PAPER SOIL-STRUCTURE INTERACTION OF PILED BRIDGE ABUTMENTS CONSTRUCTED ON SOFT CLAY

Fu/I-height piledbridge abutments constructed on soft

clay maybe vulnerable to lateral interaction effectswhich would not be consideredinroutine design. This

paper discusses the nature ofsuchmechanisms, and

demonstrates their potential significance using theresults of a series of geotechnical centrifuge tests.

Soil-structure

Approach embankment ~Abutment wall

Pile cap

m'-.~»;»»w»;:-.-:~»»»»~-»»".m~»»»»»»-=:

'ftp)l'IIMItrernbnt: . "

interaction oI'piledbridge abutggIentsconstructes on so1 t

F~f

Rgure 1.geg meeement near a pged bridge abubnenL

clayby EA Ellis, University of Cambridge Soil Mechanics Group.

~-height bridge abutments supported on foundations piled throughggsoft clay are I'requently exposed to lateral interaction effects associ-

ated with soil movement relative to the structure (Figure 1).The lat-eral structural loading and accompanying displacement arising from thesurcharging action of the embankment fill material on the clay layer willbe of particular concern at the seviceability limit state, since damage tobearings, bridge deck, or abutment structure may result (US Dept ofTransportation 1985).

Placement of the embankment fill material is assumed to occur afterabutment construction. The effect of clay layer depth and rate ofembankment construction (including the use of clay layer drainage) havebeen investigated.

E5

Ou'C

Pn 10OICl

I 15CI

nH

ysand

Immediate

2.5 years

-5 0 5 10Bending moment (MNm)

-1000 0 1000Shear force (kN)

SQNRbsasaual ISCl~fwfIn a geotechnical centrifuge test, full scale stress conditions are recreatedin a model of greatly reduced scale (Schofield 1980).A full-scale versionof the model is referred to as the 'prototype'eg Figure 1).All data dis-cussed here has been converted to prototype scale (and is expressed asforce per unit width whenever appropriate).

The clay layer was modelled using Kaolin. Depths of 6m and 10m wereused, with average undrained strength, c„-21kPa and —25kPa respec-tively (strength increasing with depth). The stiff substratum was mod-elled using Leighton Buzzard sand (equivalent prototype particle size 6-

20mm).An 8m high abutment wall, founded on two rows of 19m long, 1.27m

diameter piles (at spacing/diameter ratio of 5.25) was represented by analuminium model structure (without a bridge deck). Appropriate proto-type flexural stiffnesses for the structure were replicated in the model,which was heavily instrumented to measure displacement and bendingmoment. Typical pile bending moment data immediately after embank-ment construction and 2.5years later are shown in Figure 2 (orientationfor the sign convention as in Figure 1).Fitting spline curves to the bend-ing moment data and differentiating allows shear force and pressure dis-tributions to be derived as shown. Displacement may be obtained by inte-gration of the bending moment data.

Construction of an embankment behind the abutment wall was mod-elled during centrifuge flight by pouring sand in four 'lifts'o create a sur-charge of 140kPa on the clay.

lrhdfflflangofhhralloadlfylLateral loading is composed of three components (Figure 3).Their influ-ences are interdependent, so the resulting interaction is fundamentallycomplex.i) Embankment fill pressure (fI)Conventionally, f, is assumed to act directly on the abutment wall(since TI is not considered), and its magnitude is related to the stiffness

E

5 claym sand

R 10-0

-600 -400 -200 0 200Pressure (kPa)

aysand

0 50 100 150Displacement (mm)

Rgure2. I)fpbud plh data (gm cLay hyer, fast censlructhn): measurml bendingmemenf; derbred sbear farce, pressmeaml dlsphcennmt

of the retaining structure. In the case of compliant structures, theearth pressure coefficient (K) may be assumed to drop to the activevalue, K, (Hambly 1979).For piled structures, K is likely to lie betweenK, and the at rest value, Ka (Clayton et al 1993). The Department ofTransport (DTp 1987) design guidelines are more conservative, statingK = K, for the serviceability limit state, and 1.5Ka for the ultimate limitstate.ii) 'Passive'hrust on the piles in the clay layer (p,, pa)The soft clay displaces laterally due to the imposition of a surcharge(Figure 1), and moves past the piles. This differential movement causesthe piles to be loaded passively by the soil (see Springman & Bolton 1990).iii) Shear stress transfer mechanism (TI, T,)Although recognised by DeBeer 1977, this behaviour has not been quan-tiffied, and is likely to be unfamiliar to designers.

GROUND ENGINEERING JULY/AUGUST 1996

PAPER SOIL-STRUCTURE INTERACTION OF PILED BRIDGE ABUTMENTS CONSTRUCTED ON SOFT CLAY

r,ld

Embankment

Embankment,'Abutment structure

Displaced position

hs JjAbutmentstructure

f2

hs

Locallsed mechanism

Clay layer

Stiff substratum

Embankment

'I

Abutment structure

Displaced position

Rgure 3.Compomndsol tateral structural loading.hs

0

E= 2cd

0o 4—Eo 5—Io 6—Ldr

n 7D

nH

immediate

2.5 years

Mechanism limited by clay depth

Stiff substratum

hs

Embankment' Abutment structure

Displaced position

9-800 -600 -400 -200 0 -500 -400 -300 -200 -100

Bending moment (kNm/m) Shear force (kN/m)

0 Extrusion mechanism

lay layer

Stiff substratum

ld

o0a 4E

o 6—on 7—CI

5-200 -150 -100 -50 0 0 0.5 1

Pressure (kPa) Earth pressure coefficient

Figure 4.Ahutment wall data (6m day layer, slow construcgon): measured handingmoment; derhred shear force aml pressur».

As clay is displaced laterally beneath the embankment, shear stress (~I)will be generated on the interface with the fill material (which isrestrained against movement by the abutment structure). This will con-tribute to stability of the foundation soil against bearing capacity failureat the expense of increasing lateral pressure in the lower regions of thefill, and therefore increasing abutment loading (Figure 4).

Since clay is displaced laterally relative to the underside of the pile cap,further shear stress transfer (g) will develop in this region.

lllB $II$$FSh8$$ wdwdoa8F mIClWIISlllThe magnitude and extent of lateral deformations resulting fromundrained shearing and consolidation of the clay layer are of primaryinterest. The shear transfer force acting on the base of the embankment,T„will be determined by the length of the embankment shear transfer

Flgmu ga, h and c.Constant wlume dehnnmgonmechanhms(hasd on xenes olcongnuous shear straln, wgh sgppageat some houndarles).

zone, ln and a mean value of xi 7I mobilised across the embankment-clayinterface (Figure 3).

T, = x,lt (per unit width)

l, is dependent on the extent of lateral deformation, and will therefore beaffected by factors such as:i) q/c, (q = surcharging pressure) —lateral movements are likely tobecome particularly significant when q/c„> 3.0(Marche & Lacroix, 1972).ii)Depth of the clay layer (h,) and variation of c„with depth —a deeperlayer subject to a geometrically similar deformation mechanism exhibitsa larger deformation zone (Figure Sb, where l, = h,v'2). However, increasein c„with depth may cause the mechanism to be confined to the upperregions of the layer (Figure Sa, l, < h,v'2). Alternatively, extrusion effectsmay result in a relatively large zone of lateral deformation for a thinnerclay layer (Figure Sc, l, = h, + x). These mechanisms (which are onlyapplicable to the undrained case) are presented only as an aid to the qual-itative discussion of the extent of the mechanism, and do not necessari-ly accurately reflect the orientation of principle stress throughout thezones.iii) Stress history of the clay layer —the clay response will become con-siderably more compliant if the preconsolidation stress is exceededunder the surcharging load. Recent stress history will also be importantif a reversal of stress path is implied, since this will result in a stifferresponse to loading.iv) Long-term effects- movement associated with consolidation and creepwill cause lateral displacements to increase under constant surchargeload (Figure 6).

Lateral deformations may be reduced by pauses in the embankmentconstruction sequence, and by installing drains in the clay layer to accel-

GROUND ENGINEERING JULY/AUGUST 1996

PAPER SOIL-STRUCTURE INTERACTION OF PILED BRIDGE ABUTMENTS CONSTRUCTED ON SOFT CLAY

500

400EE

300

200

<n 100

~ immediate (post construction)o—o One year later

-100-25

i I

-20 -15 -10 -5

Distance from edge of pile cap (m)

ylguao 6.IJdeaal soll dlslalacemeutat aday layer surface (6m tday layer, fastcoustrucllon).

crate consolidation. The clay will then exhibit a stiffer response to subse-quent loading. Load bearing elements (such as granular columns) in theclay layer may also be used to decrease movements.

To calculate Tt, xt must also be estimated. An upper bound is providedby c, leading to a conservative estimate (but c„will increase with time asthe clay consolidates). The presence of a dessicated crust at the clay sur-face would add a further complication, since c„would be greatlyincreased in this region (and it would be possible that shearing wouldpreferentially occur below the crust). Analogy with pile shaft loading sug-gests that rt would be fully mobilised at relatively small displacementsacross the embankment-clay interface (based on the observation thatshaft friction is rapidly mobilised). Therefore, application of a suitablemobilisation factor (xt = ac„) irrespective of interface displacement mayprovide a pragmatic solution.

Since relative displacement across the interface is of key importance,the stiflness of the fill material and the abutment structural response tolateral loading will significantly affect both ll and rt.

T, is easier to estimate since I, is a known dimension, and the increasein c„in this region will be less significant (since it is located at the edgeof the surcharge, and partially sheltered from the increase in verticalstress by the pile cap).

EXjiea aaaaaaaaaaal ObSOMIIMSLateral loading on the abutment structure during the centrifuge testsmay be deduced from the experimental data (see Figures 2 and 4). Todetermine the contribution to total lateral loading irom shear stresstransfer, it is assumed that the abutment structure had moved sufficient-ly to mobilise K, (in the absence of shear transfer) behind the wall, andK in front of the pile cap (ft and fs, Figure 3). This assumption is validsmce the wall movement was generally of the order of 100mm.

Table 1 shows the estimated embankment-clay shear transfer force(Tt) in four centrifuge tests, with variation of clay layer thickness andembankment construction rate.

A nominal estimate of the length of shear transfer is given by Tt/c„(taking c„at the top of the clay layer). To prevent the analysis becomingtoo involved, only two values of c„are considered: the initial value, priorto embankment construction; and the final value, after consolidationunder the full surcharging pressure has occurred (factors such asstrength anisotropy, and partial consolidation are not considered). A rela-tionship of the form c„=ana (n = over-consolidation ratio) is used, takinga = 0.22 and b = 0.71 (Springman,1989). Initial and final values of c„are therefore c,. = 15.9 and c~ —— It (m) Sauslautdloumyo y,(SOhu)33.5kPa respectively. The corre- twg te2$sponding values of T,/c„h, areshown in Table 1 (using c, for t =0, and c~for t = 2,5 years). Since c„would not be fully mobilisedthroughout the shear transferzone, ti/h, > Tt/c„har However, thevalues are likely to provide a use-ful indication of the magnitude ofti/Iiir

At the end of the construction period, Tt was similar for the 'fast'nd'slow'ases. However, the subsequent increase is markedly more signifi-cant in the 'fast'onstruction tests, where the initial loading results pri-marily from undrained effects alone. A significant increase in shear trans-fer loading associated with consolidation then occurs during the post-construction period. In the 'slow'onstruction tests, the comparable initialloading is due to a combination of undrained effects and partial consoli-dation, and consequently the ultimate value is significantly reduced.

Loading of structures on the 10m clay layer was less than for the 6mlayer. This can be partially attributed to the reduced lateral structuralstiffness in this case (since the pile length was not varied, there was 4mless pile embedded in the stiff substratum); these structures underwentapproximately twice the horizontal movement (despite lower loading).However, referring to the values of Tt/c„h, it seems likely that anincrease in lt/h, as clay depth is reduced (see Figure 5) was also partiallyresponsible for this observation.

There is relatively little change between the initial and final values ofTt/cP,. Therefore, the increase in T, observed due to post-constructionconsolidation may be satisfactorily accounted for by consideration of theincrease in c„during this period.

Finally, it should be emphasised that these observations are based on ascenario where the clay foundation is surcharged close to its ultimatecapacity (q/c„- 140/20 = 7, based on the initial value of c, at mid-depth).Examination of data during intermediate stages of construction indi-cates that T,/qh, is a more meaningful variable under these circum-stances (ie T, ~ q).

The anticipated nett lateral loading on a structure of this type willdepend upon the values of earth pressure coefficient (K) assumed to actbehind and in front of it. The most conservative guideline of 1.5K,behind(DTp, 1987)and K in front results in a lateral load of 421kN/m. Using themethod adopted in the previous section to consider the component of lat-eral loading due to shear transfer in isolation (Figure 3), an equivalentloading would result f'rom a combination of K, forf, and total shear trans-fer loading (Tt + Ta) = 261kN/m. Table 1 demonstrates that Tt > 261kN/min both the 'fast'onstruction tests; and in the 'slow'ests it is likely thatTt + T, > 261kN/m.

Therefore even an apparently conservative design method (based on theassumption thatWe abutment wall movement is insufficient to mobiliseK,) may well underestimate the total loading observed at large wall dis-placements when shear transfer is of importance (in this case, where theclay layer is close to failure under the embankment surcharging load).

CeelualonsFor a full-height piled bridge abutment constructed on soft clay, currentdesign methods do not account for all potential mechanisms of lateralstructural loading. These mechanisms have been observed in a series ofgeotechnical centrifuge tests, and their potential relevance has been con-firmed.

The mechanism of shear stress transfer acting on the underside of theapproach embankment is particularly complex. However, initial studiesindicate that it can act over a length of the embankment in excess of theclay layer depth (particularly where the clay layer is of limited depth).

ACIQ'IOggISdgaasaamaaoThe centrifuge tests referred to were carried out under a contract financedby Transport Research Laboratory; and an EPSRC CASE award in con-junction with Sir William Halcrow tk Ptns. I would like to thank the tech-nical staff involved with the experimental work; and Dr SM Springman 8c

Dr CWW Ng for their intellectual contribution to the programme.

Clayton, CRI, Milititsky, J & Woods, RI (1993). 'Earth pressure and earth-retaining struc-tures'2nd ed), pp188. Blackie Academic Professional.DeBeer, EE (1977). 'Piles subjected to static lateral loads'. Proc 9th ICSMFE, Tokyo, pp1-14.DTp (1987).Highways &Traffic Departmental Standard BD30/87 'Backfilled retaining walls

and bridge abutments'. UK Department of Transport.Hambly, EC (1979).'Bridge foundations and sub-structures'. HMSO,London: Building Research Establishment Report.

T,lc,P, Marche, R &Lacroix, Y (1972). 'Stabilite des culees de points etablies

tis 6 f ia gg sur des pieux traversant une couche molle'. Canadian GeotechnicalJournal 9(1),2-24.

1.7 1.8 Schofield, AN (1980). 'Cambridge geotechnical centrifuge opera-

1 4 1 2 tions'. Geoiechnique 30(3), 227-288.Springman, SM (1989). 'The effect of surcharge loading adjacent topiles'. PhD thesis, University of Cambridge.

0 6 0 4 Springman, SM & Bolton MD (1990). 'The effect of surcharge load-ing adjacent to piled foundations'. Transport & Road ResearchLaboratory: Contractor Report 198.US Dept of Transportation (1985). 'Tolerable movement criteria forhighway bridges'. Final report. FHWA/RD-85/107 Federal HighwayAdministration, USA: US Dept of Transportation.

GROUND ENGINEERING JULY/AUGUST 1996