Time-dependent behavior of pile groups by staged construction of an adjacent embankment on soft clay

Time-dependent behavior of pile groups by staged construction of an adjacent embankment on soft clay

Sangseom Jeong,Donghee Seo,Jinhyung Lee,and Joongbai Park

Abstract:A series of centrifuge model tests were performed to investigate the behavior of pile groups subjected to lateral soil movements by surcharge loading from approach embankments.The emphasis was on quantifying the time-dependent response in terms of deflections,bending moments,and earth pressures acting on pile groups during em-bankment construction and over short-and long-term periods after embankment construction.A variety of instruments were used to examine the soil–pile interaction for pile groups adjacent to surcharge loads.Through these studies,it is found that pile cap deflections and bending moments developed to their maximum values under the short-term sur-charge loading and decreased gradually to minimum values under the long-term loading.The ground settlement reached its maximum value under long-term loading,however,due to the consolidation of soft clay.It is also found that the lateral mean pressure acting on the pile is about0.75and0.35times the surcharge load q(=γH,whereγis the unit weight of the soil and H is the height of the embankment)under short-and long-term loading,respectively.

Key words:time-dependent response,lateral soil movements,pile groups,centrifuge model tests,surcharge loads,soft clay.

Résumé:On a réaliséune série d’essais sur modèle au centrifuge pourétudier le comportement de groupes de pieux soumisàdes mouvements latéraux du sol produits par la surcharge de remblais d’approche.L’emphase aétémise sur la quantification des réponses en fonction du temps des déflexions,des moments fléchissants et des pressions des terres agissant sur les groupes de pieux durant la construction du remblai et pour des périodesàcourt etàlong terme après la construction du remblai.Une diversitéd’instrumentation aétéutilisée pour examiner l’interaction sol-pieu pour les groupes de pieux adjacents aux surcharges.àla suite de cesétudes,on a trouvéque les déflexions des chapeaux de pieux et les moments fléchissants se sont développésàleurs valeurs maximalesàcourt terme lors de la mise en place de la surcharge,et ont diminuégraduellementàdes valeurs minimalesàlong terme.Cependant,le tassement du terrain a atteint sa valeur maximaleàlong terme par suite de la consolidation de l’argile molle.On a trouvéégalement que la pression latérale moyenne agissant sur le pieu est d’environ0.75foisà0.35fois la valeur de la surcharge q(=γH)àcourt etàlong terme respectivement.

Mots clés:réaction en fonction du temps,mouvements latéraux du sol,groupes de pieux,essais sur modèle au centri-fuge,valeurs de surcharge,argile molle.

[Traduit par la Rédaction]

Introduction

When an embankment on soft clay forms an approach to an abutment supported by a pile group,time-dependent movements within the clay may produce significant lateral deflections and loads on the piles.The lateral loads resulting from the soil movements induce deflections and bending moments in the pile,which may lead to unserviceable move-ment,damage,or structural failure(Moulton et al.1985; Barker et al.1991).

The results of centrifuge model studies are generally of significant interest,since the tests are carried out under con-trolled conditions with defined geometry and material prop-erties.Though a limited number of field studies(Heyman 1965;Leussink and Wenz1969;Ingold1977)have been reported in the literature,many centrifuge model studies (Springman1989;Stewart1992;Bransby1995;Ellis1997) of passive pile loading due to adjacent surcharge on the soft clay layer have been performed and are described in Table1. The clay layer is characterized by depth(h c)and initial un-drained shear strength(c u)at mid-depth.Surcharge loads(q) were provided by either pressure systems(which do not model shear loading along the clay layer surface)or con-struction of a sand embankment inflight(Fig.1).Most stud-ies have examined the behavior of piles mainly during an embankment construction period rather than during a post-construction period.In most of the studies,the pile groups

644

Received14February2003.Accepted16February2004.

Published on the NRC Research Press Web site at

http://cgj.nrc.ca on12August2004.

S.Jeong,D.Seo,and J.Lee.Department of Civil

Engineering,Yonsei University,Seoul120-749,Korea.

J.Park.1Civil Engineering II Team,Daewoo Engineering& Construction Company,Seoul100-714,Korea.

1Corresponding author(e-mail:soj9081@yonsei.ac.kr).

had a free pile-tip condition and were located at the edge of a surcharge load with which there was no direct interaction (Figs.1a ,1b ),although the pile group carried a retaining structure in some cases (Fig.1c ).The pile group is charac-terized by the pile diameter (d p ),flexural stiffness ((EI)p ),length of embedment in a dense substratum providing active resistance (h a ),and group geometry (s 1and s 2,where s 1is the spacing along the pile row and s 2is the spacing between the rows),as shown in Table 1.

In South Korea,a number of large foundation projects such as highways,high-speed railways,and grand bridges are in progress in urban and coastal areas.Driven and drilled piles are frequently used in these areas.Over 90%of piles constructed in Korea are embedded in weathered rocks through the uppermost weathered soil deposit.This paper describes the results of a series of centrifuge model tests aimed at clarifying the behavior of pile groups embedded in rocks through soft clay.Special attention is given to the de-flections,earth pressures,and bending moments on piles with varying clay layer depth and the rate of embankment

construction.Also,the time-dependent response of pile groups is compared and discussed for the short-term condi-tion immediately after finishing embankment construction and a long-term condition representing 2years (95%consol-idated stage at prototype scale)following embankment con-struction.

Centrifuge model tests

Testing program

Six plane strain tests were performed on 1/40th scale models for different soil profiles and rates of embankment construction,as shown in Fig.2.Three piles in each of two rows were positioned normal to the section.The tests exam-ined two different model clay depths of about 280mm and 133mm (11.2m and 5.3m at prototype scale).In all cases the pile tip was fixed to simulate the piles embedded in rocks through the soft clay deposit.The rate of embankment construction was also varied,with two incremental loadings (1m per 30days and 1m per 15days)placed in six lifts and

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Table 1.Summary of results for centrifuge model tests of Springman (1989),Stewart (1992),Bransby (1995),and Ellis

(1997).

Fig.1.Centrifuge model test configurations for previous cases:(a )Springman (1989),(b )Stewart (1992),(c )Ellis (1997).

instant loading in one lift to final embankment height.The response of pile groups with time was considered by exam-ining the short-term behavior immediately after finishing embankment construction and subsequent long-term re-sponse after2years(95%consolidated stage at prototype scale)following embankment construction.Plan and sec-tional views of the test devices are given in Fig.3. Testing apparatus

The geotechnical centrifuge used in this study was an Actidyn System C65-2,with a platform radius of3.0m and a maximum capacity of120g-tons.The model tests were performed at a radial acceleration of40g.

Model piles were made of aluminum alloy with a hollow circular section,400mm embedded length,20mm outer diameter,and3mm thickness.They are equivalent to rein-forced concrete piles of length16m,outer diameter 800mm,and bending rigidity3144MN·m2at prototype scale.The model piles were arranged in two rows of three piles,and the center to center spacing of the piles was 66mm,or3.3times the pile diameter in all directions.The pile tips were rigidly socketed into a plastic block under the dense sand layer,and the pile top was attached to a pile cap, which nearly created a fixed-head condition.The pile cap was made of aluminum plate of width106mm,length 177mm,and thickness30mm.Two instrumented piles (Fig.4)were made by bonding miniature strain gauges in pairs at seven locations along the piles to measure the bend-ing moment using a fully active bridge circuit.The strain gauges were then coated with epoxy resin and sealed with a thin waterproof membrane to prevent mechanical damage.One of the instrumented piles was positioned to the side of the front row and the other was just behind it in the rear row (Fig.3).Precalibrated Kyowa PS-5KA earth pressure trans-ducers were installed on the external surface of the center piles(Fig.4)to measure the lateral earth pressures acting on the piles due to the surcharge load.

A strong sample box800mm long,198mm wide,and 500mm high was made with a stainless steel frame and a transparent front window.The water supply and drainage system in these model tests were designed to simulate the two-way drainage conditions of the soft clay layer.

A sand hopper was designed to form a uniform soil bed with the required density.It is automatically controlled by a computer system according to the traveling velocity of the hopper and the height and quantity of pluviation.

The loading system was planned to simulate the vertical surcharge load applied by an embankment,with no shear stress applied directly to the surface.This is carried out using a pressure-controlled actuator,which consists of a ver-tical loading frame with a rigid plate(197×330mm).In these model tests,the step-by-step loading was applied so that the pressures on the surface would be as uniform as pos-sible.The rigid plate is in direct contact with the soil surface and therefore is greased externally to eliminate nearly all of the shear stress that would be developed during the tests.

A variety of instruments were installed to monitor dis-placements and pore pressures during the testing.The dis-placements of the pile cap and ground surface were measured by three linear variable differential transformers (LVDTs)located on both sides(A,B)of the ground surface and the pile cap,as shown in Fig.3.The pore pressure in the

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Fig.2.Centrifuge model test configurations.

clay layer was measured using six pore pressure transducers (PPTs),and their locations in the model are also shown in Fig.3.

Sample preparation

The inside of the sample box was greased with silicon jelly to minimize wall friction.The sand layer was prepared first by pouring Joomunjin sand(Table2)from a sand hop-per to form a uniformly dense sand bed with an approximate relative density of D r=84%.Deaired water was applied to the bottom of the soil bed under a piezometric head to pre-vent hydraulic fracture and air entry while saturating the sand layer.Kimhae clays(Table2),which are identified as representative soft clays in South Korea,were selected to form a clay layer.The clay slurry was mixed mechanically at a water content of80%using deaired water.The clay slurry was then deaired for at least22h under a vacuum.The deaired clay slurry was placed carefully on top of the dense sand bed to approximately two or three times the required depth prior to the initial consolidation stage.The pre-consolidation of the slurry was performed with a gradual build up of pressure to16.9kN/m2,using the actuator mounted on the strong box.Prior to installation of the PPTs, each transducer was deaired under water to ensure its re-sponse to pore pressure.After completing preconsolidation, the deaired PPTs were inserted into the desired locations (Fig.3)by augering horizontally to half the sample width with the aid of a guide device.The holes were backfilled with the slurry using a syringe and plugged with a spiral bolt.A20mm thick saturated sand layer was then loosely

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Fig.3.Plan and section views.

prepared with D r=65%approximately as uniformly dense bed.This sand layer prevented unexpected swelling or desic-cation of the clay surface and enabled the water table to re-main at the height of the clay surface in the center of the model.

Model testing

After completing sample preparation,the strong box was mounted on the platform of the centrifuge.The progress of consolidation was monitored for22h at the40g level to al-low equilibration of pore pressures in the clay layer before the surcharge loading.Surcharge loading was applied by means of a pressure-controlled actuator.Tests were per-formed for two loading systems:(i)incremental loading ap-plied in six lifts to the final embankment height(6m),and (ii)instant loading corresponding to the final embankment height applied in one lift quickly.The stress history of incre-mental loading was chosen to be either1m per30days (180days)or1m per15days(90days),as shown in Fig.5. The typical loading histories are described in https://www.360docs.net/doc/9e18848314.html,stly,short-term tests were performed immediately after finishing embankment construction,and the subsequent long-term tests within2years(95%consolidated stage at prototype scale)following embankment construction.

At the end of each test,the centrifuge was stopped and Pilcon hand vane shear tests were performed immediately in the clay layer.Tests were carried out using both a small vane 19mm in diameter and a larger vane33mm in diameter. Tests were done in several positions with depth,and the results are shown in Fig.6.The clay was then carefully ex-cavated and the locations of the PPTs were measured to al-low for the settlement of the transducers during the data processing and analysis.

Test results and discussion

Some typical results from centrifuge model tests are pre-sented here.The data are presented in terms of prototype units scaled up according to the relevant scaling relation-ships(Schofield1980).

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Fig.4.Pile group instrumentation.(a)Strain gauge profiles,model scale.All dimensions in millimetres.(b)Earth pressure transducer profiles,model scale.All dimensions in millimetres.

Jeong et al.649 Fig.4.(concluded).

Fig.5.Construction rate of embankment.Fig.6.Vane strength profiles.

Excess pore pressure in the clay layer

Figures 7and 8show the dissipation of excess pore pres-sure for PPT2in tests 1–3.PPT2is located at about mid-depth in the clay layer in the front row side.The data are shown from the start of construction to a time period of 900days,after full embankment construction and subse-quent consolidation.In the postconstruction period of about 800days,the pore-water pressures in all cases were found to recover to hydrostatic conditions.

The immediate increase in excess pore pressure (?u i )for each loading stage was determined from each PPT.Accord-ing to traditional one-dimensional consolidation theory,the excess pore pressure (?u i )is close to the nominal increment of total vertical stress (?σv ).For PPTs 1–3,the incremental sum of all construction stages (Σ?u i )is in the range of 108–118kN/m 2,which compares well with the total stress in-crease of q =114kN/m 2.However,Σ?u i of PPTs 4–6was about 0.35q for the 11.2m clay layer and about 0.15q for the 5.3m clay layer,since PPTs 4–6were located away from the surcharge load (Fig.3).

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T a b l e 2.M a t e r i a l p r o p e r t i e s o f t e s t p i l e a n d s o i l s .

Table 3.Loading history used for the 11.2m clay layer.

Fig.7.Excess pore pressure in the clay layer (construction pe-riod).

The distinction among the rates of construction is evident.For the one-lift instant loading stage,the maximum excess pore pressure is close to the nominal embankment load of 110kN/m 2,implying virtually undrained loading.In the six-lift incremental loading stage,the maximum excess pore pressure is about 85%of the nominal embankment load for 1m per 15days and 70%for 1m per 30days.

Figure 9shows the excess pore pressure profiles for the final loading stage (short-term)and the 95%consolidated stage (long-term)in the clay layer.Excess pore pressures beneath the surcharge load were distributed typically throughout the depth of clay layer,as would be expected in a two-way drainage condition,and were found to continue to rise over the construction period,followed by 95%dissipa-tion during the postconstruction period.

Ground settlement

The soil settlement was measured using LVDTs installed on the ground surface.Figure 10shows the ground settle-ment profiles along the front row side (A)and the rear row side (B)of the pile group from the start of construction to a time period of 900days.The profiles are similar in shape,

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Fig.8.Excess pore pressure in the clay layer (long-term

period).

Fig.9.Excess pore pressure profiles (11.2m clay

layer).

Fig.10.Time-dependent ground settlement on (a )front row side A and (b )rear row side B with different soil profiles.

but there are significantly different magnitudes for different soil layers,loading sides(A,B),and the rate of embankment construction.The magnitude and extent of the ground settle-ments on side A were greater in the11.2m clay layer with instant loading(test3)than in the5.3m clay layer with in-cremental loading(test5).This effect was reversed on side B because there was ground heave adjacent to the rear piles (side B)under instant loading,where more or less constant volume conditions may be assumed.

Examination of the final loading stage of all tests with in-cremental loading suggests that about70%of the ground settlement occurred during the embankment construction pe-riod,with the remaining30%occurring during ongoing con-solidation.Test3with instant loading,however,indicates that about30%of the ground settlement occurred immedi-ately after final surcharge loading,with the remaining70% occurring during the consolidation stage.

Pile bending moment distributions

Bending moment profiles along the model piles were computed from the strain gauge data for each test.In gen-eral,similar bending moment distributions were observed for each pile for various stages of surcharge loading. Figure11shows the bending moment profiles for the front row piles(Figs.11a,11b)and rear row piles(Fig.11c) within the pile group.The distributions in each row show generally similar shapes in all cases,although the magnitude is clearly related to the rate of embankment construction. The front and rear row piles show different distributions:the maximum bending moments of the front piles were devel-

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Fig.11.Measured pile bending moment profiles:(a)front row piles,tests1and2;(b)front row piles,test3;(c)rear row piles,tests 1and2.

oped in the upper portion of the clay layer rather than at the pile head (near the pile cap),but those of the rear piles showed their maximum values at the pile head.These results compare well with the general trend observed by Stewart et al.(1994).The location of the maximum bending moment was dependent on the depth of the clay layer and the rate of embankment construction.

According to the rapid rate of embankment construction from 1m per 30days (test 1)to 1m per 15days (test 2),it was found that the degree of reduction of maximum bending moment for front row piles was increased on average from 20%to 50%during ongoing consolidation.For the one-lift instant loading (test 3),however,the degree of reduction was almost similar to the case of 1m per 15days (test 2).It is

shown that pile behavior in all tests is closely related to the average degree of consolidation,which is changed by the rate of embankment construction.

Pile deflection profiles

The measured bending moment profiles were analyzed us-ing a polynomial curve-fitting technique to examine lateral deflections.Pile displacement profiles are derived by inte-gration,using the measured pile head displacement and the fixity condition of the pile tip,which does not allow for dis-placement and rotation.

Figure 12shows the deduced lateral deflection profiles for the front row piles in tests 1and 3(Fig.12a )and test 5(Fig.12b ).Although the deduced pile displacement profiles show a more or less different trend with pile tip conditions in tests due to the extrapolation of the moment curve fit beyond the lowest data point,the overall behavior of pile groups is qualitatively identified.As a result,pile deflection was larger for instant loading (test 3)than for incremental loading (test 1,3).For tests 1and 5with incremental load-ings,the lateral pile deflections induced over the long-term period (over 95%consolidated stage)increased by about 5%–25%compared with those for the short-term period,whereas the lateral pile deflection over the long-term period caused a 20%reduction compared with the value of the short-term period for test 3with instant loading.

Lateral earth pressure profiles

Figure 13shows the lateral earth pressure distributions obtained from the measured earth pressure transducer data

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Fig.12.Deduced pile displacement profiles:(a )11.2m clay layer,front row piles;(b )5.3m clay layer,front row

piles.

Fig.13.Measured lateral earth pressure profiles.

under staged loadings(test1)and instant loading(test3). The lateral earth pressure acting on the piles was increased gradually with increasing surcharge loads.Values were much higher for instant loading than for staged loading and were also greater during the construction period(short term) than during the consolidation period(long term).

It was shown that the lateral pressure in the clay layer de-veloped more or less uniformly up to the third embankment stage;beyond that,the maximum value occurred at the mid-depth in the clay layer,implying a threshold load in which plastic flow developed between the third and fourth stages. Based on the plastic flow mechanism,the lateral mean pres-sure(p m)acting on the pile was proportional to the nearby surcharge load(γH)and is described as follows:

[1]p H

m

=αγ()

whereαis the proportional constant,γis the unit weight of the soil,and H is the height of the embankment.To calculate theαfactor,the developed lateral pressures were measured for each embankment construction stage.Table4showsαvalues for different construction stages and time-dependent short-and long-term loadings.Based on this,it is to be noted that the lateral mean pressure acting on the pile is about0.75q(=γH)during the construction period and about 0.35q at the95%consolidated stage.It is important to men-tion that the lateral earth pressure acting on piles is closely related to the degree of consolidation,which is changed by the rate of embankment construction.

Rate effect with increasing embankment load

The response of the piles with increasing embankment height is shown in Fig.14,as the pile cap deflection (Fig.14a),maximum bending moment in each pile(M max) within the group(Fig.14b),and ground settlement are plot-ted against the average vertical stress from the surcharge load(Fig.14c).Pile cap deflections,bending moments,and ground settlement increased rapidly with increasing embank-ment loads.These observations are consistent with other centrifuge data for embankment construction on soft clay (Stewart et al.1994;Bransby and Springman1997).Their values were much higher for instant loading than for staged loading,however,because pore pressures rose virtually in-stantaneously under instant loading.

Figure14also shows the consolidation effect for short-and long-term sequences.On the one hand,the highest values of pile cap deflection and moment occurred during short-term loading(undrained phase),directly after complet-ing the embankment construction.Consolidation under the long-term surcharge loads caused a20%–50%reduction in bending moments,while the cap displacement increased gradually,by around5%–25%.On the other hand,ground settlement(side A)was greater under long-term loading due to consolidation.

Concluding remarks

The main objective of the paper was to investigate experi-mentally the response of pile groups adjacent to surcharge loads.A limited study of the response of centrifuge tests was performed to examine the rate effect of embankment con-struction,two different loading histories,and fixed pile tip condition,which does not allow for displacement and rota-tion.The emphasis was on quantifying the time-dependent behavior of induced earth pressures,bending moments,and deflections on piles with varying soft clay layer depth and increasing embankment construction height.Based on the results obtained,the following conclusions are drawn: (1)Maximum values of pile cap deflections and bending

moments developed under short-term surcharge loading.

Consolidation under the long-term surcharge loads caused a20%–50%reduction in the maximum bending moments.On the contrary,ground settlement developed maximum values under long-term loading due to con-solidation.

(2)The lateral earth pressure acting on a pile is closely re-

lated to the degree of consolidation,which is changed by the rate of embankment construction.Based on model tests,the lateral mean pressure(p m)acting on the pile was0.75and0.35times the applied surcharge load-ing q(=γH)under short-and long-term loading,respec-tively.Therefore,a commonly used method like the Tschebotarioff(1973)equation for calculating the lat-eral pressure can substantially overestimate the lateral pressure acting on a pile in realistic situations.

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Table4.Values ofαfor short-and long-term loadings.

(3)Pile bending moments are of most concern for the de-

sign of the piled structure.The front and rear row piles show different bending moment distributions:the maxi-mum bending moment of the front piles was developed at the upper portion of the clay layer rather than at the pile head(near the pile cap),but that of the rear piles showed maximum values at the pile head.The location of the maximum bending moment was highly dependent on the clay layer depth and the rate of embankment con-struction.

Acknowledgements

The research described in this paper was funded by the Korea Infrastructure Safety&Technology Corporation,and the support of the Daewoo Institute of Construction Technol-ogy in the utilization of the geotechnical centrifuge is gratefully acknowledged.

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Bransby,M.F.1995.Piled foundations adjacent to surcharge loads.

Ph.D.thesis,University of Cambridge,Cambridge,UK. Bransby,M.F.,and Springman,S.M.1997.Centrifuge modelling of pile groups adjacent to surcharge loads.Soils and Founda-tions,Japanese Geotechnical Society,37(2):39–49.

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Fig.14.Pile responses with increasing embankment load:(a)pile cap deflections;(b)maximum bending moments;(c)settlements.

Ellis, E.A.1997.Soil–structure interaction for full-height piled bridge abutments constructed on soft clay.Ph.D.thesis,Univer-sity of Cambridge,Cambridge,UK.

Heyman,L.1965.Measurement of the influence of lateral earth pressure on pile foundation.In Proceedings of the6th Interna-tional Conference of Soil Mechanics and Foundation Engi-neering,Montréal,Que. A.A.Balkema,Rotterdam,The Netherlands.Vol.2,pp.257–260.

Ingold,T.S.1977.A field study of laterally loaded piles.In Pro-ceedings of the9th International Conference of Soil Mechanics and Foundation Engineering,Specialty Session10,Tokyo,Ja-pan.A.A.Balkema,Rotterdam,The Netherlands.pp.77–80. Leussink,H.,and Wenz,K.P.1969.Storage yard foundations on soft cohesive soils.In Proceedings of the7th International Con-ference of Soil Mechanics and Foundation Engineering,Mexico City,Mexico,25–29August1968.A.A.Balkema,Rotterdam, The Netherlands.Vol.2,pp.149–155.Moulton,L.K.,GangaRao,H.V.S.,and Halvorsen,G.T.1985.Tol-erable movement criteria for highway bridges.Final report FHWA/RD-85/107,U.S.Federal Highway Administration, Washington,DC.

Schofield,A.N.1980.Cambridge geotechnical centrifuge opera-tions.Géotechnique,30(3):227–268.

Springman,https://www.360docs.net/doc/9e18848314.html,teral loading of piles due to simulated embankment construction.Ph.D.thesis,University of Cam-bridge,Cambridge,UK.

Stewart,https://www.360docs.net/doc/9e18848314.html,teral loading of piled bridge abutments due to embankment construction.Ph.D.thesis,University of West-ern Australia,Perth,Australia.

Stewart,D.P.,Jewell,R.J.,and Randolph,M.F.1994.Design of piled bridge abutments on soft clay for loading from lateral soil movements.Géotechnique,44(2):277–296. Tschebotarioff,G.P.1973.Foundations,retaining and earth struc-tures.2nd ed.McGraw-Hill,New York.pp.365–414.

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