Negative skin friction on single piles and pile groups

NEGATIVE SKIN FRICTION ON SINGLE PILES AND PILE GROUPS SHEN RUIFU NATIONAL UNIVERSITY OF SINGAPORE 2008 NEGATIVE SKIN FRICTION ON SINGLE PILES AND PILE GROUPS SHEN RUIFU (BEng, Tsinghua University) (MEng, National University of Singapore) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2008 ACKNOWLEDGEMENTS I would like to express my deep and sincere gratitude to my supervisors, Professor Leung Chun Fai and Professor Chow Yean Khow for their detailed and persistent guidance and critical discussions during numerous meetings over the past six years which gradually shape the outcome of the present research work in its present form. I would also like to thank A/Prof. Phoon Kok Kwang and Prof. Tan Thiam Soon for their valuable and constructive suggestions during the course of this research project. Working on a part-time basis for my PhD thesis as a Professional Officer in the NUS Centre for Soft Ground Engineering (CSGE) enables me to have opportunities to interact closely with the brilliant geotechnical professors in CSGE whom I have the privilege to refer to as my “colleagues”. A/Prof. Tan Siew Ann is always enthusiastic in demonstrating to me the wonderful Geotechnical software suite Plaxis inside out and encouraging me to adopt Plaxis 3D Foundation for the FEM analysis of the perplexing pile-soil-pile interaction of piles subjected to NSF. From A/Prof. Lee Fook Hou, I learned Critical State Soil Mechanics and fascinating soil constitutive modelling which have been intriguing me long ago during my undergraduate years when I first came in contact with soil mechanics and geotechnical engineering. Dr Chew Soon Hoe is always very encouraging and provides valuable advices during the course of my juggling between research work and laboratory duties. I was awed from time to time by the vast knowledge and insightful views from Prof. Yong Kwet Yew. The more I learn soil mechanics and geotechnical engineering, the more my realization of my ignorance of the subject. “Our knowledge can only be finite, while our ignorance must necessarily be infinite”, to quote the wisdom word of Karl Popper (1902-1994). i As a research and laboratory staff, I mingle daily with the laboratory officers in the NUS Centrifuge and Geotechnical Laboratories: Choy Moon Nien, Foo Hee Ann, Jamilah Mohd, Loo Leong Huat, Shaja Khan Abdul Kassim, Tan Lye Heng, and Wong Chew Yuen. They always warmly and kindly extend their helping hands whenever needed and I enjoy brotherly and sisterly relationship with them which make me feel like going to the office every morning I wake up. Our conducive working ambience and productive teamwork manifest loudly with the consecutive accolades of gold medals awarded to our PILLAR team formed by the laboratory staff participating in the Work Improvement Team (WIT) competition in the Singapore National Quality Circle Convention (NQCC). The favourable policies of the National University of Singapore pertaining to NUS staff pursuing higher degrees on part-time basis are gratefully acknowledged. Thanks are due to the Department of Civil Engineering of NUS for the generous helps and various supports. Finally, I would like to dedicate this thesis to my dearest wife, Lu Yu Xia, and my loveliest daughter, Shen Yuan Yuan, who are always caring and understanding which give me a peace of mind even if I on some occasions need to run the centrifuge tests into the wee hours. January 2008 Shen Rui Fu ii TABLE OF CONTENTS Acknowledgements i Table of Contents iii Summary viii Nomenclature x List of Tables xiii List of Figures xiv CHAPTER INTRODUCTION 1.1 Background 1.2 Objective and Scope of Study 1.2 Layout of Thesis CHAPTER LITERATURE REVIEW 2.1 Introduction 2.2 Current Understanding and Uncertainties of NSF 11 2.2.1 When We Need to Consider NSF 11 2.2.2 Relative Movement Required for Mobilization of NSF 14 2.2.3 Magnitude of NSF 19 2.2.4 Location of Neutral Point (NP) 23 2.2.5 NSF on Cast-in-situ bored piles 27 Design Philosophy 29 2.3 iii 2.4 Negative Skin Friction on Pile Groups 36 2.4.1 Field Tests on Pile Group Subject to NSF 36 2.4.2 Laboratory Small-scale Tests on Pile Groups Subject to 38 2.4.3 Centrifuge Model Tests on Pile Groups Subject to NSF 39 2.5 Numerical Study of NSF on Piles 42 2.6 Concluding Remarks 46 CHAPTER EXPERIMENTAL SETUP AND PROCEDURE 3.1 Introduction 58 3.2 Model Setup 60 3.2.1 Soil Container 60 3.2.2 Supporting Frame, Slider Plate and Sand Hoppers 61 3.2.3 Hydraulic Actuators and Servo-valve Control System 61 Instrumentation and Transducers 63 3.3.1 Instrumented Model Piles 63 3.3.2 Transducers used in Model Setup 67 3.3.3 In-flight Piezocone 68 3.4 Model Ground Preparation 70 3.5 Completed Model Package 72 3.6 Experimental Procedure 74 3.6.1 Soil Self-weight Consolidation at 80g 74 3.6.2 In-flight Pile Installation 75 3.6.3 Soil Re-consolidation after Pile Installation 75 3.6.4 Simulation of Underground Water Drawdown 75 3.3 iv 3.6.5 Application of Dead Load 77 3.6.6 Surcharge Loading 77 3.6.7 Simulation of Transient Live Loads 78 3.6.8 Post-flight Tests 78 CHAPTER NSF ON SINGLE PILES 4.1 Introduction 91 4.2 Model Ground Characterization 94 4.2.1 Undrained Shear Strength Profile 94 4.2.2 Physical Properties of Clay 97 Test results on End-bearing Single Pile 98 4.3.1 Stage 1: Soil Self-weight Consolidation 98 4.3.2 Stage 2: In-flight Pile Installation 99 4.3 4.4 4.3.3 Stage 3: NSF due to Soil Re-consolidation 103 4.3.4 Stage 4: NSF due to Water Drawdown 107 4.3.5 Stage 5: Application of Dead Load on Pile 111 4.3.6 Stage 6: NSF due to surcharge 112 4.3.7 Stage 7: Effect of Live Loads on NSF 116 4.3.8 Brief Summary of Test ES 122 Test Results on Floating Pile and Socketed Pile 124 4.4.1 Stage 1: Soil Self-weight Consolidation 126 4.4.2 Stage 2: In-flight Pile Installation 126 4.4.3 Stage 3: NSF due to Soil Re-consolidation 127 4.4.4 Stage 4: NSF due to Water Drawdown 130 v 4.5 4.4.5 Stage 5: Application of Dead Load on Pile 131 4.4.6 Stage 6: NSF due to Surcharge 132 4.4.7 Stage 7: Effect of Live Loads on NSF 134 Concluding Remarks 136 CHAPTER NSF ON PILE GROUPS 5.1 Introduction 171 5.2 Boundary Effect of Pile Groups with NSF 173 5.3 Behavior of End-bearing pile groups with NSF 179 5.4 Behavior of Socketed Pile Groups 186 5.5 Comparison of Measured Dragloads on Pile Groups Against 189 Empirical and analytical Estimations 5.5 5.5.1 Empirical Methods 190 5.5.2 Analytical Methods 196 Concluding Remarks 204 CHAPTER NUMERICAL ANALYSIS OF NSF USING FEM 6.1 Introduction 222 6.2 NSF on End-bearing Single Piles 224 6.2.1 FEM Mesh and Soil Properties 224 6.2.2 Interface Elements for Pile-soil Interaction 226 6.2.3 Back-analysis Procedure and Results 233 6.2.4 MC model versus MCC model 237 6.2.5 Drained versus Consolidation Analysis 239 vi 6.3 6.4 6.2.6 Degree of Mobilization for End-bearing Piles 240 6.2.7 Effect of Transient Live Load on NSF 245 NSF on Socketed Single Piles 247 6.3.1 Back-analysis Procedure and Results 247 6.3.2 Settlement of Socketed Piles with NSF 249 6.3.2.1 Current understanding 249 6.3.2.2 Back-analysis of Socketed Pile Settlement 252 6.3.2.3 Generalized Settlement Behavior of Socketed Piles 254 Numerical Simulation of NSF on End-bearing Pile Groups 258 6.4.1 Numerical 3D Simulation Methodology 258 6.4.2 Back-analysis of End-bearing Pile Groups 259 6.4.3 Mechanism of Pile Group Effect with NSF 263 6.4.4 Boundary Effect on Pile Group with NSF 266 6.4.5 Moderation Effect of pile Cap on Pile Group 268 6.4.6 Generalization of NSF Group Reduction Factor 270 6.5 Numerical Simulation of NSF on Socketed Pile Groups 273 6.6 Concluding Remarks 276 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 7.1 Conclusions 318 7.2 Recommendations for Future Studies 325 APPENDIX A1~A11 REFERENCE R1~R12 vii SUMMARY It has long been recognized that negative skin friction (NSF) which is detrimental to piled foundations can be induced to piles installed through consolidating soils. In the present study, centrifuge model tests have been conducted to investigate the combined effects of NSF, dead load as well as transient live load on an “end-bearing” pile, a “floating” pile and a “socketed” pile, denoting the three most common pile load bearing situations in the field. An elaborate test control scheme has been developed to seamlessly incorporate sequential test stages into each model test to induce NSF on the instrumented pile through typical means, namely re-consolidation of remolded clay after pile installation, ground water drawdown as well as surcharge loading. As the entire test process can be conducted without stopping the centrifuge, the pile behavior can be scrutinized in a comprehensive and rational manner. Besides critically evaluating the understanding of NSF established in previous studies, new findings arising from the present model tests provide new insights on the mechanism of NSF on single piles. The centrifuge model study was subsequently extended to pile groups comprising 3, 5, and 16 piles connected by a rigid pile cap. The model pile shafts were instrumented with highly sensitive semi-conductor strain gauges in full-bridge configuration. As a result, the subtle difference in the induced dragload among piles in a group as well as the group effects of NSF can be qualitatively explored in a consistent and rigorous manner. These test data are invaluable in view of the dearth of such data in the literature and are readily utilized to evaluate the appropriateness viii Downdrag loads (kN) Appendix 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 -100 Soil Water reconsolidation Surcharge drawdown level1 level2 level3 level4 level5 level6 level7 level8 level9 100 200 300 400 500 600 700 800 900 1000 1100 1200 Time after pile installation (days) Downdrag loads (kN) Fig. A13 Development of downdrag loads with time at various elevations along pile shaft for the corner pile in socketed 5-pile group test (Test SE-5) 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 -100 Soil Water reconsolidation Surcharge drawdown level1 level2 level3 level4 level5 level6 level7 level8 level9 100 200 300 400 500 600 700 800 900 1000 1100 1200 Time after pile installation (days) Fig. A14 Development of downdrag loads with time at various elevations along pile shaft for the inner pile in socketed 5-pile group test (Test SE-5) A8 Appendix 1300 1200 Soil Water reconsolidation drawdown Surcharge 1100 level1 level2 level3 level4 level5 level6 level7 level8 level9 Downdrag loads (kN) 1000 900 800 700 600 500 400 300 200 100 -100 100 200 300 400 500 600 Time after pile installation (days) Fig. A15 Development of downdrag loads with time at various elevations along pile shaft for the corner pile in socketed 9-pile group test (Test SE-9) 1300 Water Soil 1200 reconsolidation Surcharge drawdown 1100 level1 level2 level3 level4 level5 level6 level7 level8 level9 Downdrag loads (kN) 1000 900 800 700 600 500 400 300 200 100 -100 100 200 300 400 500 Time after pile installation (days) Fig. A16 Development of downdrag loads with time at various elevations along pile shaft for the side pile in socketed 9-pile group test (Test SE-9) A9 600 Appendix 1300 1200 Soil Water reconsolidation drawdown Surcharge 1100 level1 level2 level3 level4 level5 level6 level7 level8 level9 Downdrag loads (kN) 1000 900 800 700 600 500 400 300 200 100 -100 100 200 300 400 500 600 Time after pile installation (days) Fig. A17 Development of downdrag loads with time at various elevations along pile shaft for the inner pile in socketed 9-pile group test (Test SE-9) 1300 Soil 1200 reconsolidation Water drawdown Surcharge 1100 level1 level2 level3 level4 level5 level6 level7 level8 level9 Downdrag loads (kN) 1000 900 800 700 600 500 400 300 200 100 -100 100 200 300 400 500 600 700 800 900 Time after pile installation (days) Fig. A18 Development of downdrag loads with time at various elevations along pile shaft for the corner pile in socketed 16-pile group test (Test SE-16) A10 Appendix 1300 Soil 1200 reconsolidation Water drawdown Surcharge 1100 level1 level2 level3 level4 level5 level6 level7 level8 level9 Downdrag loads (kN) 1000 900 800 700 600 500 400 300 200 100 -100 100 200 300 400 500 600 700 800 900 Time after pile installation (days) Fig. A19 Development of downdrag loads with time at various elevations along pile shaft for the side pile in socketed 16-pile group test (Test SE-16) 1300 Soil 1200 reconsolidation Water drawdown Surcharge 1100 level1 level2 level3 level4 level5 level6 level7 level8 level9 Downdrag loads (kN) 1000 900 800 700 600 500 400 300 200 100 -100 100 200 300 400 500 600 700 800 900 Time after pile installation (days) Fig. A20 Development of downdrag loads with time at various elevations along pile shaft for the inner pile in socketed 16-pile group test (Test SE-16) A11 References REFERENCES Acar, Y. 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Comparison of NSF group reduction factors for piles within capped and uncapped pile group 311 Figure 6.38 Typical dragload profiles of pile groups from 3D FEM analysis 312 Figure 6.39 Variation of average NSF group reduction factor under some pile and soil conditions for (a) stocky pile groups; and (b) slender pile groups Figure 6.40 Variation of average NSF group reduction factor under various pile and. .. available on pile groups, contradictory observations have been presented by different researchers Therefore there exists a need for further research on the behavior of piles under realistic loading conditions, in particular for pile groups Conducting field studies to investigate the behavior of single piles and pile groups subjected to axial force and dragload is obviously very costly and requires a very long... various elevations of (a) corner pile; (b) side pile; and (c) inner pile of the end-bearing 16 -pile group 212 Figure 5.8 Profiles of downdrag load along pile shaft for end-bearing 16 -pile group 213 Figure 5.9 Pile Variation of dragload and NSF group reduction factor for (a) corner pile; (b) side pile and (c) inner pile within pile groups 214 Figure 5.10 Variation of averaged dragloads and group factors... NSF on single piles 7 Chapter 1⎯ Introduction 4) The centrifuge model study was subsequently extended to the study of pile groups comprising 3, 5, 9 and 16 piles Emphasis has been placed on the subtle difference of distribution of dragload among piles in a group connected by a rigid pile cap By examining the dragload in each pile within the pile groups against that of a single pile, the pile group effect... 6 months which required costly remedial work However, on a neighboring site, the sequence of construction was altered such that piles were installed after consolidation of the clay under backfill was essentially completed It was found that overloading on piles due to negative skin friction had not occurred after this alteration of construction sequence Ho and Mak (1994) also reported a long-term monitoring... number of piles within (a) end-bearing; and (b) socketed pile groups 215 Figure 5.11 Profiles of downdrag loads along pile shaft at various test stages for the socketed 16 -pile group 216 Figure 5.12 Variation of dragloads and group factors with number of piles for (a) corner pile; (b) side pile and (c) inner pile within socketed pile groups Figure 5.13 Illustration of empirical methods for calculation of... Furthermore, piles are more commonly installed as a group connected by a rigid pile cap under a loaded column supporting the superstructure However, field studies reported so far mainly concentrated on the development of NSF on single piles only, and in most cases, without application of external loads Test data on pile groups are especially rare in the literature since it is extremely onerous to conduct... Mobilization of net downdrag loads along pile shaft during soil reconsolidation after pile driving (Test SS) 163 Figure 4.33 Overall axial load distribution along pile shaft during soil reconsolidation after pile driving (Test SS) 163 Figure 4.34 Downdrag loads along pile shaft during water drawdown stage for (a) Test FS on floating pile; (b) Test SS for socketed pile 164 Figure 4.35 Application of additional... relationship of aluminum tube used for the fabrication of model piles 84 Figure 3.9 Schematic configuration for each level of strain gauge station along the pile shaft 84 Figure 3.10 Schematic of a instrumented model pile (unit in mm) 85 Figure 3.11 Completed instrumented model piles and dummy piles 86 Figure 3.12 Pile head assembly with coupling connector 86 Figure 3.13 In-flight miniature piezocone... consideration for socketed piles based on allowable settlement; the adverse implication of the unbalanced stresses inside and outside a pile group due to NSF; the moderation effect of a rigid pile cap as well as the variation of NSF group reduction factors with the pile- soil conditions Keywords: Negative skin friction; Dragload; Downdrag settlement; Single Pile; Pile group; Centrifuge model test; Finite . NEGATIVE SKIN FRICTION ON SINGLE PILES AND PILE GROUPS SHEN RUIFU NATIONAL UNIVERSITY OF SINGAPORE 2008 NEGATIVE SKIN FRICTION. iv 2.4 Negative Skin Friction on Pile Groups 36 2.4.1 Field Tests on Pile Group Subject to NSF 36 2.4.2 Laboratory Small-scale Tests on Pile Groups Subject to 38 2.4.3 Centrifuge Model Tests on Pile. insights on the mechanism of NSF on single piles. The centrifuge model study was subsequently extended to pile groups comprising 3, 5, 9 and 16 piles connected by a rigid pile cap. The model pile