ultimate soil pressures for piles subjected to lateral soil movements

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TECHNICAL NOTES

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Ultimate Soil Pressures for Piles Subjectedto Lateral Soil Movements

J. L. Pan1; A. T. C. Goh2; K. S. Wong3; and C. I. Teh4

Abstract: A series of laboratory model tests in soft clay was conducted to investigate the behavior of coupled piles subjectedsoil movements~‘‘passive’’ piles!, and to determine the ultimate soil pressure acting on the pile shaft. Two piles in a row~center-to-center‘‘joining’’ line being perpendicular to the direction of the applied soil movements! and in a line~center-to-center ‘‘joining’’ line being inthe direction of the applied soil movements! were considered. The ultimate soil pressures along the pile shaft for two piles in a rowin a line with pile spacings of three and five times the pile width~B520 mm! were lower than those for single passive piles. Group effestill existed even with a pile spacing of 5 B for coupled piles in a row and in a line. Group factors decrease as pile spacing decreapiles in a row. The test results also indicated that different distributions of limiting soil pressures along the pile shaft were develthe single and coupled passive piles.

DOI: 10.1061/~ASCE!1090-0241~2002!128:6~530!

CE Database keywords: Clays; Lateral loads; Soil pressure; Pile groups.

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Introduction

The movement of soil laterally past existing piles~‘‘passive’’piles! induces forces and bending moments in the piles thatlead to serviceability problems or even failure of the passive piTypical examples are piles adjacent to deep basement excavaand tunnel operations~Poulos and Chen 1997; Loganathan et2000!, piles used to stabilize slopes~Viggiani 1981!, and pilessupporting bridge abutments adjacent to approach embankm~Stewart 1992; Bransby 1995!. Knowledge of the pressure actinon the pile from the moving soil is of primary importancedesign to determine the pile behavior and the bending momenthe pile.

In the analysis of single passive piles, the values of ultimsoil pressure in the range of 9 – 12su ~su5undrained shearstrength of soil! are commonly adopted~Randolph and Houlsby1984; Chen 1994!. These values are similar to those from ‘‘a

1Postdoctoral Research Associate, School of Civil EngineeringEnvironmental Science, Univ. of Oklahoma, 202 W. Boyd, Room 3Norman, Oklahoma 73019-1024. E-mail: [email protected]

2Associate Professor, Geotechnical Research Centre, School of& Structural Engineering, Nanyang Technological UniSingapore 639798. E-mail: [email protected]

3Associate Professor, Geotechnical Research Centre, School of& Structural Engineering, Nanyang Technological UniSingapore 639798. E-mail: [email protected]

4Associate Professor and Director, Geotechnical Research CeSchool of Civil & Structural Engineering, Nanyang Technological UniSingapore 639798. E-mail: [email protected]

Note. Discussion open until November 1, 2002. Separate discussmust be submitted for individual papers. To extend the closing dateone month, a written request must be filed with the ASCE ManagEditor. The manuscript for this technical note was submitted for revand possible publication on August 14, 2000; approved on Novembe2001. This technical note is part of theJournal of Geotechnical andGeoenvironmental Engineering, Vol. 128, No. 6, June 1, 2002. ©ASCEISSN 1090-0241/2002/6-530–535/$8.001$.50 per page.

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tive’’ pile studies where the pile is subjected to lateral loadingthe pile head~Broms 1964; Matlock 1970; Murff and Hamilton1993!. However, some researchers have suggested much lovalues of ultimate soil pressure for passive piles. For examViggiani ~1981! in the back-analysis of piles in unstable slopsuggested that the ultimate soil pressure is between 2.8 andsu .Poulos ~1995! suggested that the ultimate soil pressure forpassive pile design increases linearly from 2su at the ground sur-face to 9su at a depth of 3.5 times the pile width. The low valuof 2su at the ground surface was adopted taking into accountnear-surface effects. In contrast, Chen~1994! and Bransby andSpringman~1999! in their two-dimensional numerical passivpile analysis indicated that the ultimate soil pressure is 11.711.75su , respectively.

In the case of pile groups loaded passively, a number ofperimental and numerical studies have been carried out~Matsuiet al. 1982; Springman 1989; Stewart 1992; Chen 1994; Bran1995!. However, most of the above studies focused on the lodeflection behavior of the piles and provided limited insightsthe ultimate soil pressure acting on the pile shaft.

This paper presents the results of model tests on ‘‘couplpiles. The results focus on the values of ultimate soil pressacting along the length of the pile. This would enable a beunderstanding of the interaction between the moving soil andpiles. Knowledge of the limiting soil pressures along the pile shcould be used, for example, to carry out a ‘‘p-y’’ type analysisstudy the pile behavior~Byrne et al. 1984; Goh et al. 1997!. Theterm ‘‘coupled’’ refers to two piles that are adjacent to each otheither in a row~center-to-center ‘‘joining’’ line of the piles beingperpendicular to the direction of the applied soil movements! orin a line ~center-to-center ‘‘joining’’ line of the piles being in thedirection of the applied soil movements!.

Experimental DetailsFor brevity, the details of many aspects of the experimental sesuch as the sample preparation have been omitted. Further d

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can be found in Pan~1998! and Pan et al.~2000!. The testingprocedure essentially involved a uniform lateral translation of spast two piles as shown schematically in Fig. 1.

Each of the stainless steel model piles is 295 mm long, 20wide ~B520 mm!, and 6 mm thick. The small thickness of 6 mwas used to minimize the side shear resistance. The piledesigned to behave fairly rigidly in order to examine how‘‘stiff’’ pile would behave when it is subjected to lateral somovements. The tests were carried out in saturated soft cunder undrained loading with the pile head and tip fully fixagainst movement and rotation. The clay deposits were prepfrom kaolin slurry by consolidating the slurry in a consolidometer. Based on unconsolidated undrained triaxial tests, the samhad an averagesu of about 18 kPa.

In these tests, the maximum soil pressure is defined aspressure beyond which the pressure-displacement curve tpractically a linear form showing a large displacement for a smincremental load~Pan et al. 2000!. Since the maximum soil pressure may not be uniform along the length of the pile, the mamum soil pressure at any particular depth of the pile shafdefined as the limiting soil pressurepL . The ultimate soil pres-surepu refers to the maximum value of the limiting soil pressuralong the length of the pile shaft. In these tests, the measpressure is the contact pressure acting on the pile shaft ratherthe net lateral resistance, which comprises the contribution fthe side shear resistance in addition to the differential pressurthe front and rear faces of the pile~Pan et al. 2000!. The signconvention used in presenting the experimental data was chfor convenient graphical representation. The front face of a pilthe face of the pile far from the source of the applied soil moments, whereas the rear face of a pile is the face of the pile cto the source of the applied soil movements. For piles in a r

Fig. 1. Apparatus for stiff passive pile tests

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the terms ‘‘left’’ and ‘‘right’’ pile describe the location of the pilesviewed from the source of the applied soil movements. For pin a line, the terms ‘‘near’’ and ‘‘far’’ pile describe the location othe pile relative to the source of the applied soil movemenHence a ‘‘near’’ pile encounters the soil movements before‘‘far’’ pile. As the lateral soil movements increased, the soil in trear side of the pile separated from the pile and thus no sucwas induced in the rear face of the pile. Therefore only the frface of the pile was instrumented with five soil pressure transders.

Test Results

This section presents the results of the model tests on coupiles in a row and in a line with pile head and tip fully fixeagainst movement and rotation. For brevity, only some ofplots have been illustrated. In all the tests, the same soil displment rate of 1.5 mm/min was used to ensure undrained condit~Pan et al. 2000!.

Coupled Piles in a Row

Two pile spacings of three times the pile width~3 B! and fivetimes the pile width~5 B! were investigated in the tests. Since tmeasured soil pressures on the shaft of each pile showed sdiscrepancies~less than 6%!, the average values from the mesurements are presented.

Fig. 2 shows the normalized pressure-soil displacement cufor the piles spaced 3 B apart~R3B test! and the locations of thesoil pressure transducers along the pile shaft. The lateral soil psures for all the transducers except D06 & 12 increased at almthe same rate, but the rate decreased after a soil displaceme3.2 mm~0.16 B!.

Coupled Piles in a Line

Figs. 3 and 4 show the normalized pressure-soil displacemcurves for the ‘‘near’’ and ‘‘far’’ piles, respectively, spaced 3apart~L3B test!. As shown in Fig. 3, the strain-hardening shapethe plots for all the transducers for the ‘‘near’’ pile was similaand the normalized pressure increased in a progressively decing rate from the tip to the head of the pile. The strain-softenshape of the plots for all the transducers for the ‘‘far’’ pile walso similar as shown in Fig. 4.

Pressure Distributions along Pile Shaft

The distributions of the limiting soil pressurespL along the pileshaft for the coupled pile tests, as well as the single pile test~SP!reported in Pan et al.~2000! are shown in Fig. 5. The differenshapes of the plots with depth suggest that the deflections opiles in the soil followed different patterns and as a result pduced different limiting soil pressurespL along the pile shaft.

Discussion

For coupled piles in a row, the ultimate soil pressurespu were7.1su for pile spacing of 3 B and 8.6su for pile spacing of 5 B.The ultimate soil pressurespu were reached after the soil hatranslated approximately 0.63 B for a pile spacing of 3 B and 0.68B for a pile spacing of 5 B. For coupled piles in a line with a p

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Fig. 2. Normalized soil pressure-soil displacement curves for R3B test

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spacing of 3 B, the ultimate soil pressurespu were 8.2su for the‘‘near’’ pile and 4.3su for the ‘‘far’’ pile. The ultimate soil pres-surespu were reached after the soil had translated approxima0.6 B for the ‘‘near’’ pile and 0.38 B for the ‘‘far’’ pile. Forcoupled piles in a line with a pile spacing of 5 B, the ultimate spressurespu were 7.1su for the ‘‘near’’ pile and 8.1su for the‘‘far’’ pile. The ultimate soil pressurespu were reached after thesoil had translated approximately 0.24 B for the ‘‘near’’ pile a0.58 B for the ‘‘far’’ pile.

Previous tests for single piles indicated that thepu was ap-proximately 10su ~Pan et al. 2000!. The pu was reached after thesoil had translated approximately 0.48 B, which is similar tovalue obtained by Matsui et al.~1982! in their model tests.



Group effects on the lateral response of vertical piles to latesoil movements have been studied numerically by Chen~1994!and Chen and Poulos~1997!. In Chen ~1994!, the group factorFm, which was calculated in terms of the maximum bending mment, was obtained from model tests on groups of two pilesrow embedded in calcareous sand. In Chen and Poulos~1997!, thegroup factor f p , which was calculated in terms of the limitinpile–soil contact pressure, was based on plane-strain finitement analysis of piles embedded in clay arranged in an infinilong row.

In the present study, the results of the coupled pile tests wcompared with those from the single pile tests in order to inv

Fig. 3. Normalized soil pressure-soil displacement curves for ‘‘near’’ pile in L3B test

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Fig. 4. Normalized soil pressure-soil displacement curves for ‘‘far’’ pile in L3B test

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Fig. 5. Variations of limiting soil pressurespL with depth forcoupled pile tests
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tigate the pile–soil interaction behavior. The group effect wassessed by a group factorFp , based on the measuredpu .

Fp5puc

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in which puc5ultimate soil pressure of a pile from a coupled pitest andpus5ultimate soil pressure of a pile from a single pitest.

Table 1 summarizes the group factors from the literaturethe present tests. The test results for the coupled piles in ashow thatFp is smaller for a pile spacing of 3 B than 5 B. Witha pile spacing of 5 B, the values ofFp are still less than unity.This suggests that group effects still exist even with a pile spacof 5 B. In the plane-strain finite element analysis by Chen aPoulos~1997! for an infinitely long row of piles in clay with pilespacings of 3 and 4 B, the group factorf p was greater than unityThis could be because the pile end fixity conditions may hasome effect on the group factors. The group factorFp based onthe present tests on piles in a row embedded in clay is consiswith the group factorFm obtained by Chen~1994! based onmodel tests on capped-head piles in a row embedded in calcous sand. Both the present tests and Chen’s results indicatethe group factors decrease as pile spacing decreases for pilerow, which agrees with the test results from Cox et al.~1984!,Shibata et al.~1989!, Adachi et al.~1994!, and Rao et al.~1996!.

Conclusions

A series of laboratory model tests was carried out on singlecoupled stiff passive piles and the results are presented inpaper. Only the front face of the pile was instrumented with spressure transducers because there was no suction inducedrear face of the pile as the lateral soil movements increased.major findings from the model tests are summarized below.1. For coupled piles in a row the ultimate soil pressurespu

were 7.1su for a pile spacing of 3 B and 8.6su for a pilespacing of 5 B;

2. For coupled piles in a line with a pile spacing of 3 B, thultimate soil pressurespu were 8.2su for the ‘‘near’’ pile and


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Table 1. Summary of Group Factors

Soil type Test/analysis name End fixity conditions Pile spacing~B! Group factor Factor value Researcher

Clay R3B Head-tip-fixed 3 Fp 0.67 Present testsR5B Head-tip-fixed 5 0.81L3B Head-tip-fixed 3 Near: 0.77

Far: 0.41L5B Head-tip-fixed 5 Near: 0.67

Far: 0.76

Clay Piles in one infinitely long row Free-head 3 f p 1.2 Chen and Poulos~1997!Free-head 4 1.1Free-head 8 1.0

Sand Two piles in a row Free-head 2.5 Fm 0.81 Chen~1994!Free-head 5.0 0.88Free-head 7.5 0.98

Capped-head 2.5 0.72Capped-head 5.0 0.78Capped-head 7.5 0.84

Sand Two piles in a line Free-head 2.5 Near: 1.31Far: 1.01

Chen~1994!

Free-head 5.0 Near: 1.59Far: 1.10

Free-head 7.5 Near: 1.20Far: 0.69

Capped-head 2.5 Near: 0.93Far: 0.92



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4.3su for the ‘‘far’’ pile. For coupled piles in a line with apile spacing of 5 B, the ultimate soil pressurespu were 7.1su

for the ‘‘near’’ pile and 8.1su for the ‘‘far’’ pile;3. The magnitude of the soil translations required to fully m

bilize the ultimate soil pressurespu ranged from 0.2 to 0.7 Bfor coupled piles;

4. Group effects still existed even with a pile spacing of 5 B forcoupled piles in a row and in a line;

5. Group factors decrease as pile spacing decreases for pila row; and

6. Different distributions of limiting soil pressurespL along thepile shaft were developed for coupled piles in a row and iline.

Further experimental study would be useful to investigate difent soil types, soil strength, pile configuration, and pile rigidand fixity conditions on the ultimate soil pressurespu .

Acknowledgments

The results presented in this paper formed part of a thesis subted to the Nanyang Technological University for the degreeDoctor of Philosophy of Engineering. The first writer is grateto the Nanyang Technological University for providing a reseascholarship to carry out the research.

References

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