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INDIAN GEOTECHNICAL SOCIETY CHENNAI CHAPTER

ABSTRACT: This paper presents the results of model tests conducted on piles embedded in loose and medium dense sand behind aretaining wall. It is observed that the maximum pile head deflection is higher for shorter pile (L/D=10) in loose sand whereas it is higherfor longer pile (L/D=27) in medium dense sand. The induced bending moment in piles is more for longer piles than shorter pilesirrespective of the density of sand bed. It is also found that the pile head deflection and the bending moment are found to decreaseexponentially with increasing distance between the pile and the wall. In case of pile groups, the test results reveal that the pilesarranged in a line perpendicular to retaining wall show lesser deflection than single pile and also for the piles of pile group arranged in arow parallel to retaining wall.

KEYWORDS: Pile, Model tests, Retaining Wall, Ground movement, Pile head deflection, Bed density, Sand bed

Effect of Ground Movement on the Performance of Pile FoundationR.K. Madhumathi1 and K. Ilamparuthi2

Post graduate Student, Department of Soil Mechanics and Foundation Engineering, Anna university Chennai, Chennai-600 025,Email: [email protected] and head, Department of Soil Mechanics and Foundation Engineering, Anna university Chennai, Chennai-600 025,

Email: [email protected]

IntroductionPile foundations are used to transfer the load to

deeper soil of high bearing capacity avoiding shallow soilof lower bearing capacities. They subjected to bothlateral and vertical loads. In most of the situations lateralload govern the pile design and the condition is criticalwhen piles are subjected to ground movement(lateralspread). The lateral movement of soil may be attributedto ground instability and change in stress state. In manycases the piles are not designed to sustain lateral soilmovements. More over soil movement brings in morecomplexity in the pile analysis and design is becomingmore difficult. Though the geotechnical engineers know

about the problems to the foundation due to groundmovement, their attention is not drawn to these problemsuntil recently except the situations like construction ofberthing structures and foundation on or in slopingground. Of late in urban areas where construction of highrise buildings with multiple basements are quite common,and foundations of neighbouring structures are subjectedto ground movement because of deep excavations. Inorder to understand the response of foundations due toground movement numerous researchers have workedon the problem of pile subjected to lateral movement of

field is very limited since difficulty involved in simulating

the field condition for conducting tests and complexityinvolved in numerical modeling.

In the present study, tests were carried out onmodel piles to examine the influence of groundmovement on the behaviour of piles and to bring out theinfluence of parameters such as distance between thepile and the excavation face, depth of cut versus lengthof pile, relative stiffness of pile etc. on the displacementand moment on pile. The performance of pile groups( twopiles) were also studied by varying the spacing betweenthe piles. The test results thus obtained are interpretedand presented in detail in this paper.

Experimental Setup and Procedure

Model tests were conducted on steel tank of size650 mm x 400 mm x 600 mm which was stiffened atdifferent levels to avoid volume change duringpreparation of sand bed. The tank was fabricated usingsteel plates of 6 mm thickness and one long side of thetank (650 mm x 600 mm) was made transparent by fixingperspex sheet of 10 mm thick. Figure 1 shows theschematic setup of the model. Hollow aluminium tubes of

ground. Experimental studies through 1g modelsand centrifuge model tests on piles behindsupported excavations were conducted byPoulos and Chen (1997), Chen et al (1997),Leung et al (2000 and 2003). Tests wereconducted on simulated field conditions and theinfluence of depth of excavation and distancebetween the pile ( or pile of pile group) and faceof excavation were brought out. But influence ofpile length, relative stiffness and density of soilon response of pile is not adequately dealt in theliterature. In the theoretical studies of Poulosand Chen (1997) some of these efforts havebeen addressed through boundary elementmethod (PALLAS) and design charts wereproposed for various relative stiffness of pile-soilsystem. However, research contribution in this Fig. 1 The Model Setup


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STUDENTS PAPER COMPETITION 200940

various diameter were selected as model piles. Thelength and diameter of piles are varied to perform tests atdifferent L/D ratios. Aluminum sheets of 0.5 mmthickness were used as retaining wall. The piles ofdiameters 25.4mm,19.05mm and 15mm and lengths254mm, 381mm and 405mm are used as model piles.

Air dried sand was compacted in layers in the tankby adopting sand raining or combination of raining andtamping technique depending on the density required.The sand bed up to the base of the pile was preparedand model pile was placed in position. Preparation ofsand bed was continued upto the base level of retainingwall and the retaining wall was installed in position.Further preparation of sand bed was continued up to thetop level of the model pile in layers of 100mm thick formedium dense sand beds; for loose sand beds sandraining technique was adopted. Sufficient thickness ofsand bed below the pile was always provided in all theexperiments. The deformation dial gauges were set inposition much before the commitment of excavation. The

output cables of the strain guages were connected to thestrain indicator. The strain indicator reading was made tozero before starting the excavation. The strain gaugeswere calibrated to obtain the relationship betweenbending moment and strain. Ground movement wasinduced by excavating the soil in the front face ofretaining wall. The removal of wooden planks in stagesinduced soil movement. The displacement of the pile andthe retaining wall at the top (at the surface level of thesand bed) and their strains for each excavation depthwere recorded at predetermined time intervals (i.e. at theinterval of ten minutes)

Pile Head Deflection

Figure 3 shows the variation of pile headdeflection with excavation depth for pile of three L/Dratios embedded in loose sand at a distance of 2d fromthe retaining wall. It can be observed that the rate ofincrease in deflection is high for pile of L/D=10 whencompared to 20 and 27 in case of piles embedded inloose sand. This indicates that short pile deflects morewhen compared to long pile. And the pile deflectiondecreases exponentially as the distance from theretaining wall increases. It is also observed that the piledeflection increases gradually upto the depth ofexcavation of 140mm and rate of increase of piledeflection is reduced thereafter.

Response of Pile

The first series of tests were conducted on piles ofthree L/D ratios such as 10, 20 and 27.The tests werecarried out in both loose and medium dense sand bed forpiles placed at distances of 2d, 3d and 4d from the faceof retaining wall. The retaining wall is placed at 20cmfrom the excavation face. The Figure 2 shows thevariation of retaining wall deflection with the excavationdepth in loose sand and medium dense sand bed. It canbe observed that the deflection of wall increases as theexcavation depth increases. The responses also showsthat the deflection is more in loose sand compared tomedium dense sand bed

Figure 4 shows the variation of pile headdeflection with excavation depth for pile of three L/Dratios embedded in medium sand at a distance of 2dfrom the retaining wall. The results show that the higherL/D exhibits higher deflection in case of pile embedded indense sand whereas in case of piles in loose sand thelower L/D ratio exhibits higher deflection. This may beattributed to higher resistance of medium sand againstexcavation induced soil movement and resistance offeredby the larger diameter pile.

Fig. 2 Variation of Retaining Wall Deflection with theExcavation Depth in Loose Sand and Medium Dense Sand

Fig. 3 Variation of Pile Head Deflection with ExcavationDepth for Pile in Loose Sand

Fig. 4 Variation of Pile Head Deflection with ExcavationDepth for Pile in Medium Dense Sand


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EFFECT OF GROUND MOVEMENT ON OF PILE FOUNDATION 41

Variation of Maximum Pile Deflection withDistance of Pile from Retaining Wall

Figures 5 and 6 show the variation of maximumpile deflection with distance of pile from retaining wall for

piles tested with L/D ratios 10, 20 and 27 in loose andmedium dense sands respectively. It can be observedthat on increase in the distance from the retaining wall,the magnitude of maximum deflection of pile headreduced almost linearly irrespective of the density anddistance from the wall and the pile tested.

2d from retaining wall. The bending moment in the pileincreased with excavation depth and the location ofmaximum bending moment was observed at a length of190 mm from the pile head for pile embedded in bothloose and medium dense sand. The piles placed at 3dand 4d from the face of retaining wall exhibits an identical

behaviour as that of piles placed at a distance of 2d fromthe retaining wall. The response observed in the testconducted in medium dense sand is similar to that of testconducted in loose sand with identical test conditions.The basic difference is only in the magnitude of bendingmoment of pile for the given depth of excavation.

Variation of Max.Pile bending moment with L/D

Figures 8 and 9 show the variation of maximumpile bending moment with the L/D ratios of pilesembedded in loose and medium dense sandsrespectively. It is observed that the piles of higher L/Dratio exhibits higher bending moment than the piles oflower L/D ratios irrespective of the density of sand. It can

be noted that the pile which are at a distance of 2d fromthe retaining wall shows higher bending moment than thepiles at the distance of 3d and 4d .

Bending Moment

Figure 7 shows the variation of bending momentalong the pile length with excavation depth for a pile ofL/D =10 embedded in loose sand. The pile is located at

Response of Two Pile Group

Figure 10 show the comparisons of deflectionbetween single and two pile group tested in loose sand .The results presented are for the piles embedded at thedistance of 3d and two piles in a row parallel to retainingwall. It is noted that deflection of single pile is more thanthe deflection of two pile group irrespective of the depthof excavation for various piles tested. The response

Fig. 6 Distance of Pile from Retaining Wall Vs MaximumPile Deflection for Pile Embedded in Medium Dense Sand

Fig. 5 Distance of Pile from Retaining Wall Vs Maximum PileDeflection for Pile Embedded in Loose Sand

Fig. 7Bending Moment Vs Depth for Pile Located At 2dfrom Retaining Wall (L/D=10) in Loose Sand

Fig. 8 L/D ratio vs Maximum Pile Bending Moment for PileEmbedded in Loose Sand

Fig. 9 L/D ratio vs Maximum Pile Bending Moment for PileEmbedded in Medium Dense Sand


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STUDENTS PAPER COMPETITION 200942

observed in the test conducted in medium dense sand issimilar to that of test conducted in loose sand. But themagnitude of deflection is lesser due to the higherresistance of medium dense sand against deformation

than loose sand. Due to this ability the deformation ofretaining wall is far lesser in medium sand than loosesand irrespective of the depth of excavation for which themodel tests are conducted in this study.

Conclusions

The maximum pile head deflection is higher forshorter pile (L/D=10) in loose sand whereas it is higherfor longer pile (L/D=27) in medium dense sand. Thepercentage decrease in maximum deflection is 6.6% and12.1% for L/D of 20 and 27 respectively while comparingwith the deflection of pile of L/D =10 in loose sand.Similarly the reduction in minimum deflection is 7.5% and

12.5% for L/D of 20 and 10 respectively in medium sandwhile comparing with the pile of L/D =27. The inducedbending moment in piles is more for longer piles thanshorter piles irrespective of the density of sand bed. The

pile head deflection and the bending moment are foundto decrease exponentially with increasing distancebetween the pile and the wall.

The pile group tests reveal that the response of apile within a pile group due to excavation induced soil

depends on its distance from the retaining wall. The pilegroup arranged in a row parallel to retaining wall exhibitlesser deflection than the single pile. The pile deflectionratio is almost same for both loose and medium denseconditions for spacing less than 3.5d. In case of pilesarranged in a line perpendicular to retaining wall, the pilegroup exihibits lesser deflection than single pile and thepiles of pile group arranged in a row parallel to retainingwall.

References

Poulos H. G., and Chen L. T., (1997) Pile response dueto excavation-induced lateral soil movements, Journal o

Geotechnical and Geoenvironmental Engineering.,ASCE, Vol.23 (2), pp 94-99.

Chen, L. T., Poulos, H. G., and Hull, T. S., (1997). Modeltests on pile groups subjected to lateral soil movement,Soils and Foundations, Vol. 37 (1), pp 1-12.

Leung, C.F., Chow, Y. K., and Shen R. F. (2000)Behaviour of pile subjected to excavation-induced soilmovements, Journal of Geotechnical andGeoenvironmental Engineering., ASCE, Vol.126 (11),pp 947-954.

Leung, C.F., Lim, J. K., Chow, Y. K., and Shen R. F.(2003), Behaviour of pile groups subjected to

excavation-induced soil movements, Journal OGeotechnical and Geoenvironmental Engineering.,ASCE, Vol.129 (1), pp 58-65.

Fig. 9 Comparisons of Deflection between Single and TwoPile Group Tested in Loose sand

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