British Standards Institution (1994). Eurocode 7:Geotechnical design, Part 1, DD ENV 1997-1:1995.London: British Standards Institution.

British Standards Institution (1994). Code of practice forearth retaining structures BS8002. London: BritishStandards Institution.

British Standards Institution (1999). BS5930: Code ofpractice for site investigation. London: British StandardsInstitution.

Broms B B (1964). Lateral resistance of piles in cohesivesoils. J Soil Mech Foundn Div, ASCE, vol. 90, no. SM2,pp. 27-63.

Brown D A and Shie C-F (1990). Three dimensionalfinite element model of laterally loaded piles. Computersand Geotechnics, vol. 10, no. 1, pp. 59-79.

Brown D A and Shie C-F (1991). Some numericalexperiments with a three dimensional finite element modelof a laterally loaded pile. Computers and Geotechnics,vol. 12, no. 2, pp. 149-162.

16

Cantoni R, Collotta T, Ghionna V N and Moretti P C(1989). A design method for reticulated micropilestructures in sliding slopes. Ground Engineering, vol. 22,no. 4, pp. 41-45.

Carder D R and Barker K J (2005a). The performanceof a single row of spaced bored piles to stabilise a GaultClay slope on the M25. TRL Report TRL627. Crowthorne:TRL Limited.

Carder D R and Barker K J (2005b). The performanceof a single row of spaced bored piles to stabilise a LondonClay slope on the A12. ISS/01/05. Crowthorne: TRL Limited.(Unpublished report available on direct personalapplication only)

Carder D R and Bush D I (2001). Development andtesting of load cell pressuremeter. Proc Institution CivilEngineers, vol. 149, no. 3, pp. 141-142.

Carder D R and Easton M R (2001). Analysis ofperformance of spaced piles to stabilise embankmentand cutting slopes. TRL Report TRL493. Crowthorne:TRL Limited.

Carder D R and Temporal J (2000). A review of the useof spaced piles to stabilise embankment and cutting slopes.TRL Report TRL466. Crowthorne: TRL Limited.

Chen L and Poulos H G (1993). Analysis of pile-soilinteraction under lateral loading using infinite and finiteelements. Computers and Geotechnics, vol. 15, no. 4,pp. 189-220.

Crabb G and Atkinson J (1991). Determination of soilstrength parameters for the analysis of highway slopefailures. In: Proceedings of the International Conference onSlope Stability, ‘Slope stability engineering: Developmentsand applications’ (Chandler R J Ed), pp. 13 –18. London:Thomas Telford.

Day R W (1999). Discussion on ‘Design method forstabilization of slopes with piles’. J Geotechnical andGeoenvironmental Engng, ASCE, vol. 125, no. 10,pp. 910-911.

Dunnicliff J (1988). Geotechnical instrumentation formonitoring field performance. New York: John Wiley.

Fleming W G K, Weltman A J, Randolph M F andElson W K (1994). Piling Engineering. Glasgow: BlackieGroup, Bishopbriggs.

Gabr M A and Borden R H (1990). Lateral analysis ofpiers constructed on slopes. J Geotechnical Engineering,ASCE, vol. 116, no. 12, pp. 1831-1850.

Geotechnical Consulting Group (1993). The design ofslopes for highway cuttings and embankments. ProjectReport PR/CE/37/93. Crowthorne: TRL Limited.(Unpublished report available on direct application only).

Hanna T H (1985). Field instrumentation in GeotechnicalEngineering. Clausthal-Zellerfield, Germany: Trans. Tech.Publications.

Hassiotis S, Chameau J L and Gunaratne M (1997).Design method for stabilization of slopes with piles. JGeotechnical and Geoenvironmental Engng, ASCE, vol.123, no. 4, pp. 314-323.

Hayward T, Lees A, Powrie W, Richards D J andSmethurst J (2000). Centrifuge modelling of a cuttingslope stabilised by discrete piles. TRL Report TRL471.Crowthorne: TRL Limited.

Ito T and Matsui T (1975). Methods to estimate lateralforce acting on stabilising piles. Soils and Foundations,vol. 15, no. 4, pp. 43-59.

Ito T, Matsui T and Hong W P (1981). Design methodfor stabilizing piles against landslide – one row of piles.Soils and Foundations, vol. 21, no. 1, pp. 21-37.

MacNeil D J, Steele D P, McMahon W and Carder D R(2001). Vegetation for slope stability. TRL Report TRL515.Crowthorne: TRL Limited.

Matlock H and Reese L C (1960). Generalised solutionsfor laterally loaded piles. J Soil Mech Foundn Div, ASCE,vol. 86, no. SM5, pp. 63-91.

Meigh A C (1987). Cone penetration testing: methods andinterpretation. CIRIA Ground Engineering Report: In-situTesting. London: Butterworths.

Morgenstern N R and Price V E (1965). The analysis ofthe stability of general slip surfaces. Geotechnique, vol. 15,pp. 79-93.

Perry J (1989). A survey of slope condition on motorwayearthworks in England and Wales. Research Report RR199.Crowthorne: TRL Limited.

Popescu M E (1991). Landslide control by means of a rowof piles. Slope stability engineering, pp. 389-394. London:Thomas Telford.

Potts D M, Kovacevic N and Vaughan P R (1997).Delayed collapse of cut slopes in stiff clay. Geotechnique,vol. 47, no. 5, pp. 953-982.

Poulos H G (1973). Analysis of piles in soil undergoinglateral movement. J Soil Mech Foundn Engng Div, ASCE,vol. 99, no. SM5, pp. 391-406.

Poulos H G and Davis E H (1980). Pile foundationanalysis and design. New York: John Wiley & Sons.

Ridley A, Brady K C and Vaughan P R (2003). Fieldmeasurement of pore water pressures. TRL Report TRL555.Crowthorne: TRL Limited.

17

Rowe R K and Poulos H G (1979). A method forpredicting the effect of piles on slope behaviour. ResearchReport No R340, University of Sydney, Australia.

Skempton A W (1964). Long-term stability of clay slopes.Geotechnique, vol. 14, no. 2, pp. 77-101.

Vaughan P R (1994). Assumption, prediction and realityin geotechnical engineering. Geotechnique, vol. 44, no. 4,pp. 573-609.

Viggiani C (1981). Ultimate lateral load on piles used tostabilise landslides. Proc 10th Int Conf Soil Mech FoundnEngng, Stockholm, vol. 3, pp. 555-560.

Wang W L and Yen B C (1974). Soil arching in slopes.J Geotechnical Engineering Division, ASCE, vol. 100,no. GT1, pp. 61-78.

Yegian M and Wright S G (1973). Lateral soilresistance-displacement relationships for pile foundationsin soft clays. Proc 5th Annual Offshore Tech Conf, Houston,vol. 2, pp. 663-676.

18

Abstract

A single row of bored piles spaced at intervals along a clay slope provides an effective remedial measure toimprove the stability of slopes that have either failed or are showing incipient signs of failure. The technique canalso be used to steepen slopes so that highway corridors can be widened within existing boundaries toaccommodate extra vehicular lanes or other modes of transport. Although the piling technique is now becomingmore frequently used on the highway network, further design guidance is needed to ensure economy of design andsafe practice. This report provides additional guidance based on a programme of research which has included areview of current practice, centrifuge and analytical modelling, and a full scale instrumented trial on the network.

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TRL627 The performance of a single row of spaced bored piles to stabilise a Gault Clay slope on the M25by D R Carder and K J Barker. 2005 (price £30, code EX)

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