NUMERICAL MODELLING OF TIME DEPENDENT PORE PRESSURE RESPONSE INDUCED BY HELICAL PILE INSTALLATION by ALEXANDER M. VYAZMENSKY

Diploma Specialist in Civil Engineering (B.Hons. equivalent) St. Petersburg State University of Civil Engineering and Architecture, 1997

A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (Civil Engineering)

THE UNIVERSITY OF BRITISH COLUMBIA

February 2005

Alexander M. Vyazmensky, 2005

Abstract.

ABSTRACT.The purposes of this research are to apply numerical modelling to prediction of the pore water pressure response induced by helical pile installation into fine-grained soil and to gain better understanding of the pore pressure behaviour observed during the field study of helical pile soil interaction, performed at the Colebrook test site at Surrey, B.C. by Weech (2002). The critical state NorSand soil model coupled with the Biot formulation were chosen to represent the behaviour of saturated fine-grained soil. Their finite element implementation into NorSandBiot code was adopted as a modelling framework. Thorough verification of the finite element implementation of NorSandBiot code was conducted. Within the NorSandBiot code framework a special procedure for modelling helical pile installation in 1-D using a cylindrical cavity analogy was developed. A comprehensive parametric study of the NorSandBiot code was conducted. It was found that computed pore water pressure response is very sensitive to variation of the soil OCR and its hydraulic conductivity kr. Helical pile installation was modelled in two stages. At the first stage expansion of a single cavity, corresponding to the helical pile shaft, was analysed and on the second stage additional cavity expansion/contraction cycles, representing the helices, were added. The pore pressure predictions were compared and contrasted with the pore pressure measurements performed by Weech (2002) and other sources. The modelling showed that simulation of helical pile installation using a single cavity expansion within NorSandBiot framework provided reasonable predictions of the pore pressure response observed in the field. More realistic simulation using series of cavity expansion/contraction cycles improves the predictions. The modelling confirmed many of the field observations made by Weech (2004) and proved that a fully coupled NorSandBiot modelling framework provides a realistic environment for simulation of the fine-grained soil behaviour. The proposed modelling approach to simulation of helical pile installation provided a simplified technique that allows reasonable predictions of stresses and pore pressures variation during and after helical pile installation.

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Table of contents. TABLE OF CONTENTS. ABSTRACT ...................................................................................................................................ii TABLE OF CONTENTS ............................................................................................................iii LIST OF TABLES ......................................................................................................................vii LIST OF FIGURES ...................................................................................................................viii ACKNOWLEDGEMENTS ......................................................................................................xiii 1.0. INTRODUCTION ..............................................................................................................1 1.1. CHALLENGES IN AXIAL PILE CAPACITY PREDICTIONS IN SOFT FINE-GRAINED SOILS .........1 1.2. HELICAL PILES ..................................................................................................................2 1.3. PURPOSES AND OBJECTIVES OF RESEARCH........................................................................4 1.4. SCOPE AND LIMITATIONS OF STUDY ..................................................................................4 1.5. THESIS ORGANIZATION .....................................................................................................6 2.0. OVERVIEW OF FIELD STUDY OF HELICAL PILE PERFORMANCE IN SOFT SENSITIVE SOIL ..............................................................................................................8 2.1. INTRODUCTION ...................................................................................................................8 2.2. SCOPE OF WEECH'S STUDY .................................................................................................8 2.3. SITE SUBSURFACE CONDITIONS..........................................................................................9 2.3.1. SITE STRATIGRAPHY. ..................................................................................................9 2.3.2. SOIL PROPERTIES ......................................................................................................10 2.3.2.1. FIELD INVESTIGATION BY MINISTRY OF TRANSPORTATION AND HIGHWAYS .10 2.3.2.2. RESEARCH BY UNIVERSITY OF BRITISH COLUMBIA (1). ................................10 2.3.2.3. RESEARCH BY UNIVERSITY OF BRITISH COLUMBIA (2). ................................11 2.4. HELICAL PILES AND PORE PRESSURE MEASURING EQUIPMENT .......................................12 2.4.1. TEST PILES GEOMETRY AND INSTALLATION DETAILS...............................................12 2.4.2. MEASURING EQUIPMENT ..........................................................................................13 2.5. SUMMARY OF WEECHS STUDY RESULTS........................................................................14 2.5.1. PORE WATER PRESSURE RESPONSE DURING HELICAL PILE INSTALLATION..............14 2.5.2. PORE WATER PRESSURE DISSIPATION AFTER HELICAL PILE INSTALLATION. ...........15 2.6. SUMMARY ........................................................................................................................17 3.0 LITERATURE REVIEW ..................................................................................................30 3.1. INTRODUCTION. ...............................................................................................................30 iii

Table of contents. 3.2. PORE PRESSURE RESPONSE INDUCED BY PILE INSTALLATION INTO FINE GRAINED SOIL AND ITS INFLUENCE ON PILE CAPACITY ...........................................................................30 3.2.1. FIELD GENERATION OF EXCESS PORE PRESSURE. ......................................................30 3.2.2. FIELD DISSIPATION OF EXCESS PORE PRESSURE........................................................31 3.2.3. OBSERVED AXIAL PILE CAPACITY AS FUNCTION OF DISSIPATION OF EXCESS PORE PRESSURE..................................................................................................................33 3.3. PREDICTION OF TIME-DEPENDENT PORE PRESSURE RESPONSE ........................................34 3.3.1. PREDICTION METHODS..............................................................................................34 3.3.2. BASIC CONCEPTS BEHIND EXISTING PREDICTION SOLUTIONS. .................................37 3.3.2.1. MODELLING ANALOGUEFOR

SIMULATION OF PILE OR CONE PENETRATION ....37

3.3.2.2. MODELLING FRAMEWORK .............................................................................38 3.3.3. OVERVIEW OF EXISTING PREDICTION SOLUTIONS.....................................................39 3.3.3.1. CAVITY EXPANSION SOLUTIONS ....................................................................39 3.3.3.2. SOLUTIONS BASED ON STRAIN PATH METHOD ..............................................42 3.4. SUMMARY ........................................................................................................................42 4.0. FORMULATION OF MODELLING APPROACH ....................................................49 4.1. INTRODUCTION. ..............................................................................................................49 4.2. MODELLING APPROACH TO SIMULATION OF HELICAL PILE INSTALLATION INTO FINE GRAINED SOIL .................................................................................................................49 4.2.1. MODELLING FRAMEWORK ........................................................................................49 4.2.2. MODELLING PROCEDURE FOR SIMULATION OF HELICAL PILE INSTALLATION. .........50 4.3. NORSANDBIOT FORMULATION. ......................................................................................52 4.3.1. NORSAND CRITICAL STATE MODEL .........................................................................52 4.3.1.1. MODEL DESCRIPTION ......................................................................................52 4.3.1.2. MODEL PARAMETERS ......................................................................................55 4.3.1.3. BEYOND SAND ................................................................................................56 4.3.2. BIOT COUPLED CONSOLIDATION THEORY .................................................................57 4.3.3. FINITE ELEMENT IMPLEMENTATION OF NORSANDBIOT FORMULATION ....................58 4.3.4. FINITE ELEMENT CODE VERIFICATION ......................................................................58 4.4. SUMMARY .......................................................................................................................59 5.0. SELECTION OF SITE-SPECIFIC SOIL PARAMETERS FOR MODELLING .....67 5.1. INTRODUCTION. ..............................................................................................................67 5.2. SOIL PARAMETERS FOR MODELLING. .............................................................................67 5.2.1. ELASTIC PROPERTIES G, . .......................................................................................67 iv

Table of contents. 5.2.2. OVERCONSOLIDATION RATIO OCR. .........................................................................69 5.2.3. COEFFICIENT OF LATERAL EARTH PRESSURE K0. .......................................................70 5.2.4. HYDRAULIC CONDUCTIVITY DERIVATION. ...............................................................71 5.2.4.1. COEFFICIENT OF CONSOLIDATION ....................................................................71 5.2.4.2. COEFFICIENT OF VOLUME CHANGE, mv ...........................................................73 5.2.4.3. RADIAL HYDRAULIC CONDUCTIVITY, kr..........................................................74 5.2.5. VERTICAL EFFECTIVE STRESS vo AND EQUILIBRIUM PORE PRESSURE uo. ..............74 5.2.6. NORSAND MODEL PARAMETERS DERIVATION ..........................................................74 5.2.6.1. CRITICAL STATE COEFFICIENT, Mcrit ...............................................................75 5.2.6.2. STATE DILATANCY PARAMETER, .................................................................75 5.2.6.3. HARDENING MODULUS, Hmod ..........................................................................75 5.2.6.4. SLOPE OF CRITICAL STATE LINE, .................................................................75 5.2.6.5. INTERCEPT OF CRITICAL STATE LINE AT 1 KPA STRESS, .............................77 5.2.6.6. STATE PARAMETER, .....................................................................................77 5.2.7. NORSAND PARAMETERS ANALYSIS ..........................................................................79 5.3. SUMMARY. .....................................................................................................................80 6.0. NORSAND-BIOT CODE PARAMETRIC STUDY .....................................................95 6.1. INTRODUCTION. ..............................................................................................................95 6.2. MODELLING PARTICULARS. ............................................................................................95 6.3. REFERENCE RESPONSE. ...................................................................................................96 6.4. PARAMETRIC STUDY SCENARIOS. ...................................................................................98 6.5. PARAMETRIC STUDY RESULTS. . ...................................................................................100 6.5.1. INFLUENCE OF COEFFICIENT OF LATERAL EARTH PRESSURE ..................................102 6.5.2. INFLUENCE OF MEASURES OF SOIL OCR ................................................................103 6.5.3. INFLUENCE OF ELASTIC PROPERTIES ......................................................................106 6.5.4. INFLUENCE OF CRITICAL STATE LINE PARAMETERS................................................108 6.5.5. INFLUENCE OF HARDENING MODULUS ....................................................................109 6.5.6. INFLUENCE OF STATE DILATANCY PARAMETER ......................................................110 6.5.7. INFLUENCE OF HYDRAULIC CONDUCTIVITY ............................................................110 6.6. CONCLUDING REMARKS ON PARAMETRIC STUDY RESULTS ..........................................111 6.7. SUMMARY .....................................................................................................................113 7.0. MODELLING OF PORE PRESSURE CHANGES INDUCED BY PILE INSTALLATION IN 1-D ..............................................................................................138 v

Table of contents. 7.1. INTRODUCTION. ............................................................................................................138 7.2. 1-D SIMULATIONS. .......................................................................................................138 7.2.1. STAGE I. MODELLING OF HELICAL PILE INSTALLATION AS SINGLE CAVITY EXPANSION..............................................................................................................139 7.2.1.1. COMPARISON OF MODELED AND FIELD PORE PRESSURE RESPONSES .........139 7.2.1.2. NORSANDBIOT BEST FIT WITH FIELD DATA ..........................................141 7.2.2. STAGE II. MODELLING OF HELICAL PILE AS SERIES OF CAVITY EXPANSIONS ..146 7.2.2.1. DETAILS OF HELIX MODELLING .................................................................146 7.2.2.2. EFFECT OF CAVITY EXPANSION/CONTRACTION CYCLING ON PORE PRESSURE RESPONSE................................................................................................................147 7.3. IMPLICATIONS FROM 1-D MODELLING. .........................................................................153 7.3.1. PREDICTED VERSUS MEASURED/INTERPRETED PORE PRESSURE RESPONSE .......153 7.3.2. FROM PORE PRESSURE RESPONSE PREDICTIONS TO PILE BEARING CAPACITY ..155 7.4. SUMMARY .....................................................................................................................156 8.0. CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER STUDY ..........175 8.1. SUMMARY AND CONCLUSIONS. ...................................................................................175 8.2. RECOMMENDATIONS FOR FURTHER RESEARCH. ..........................................................177 8.2.1. LABORATORY STUDY .........................................................................................177 8.2.2. 2-D NUMERICAL MODELLING ...........................................................................178 REFERENCES .........................................................................................................................180 NOTATION ..............................................................................................................................187 APPENDIX A. SOURCES OF SUBSURFACE INFORMATION FOR COLEBROOK SITE .....................189 APPENDIX B. PIEZOMETERS RESPONSE ...................................................................................191 APPENDIX C. VALIDATION OF NORSAND MODEL AGAINST BONNIE SILT .................................193 APPENDIX D. NORSAND-BIOT COUPLING ..............................................................................197 APPENDIX E. NORSAND-BIOT CODE VERIFICATION. ..............................................................200 APPENDIX F. COUPLED MODELLING OF OBSERVED PORE PRESSURE DISSIPATION AFTER HELICAL PILE INSTALLATION (PAPER) ...............................................................209

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List of tables.

LIST OF TABLES

TABLE

PAGE

2.1. Average index properties of clayey silt/silty clay layer ..................................................... 11 3.1. Solutions for prediction of pore response induced by penetration of piles and piezocones.. 36 4.1. NorSand model formulation ............................................................................................... 55 4.2. NorSand code input parameters.......................................................................................... 55 5.1. List of correlations used to estimate K0 from CPT test data .............................................. 70 5.2. Calculation of radial hydraulic conductivity, kr ................................................................ 74 5.3. Estimation of slope of critical state line, , based on laboratory derived values of Cc reported by Crawford & Campanella (1991)...................................................................... 77 5.4. Summary of NorSand parameters for Colebrook silty clay ............................................... 79 5.5. Undrained shear strength and sensitivity estimated from field measurements and NorSand simulation of triaxial test ................................................................................................... 79 5.6. NorSand-Biot input parameters for Colebrook silty clay................................................... 80 6.1. List of scenarios for NorSandBiot code sensitivity analysis .............................................. 99 6.2. Parametric study results.................................................................................................... 101 6.3. Ranking of NorSandBiot formulation input parameters .................................................. 111 7.1. Modelling parameters for base case and best fit simulations.................................... 142 7.2. Undrained shear strength and sensitivity estimated from simulation of triaxial test with base case and best fit set of parameters..................................................................... 142 7.3. Pore pressure response for base case, best fit and field data (Weech, 2002) ............ 143 7.4. Variation of effective stresses with time for base case and best fit simulations........ 144 7.5. Piezometers considered for the analysis .......................................................................... 148 7.6. Final stress state for base case, best fit and Case A simulation with 5 helices ......... 152

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List of figures. LIST OF FIGURES Figure 1.1. 2.1. 2.2. 2.3. 2.4. 2.5. 2.6. 2.7. 2.8. 2.9. Page

Helical piles .......................................................................................................................... 7 Helical pile performance research site location.................................................................. 18 Site subsurface conditions at the research site .................................................................. 18 Approximate locations of subsurface investigations at the Colebrook site........................ 19 Location of CPT tests and solid-stem auger holes ............................................................. 19 Variation of field vane shear strength test results with elevation....................................... 20 Example of cone penetration test results (CPT-7).............................................................. 21 Helical piles geometry ....................................................................................................... 22 Helical piles locations......................................................................................................... 23 Variation of excess pore pressure with pile tip depth, S/D=1.5 ......................................... 24

2.10. Variation of excess pore pressure with pile tip depth, S/D=3 ............................................ 25 2.11. Radial distribution of excess pore pressure generated by penetration of pile shaft .......... 26 2.12. Radial distribution of maximum excess pore pressure after penetration of helices .......... 27 2.13. Radial distribution of excess pore pressure around helical piles (above level of bottom helix) during dissipation process ....................................................................................... 28 2.14. Radial distribution of excess pore pressure above & below level of bottom helix during dissipation process ............................................................................................................. 28 2.15. Average dissipation trends for different radial distances from pile .................................. 29 2.16. Dissipation curves from piezometers/piezo-ports located at different radial distances from pile ................................................................................................................................ 29 3.1. 3.2. 3.3. 3.4. 3.5. 3.6. 3.7. 3.8. 3.9. 4.1. Effect of pile installation on soil conditions ...................................................................... 44 Measured excess pore pressures due to installation of piles ............................................. 44 Typical pore pressure dissipation measured during CPTU tests ....................................... 45 Increase in pile bearing capacity with time ....................................................................... 46 Increase in pile bearing capacity and pore pressure dissipation ........................................ 46 Comparison of variation of pile bearing capacity with time and theoretical decay of excess pore pressure .......................................................................................................... 47 Idealized schematics of soil set-up phases ........................................................................ 47 Cavity expansion zones along pile .................................................................................... 48 Comparison of measured and theoretical soil displacements due to pile penetration ....... 48 Schematic representation of 2-D modelling approach ...................................................... 60viii

List of figures. 4.2. 4.3. 4.4. 4.5. 4.6. 4.7. 4.8. 4.9. 5.1. 5.2. 5.3. 5.4. 5.5. 5.6. 5.7. 5.8. 5.9. Conceptual representation of modelling of helical pile installation as an expansion of cylindrical cavity in 2-D .................................................................................................... 61 Conceptual representation of modelling of helical pile installation as an expansion of cylindrical cavity in 1-D .................................................................................................... 61 Normal compression lines from isotropic compression tests on Erksak sand ................... 62 Definition of NorSand parameters , , , and R ........................................................... 62 Definitions of internal cap, pi, pc, Mtc, Mi and L on yield surface for a very loose sand .. 63 Conventional and NorSand representation of overconsolidation ratio for soil initially at p = 500 kPa subject to decreasing mean stress ..................................................................... 63 NorSand fit to Bothkennar Soft clay in CK0U triaxial shear ............................................ 64 NorSand simulation fit to constant p=80kPa drained triaxial test on Bonnie silt ............. 65 Typical shear modulus reduction with strain level for plasticity index between 10% and 20% 81 Level of shear strain for various geotechnical measurements ........................................... 81 Variation of small strain shear modulus Gmax with elevation ............................................ 82 Inferred variation of rigidity index with depth .................................................................. 83 Variation of shear modulus G with elevation .................................................................... 84 Range of overconsolidation ratio OCR with elevation ...................................................... 85 Variation of coefficient of earth pressure K0 with elevation ............................................. 86 Variation in estimated coefficient of horizontal consolidation with depth ....................... 87 Variation in estimated coefficient of horizontal consolidation with elevation with corrected CPTU derived values ........................................................................................ 88

4.10. Flow chart for large strain numerical code ........................................................................ 66

5.10. Variation of vertical effective stress with elevation .......................................................... 89 5.11. Variation of equilibrium pore water pressure with elevation ............................................ 90 5.12. Probable range of slope of critical state line, .................................................................. 91 5.13. Variation of void ratio with mean effective stress based on data reported by Crawford & Campanella (1988) ............................................................................................................ 92 5.14. Variation of state parameter and overconsolidation ratio with mean effective stress ....... 92 5.15. Simulation of drained triaxial test with NorSand model, using base case set of input parameters .......................................................................................................................... 93 5.16. Simulation of undrained triaxial test with NorSand model, using base case set of parameters .......................................................................................................................... 94 6.1. 6.2. 6.3. FE Mesh for Parametric Study ........................................................................................ 114 Cylindrical cavity expansion from non-zero radius ........................................................ 114 Radial distribution of generated excess pore water pressure at the end of cavity expansion for base case scenario ................................................................................................... 115ix

List of figures. 6.4. 6.5. 6.6. 6.7. 6.8. 6.9. Time dependent pore pressure response at cavity wall for base case scenario ........... 115 Stress path for base case scenario ................................................................................ 116 Variation of void ratio, e, with mean effective stress, p for base case simulation ..... 116 Variation of e with p for base case, 20 & 21 scenarios ............................................... 117 Effect of K0 on radial distribution of generated excess pore pressure at the end of cavity expansion ......................................................................................................................... 117 Effect of K0 on time dependent pore water pressure response at cavity wall .................. 118

6.10. Stress paths for base case, 1 & 2 scenarios .................................................................. 118 6.11. Effect of coupled R & on radial distribution of excess pore pressure response at the end of cavity expansion .......................................................................................................... 119 6.12. Effect of coupled R & on time dependent pore water pressure response at cavity wall 119 6.13. Effect of uncoupling R & on radial distribution of excess pore water pressure response at the end of cavity expansion, for simulations with positive ...................................... 120 6.14. Effect of uncoupling R & on time dependent pore water pressure response at the cavity wall, for simulations with positive ............................................................................... 120 6.15. Effect of uncoupling R & on time dependent pore pressure response at the cavity wall, for simulations with negative . ...................................................................................... 121 6.16. Generation of excess pore pressure during cavity expansion for the first mesh element adjacent to the cavity, presented in terms of pore pressure components ......................... 121 6.17. Effect of uncoupling R & on radial distribution of excess pore water pressure response at the end of cavity expansion, for simulations with negative . .................................... 122 6.18. Radial distribution of different excess pore pressure components for scenario 5a ......... 122 6.19. Radial distribution of generated pore pressure, for scenario 5a, at different levels cavity expansion ......................................................................................................................... 123 6.20. Initial conditions in e-ln (p) space for scenarios 3..6 and base case .............................. 123 6.21. Stress paths for scenarios 36 and base case .................................................................. 124 6.22. Variation of e with p for scenarios 36 and base case ................................................... 124 6.23. Effect of G on radial distribution of excess pore pressure at the end of cavity expansion .125 6.24. Effect of G on time dependent pore pressure response at cavity wall ............................. 125 6.25. Stress paths for scenarios base case, 7, 8 & 9 ............................................................... 126 6.26. Effect of on radial distribution of excess pore pressure at the end of cavity expansion .. 126 6.27. Effect of on time dependent pore water pressure response at cavity wall .................... 127 6.28. Stress paths for scenarios base case, 22 & 23. .............................................................. 127 6.29. Effect of on radial distribution of excess pore water pressure at the end of cavity expansion ......................................................................................................................... 128 6.30. Effect of on time dependent pore water pressure response at cavity wall .................... 128x

List of figures. 6.31. Stress paths for scenarios base case, 10 & 11 ................................................................ 129 6.32. Effect of & on radial distribution of excess pore pressure at the end of cavity expansion.......................................................................................................................... 129 6.33. Effect of & on time dependent pore water pressure response at cavity wall ............. 130 6.34. Stress paths for scenarios base case, 12 & 13 ................................................................ 130 6.35. Effect of Mcrit on radial distribution of excess pore pressure at the end of cavity expansion .131 6.36. Effect of Mcrit on time dependent pore water pressure response at cavity wall. .............. 131 6.37. Stress paths for scenarios base case, 14 & 15 ................................................................ 132 6.38. Effect of Hmod on radial distribution of excess pore pressure at the end of cavity expansion 132 6.39. Effect of Hmod on time dependent pore water pressure response at cavity wall ............... 133 6.40. Stress paths for scenarios base case, 14 & 15 ................................................................ 133 6.41. Effect of on radial distribution of excess pore pressure at the end of cavity expansion...134 6.42. Effect of on time dependent pore water pressure response at cavity wall .................... 134 6.43. Stress paths for simulations with base case, scenario 18 & 19 set of input parameters .. 135 6.44. Effect of permeability, k, on radial distribution of excess pore pressure at the end of cavity expansion.......................................................................................................................... 135 6.45. Effect of permeability, k, on time dependent pore pressure response at cavity wall ........ 136 6.46. Stress paths for scenarios base case, 20 & 21. .............................................................. 136 6.47. Location of final stress state in q-p space, at the end of pore pressure dissipation, in relation to critical state line ............................................................................................................ 137 7.1. 7.2. 7.3. 7.4. 7.5. 7.6. 7.7. 7.8. Radial pore pressure distribution at the end of pile installation reported by Levadoux & Baligh (1980), measured by Weech (2002) and simulated with base case parameters .. 158 Time-dependent pore pressure response at the pile shaft/soil interface measured by Weech (2002) and simulated with base case parameters.......................................................... 158 Comparison of modelled undrained triaxial response for best fit and base case sets of NorSandBiot input parameters ........................................................................................ 159 Radial pore pressure distribution at the end of pile installation reported by Levadoux & Baligh (1980), measured by Weech (2002) and simulated with best fit parameters .... 160 Time-dependent pore pressure response at the pile shaft/soil interface measured by Weech (2002) and simulated with best fit parameters.............................................................. 160 Comparison of u/v0 and v/v0 vs. time for best fit and base case simulation and the field measurements ........................................................................................................... 161 Stress path plot for central gaussian point of the mesh element adjacent to the cavity wall (r/Rshaft = 1.08) for simulation of helical pile shaft installation with best fit parameters. 161 Void ratio versus mean stress (e-ln(p)) plot for central gaussian point of the mesh element adjacent to the cavity wall (r/Rshaft = 1.08) for simulation with best fit parameters ........................................................................................................................ 162xi

List of figures. 7.9. Modelling cases considered in the analysis of the effect of the helices ........................... 163

7.10. Modelling algorithm of helical piles installation in 1-D ................................................. 163 7.11. Comparison of time dependent pore pressure response during helical pile installation measured in the field and simulated using NorSandBiot formulation (Case A). ............ 164 7.12. Comparison of time dependent pore pressure response during helical pile installation measured in the field and simulated using NorSandBiot formulation (Case B). ............. 165 7.13. Comparison of radial pore distribution for simulations with and without helices and the field measurements........................................................................................................... 166 7.14. Radial pore pressure distribution during first helix expansion (Case A).......................... 166 7.15. Radial pore pressure distribution during first helix contraction (Case B)........................ 167 7.16. Radial pore pressure distribution during expansion/contraction cycles for simulation of helical pile with 5 helices (Case A).................................................................................. 167 7.17. Radial pore pressure distribution during expansion/contraction cycles for simulation of helical pile with 3 helices (Case A).................................................................................. 168 7.18. Radial pore pressure distribution during expansion/contraction cycles for simulation of helical pile with 5 helices (Case B). ................................................................................. 168 7.19. Radial pore pressure distribution during expansion/contraction cycles for simulation of helical pile with 3 helices (Case B). ................................................................................. 169 7.20. Time dependent pore pressure response at the cavity wall for simulation of helical pile with 5 helices (Case A)..................................................................................................... 170 7.21. Time dependent pore pressure response at the cavity wall for simulation of helical pile with 3 helices (Case A). ................................................................................................... 170 7.22. Time dependent pore pressure response at the cavity wall for simulation of helical pile with 5 helices (Case B)..................................................................................................... 171 7.23. Time dependent pore pressure response at the cavity wall for simulation of helical pile with 3 helices (Case B)..................................................................................................... 171 7.24. Stress path plot for mesh element adjacent to the cavity wall (r/Rshaft = 1.08) for simulation of helical pile shaft installation....................................................................... 172 7.25. Void ratio versus mean stress (e ln(p)) plot for mesh element adjacent to the cavity wall (r/Rshaft = 1.08).................................................................................................................. 172 7.26. Comparison of stress paths for central gaussian point of the mesh element adjacent to the cavity wall (r/Rshaft = 1.08) for simulations with different set of input parameters and modelling schemes ............................................................................................................ 173 7.27. Radial pore pressure distribution during expansion/contraction cycles for simulation of helical pile with 5 helices (Case A. Assumption 2)......................................................... 174

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Acknowledgements.

ACKNOWLEDGEMENTS. I wish to thank my scientific supervisors, Dr. Dawn Shuttle and Dr. John Howie for their invaluable guidance throughout this project. Dr. Shuttle was always willing to assist with solving the most challenging problems and had always been a source of brilliant ideas. Her ability to explain complex concepts with clarity and ease and her truly endless patience are greatly appreciated. Dr. Shuttles enthusiasm for this project had never run out and her pressure, in a good sense, kept me going. My study at the University of British Columbia was a great learning experience. I would like to thank Dr. Howie for taking me into the UBC Geotechnical Group. It was always a great pleasure to work with him. Thoughtful contributions of Dr. Howie to many discussions related to this project are sincerely appreciated. I would like to express my gratitude to Dr. Michael Jefferies for shearing the code and for his valuable suggestions. Special thanks for the ideas and helpful information belongs to my fellow graduate students: Sung Sik Park, Mavi Sanin, Ali Amini and Somasundaram Sriskandakumar. My deep appreciation goes to my fianc Valeria and my stepson Vadim, who inspired me all the way through. Their patience and moral support are greatly acknowledged. Most of all, I would like to thank my parents Sofia & Mikhail, and my elder brother Alexei. Their unconditional love has always been there for me. I am indebt for their steadfast backing of my intellectual and spiritual growth. This thesis is one of the fruits of their dedication and love. There will be many more to come. I dedicate this work to my beloved family.

PER ASPERA AD ASTRA

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Chapter 1. Introduction. 1. INTRODUCTION.

1.1. CHALLENGES IN AXIAL PILE CAPACITY PREDICTIONS IN SOFT FINE-GRAINED SOILS. Piles are relatively long and normally slender structural foundation units that transfer superstructure loads to underlying soil strata. Presently there are more than 100 different types of piles. The major share in piling foundations belongs to driven or jacked piles of various shapes, which are often referred to as traditional piles. In geotechnical practice, piles are usually employed when soil conditions are not suitable for use of shallow foundations, i.e. when the upper soil layers are too weak to support heavy vertical loads from the superstructure. Piles transfer vertical loads by friction along their surface and/or by direct bearing on the compressed soil at, or near, the pile tip. Given that the pile material is not over-stressed, the ultimate axial load capacity of a pile is equal to the sum of end bearing and side friction. The amount of resistance contributed by each component varies according to the nature of load support, soil properties and pile dimensions. Prediction of pile capacity is complicated by the fact that during installation the soil surrounding the pile is severely altered. This is particularly relevant for piles installed in thick deposits of soft fine-grained soils, where the friction along the shaft is usually a prime factor governing the pile capacity. Soft-fine grained soils are known for their tendency to lose strength when disturbed, and their slow rate of strength recovery following disturbance. Gradual gain of pile capacity with time after pile installation is a well-known occurrence. Although factors such as thixotropy and aging contribute to this phenomenon, the most significant cause for gain of capacity with time is associated with the dissipation of the excess pore water pressure generated during pile installation. The processes occurring during and after pile installation has a very limited analytical treatment and pile design is still largely relies on empirical correlations. At a recent

symposium on pile design (Ground Engineering, 1999) the participants were asked to provide a prediction of the capacity of a single driven steel pile. The general success rate was very poor with only 2 of 16 teams getting within 25% of the correct capacity. The best prediction of the piles capacity was obtained from compensating errors; a too low side friction capacity1

Chapter 1. Introduction. was balanced by a too high end bearing. Randolph in his Rankine lecture (2003) also Due to shortcomings in pile capacity

recognized the lack of accuracy in pile design.

predictions geotechnical engineers have to rely on pile load tests to refine final piling foundation design. The ability to accurately predict the variation of stresses and pore pressures in fine-grained soil due to pile installation is a key to improving pile capacity prediction capabilities. The problem of predicting the variation of pile capacity in fine-grained soils is one of predicting the excess pore pressure and associated stresses at the pile shaft as a function of time. It appears that development of a robust technique for evaluation of pore pressure changes due to pile installation will provide a basis from which a method accounting for capacity gain with time in design and testing can be developed. This study is concerned with modelling the time-dependent pore pressure response due to helical pile installation in soft fine-grained soil. 1.2. HELICAL PILES.

A helical pile is an assembly of mechanically connected steel shafts with a series of steel helical plates welded at particular locations on the lead section, as shown in Fig. 1.1.a. Historically helical piles have evolved from early foundations known as screw piles. The screw piles have been in use since the early 19th century. Early applications of these piles were based on hand installation. The first power installed screw piles were employed during construction of a series of lighthouses in England in 1833 (Wilson & Guthlac, 1950). Generally, the screw

piles had a very limited use until the 1960s; when reliable truck mounted hydraulic torque motors became readily available. Nowadays the major helical piles manufacturer is a USA based company - AB Chance Ltd. They manufacture piles with the shaft 3.8 25 cm and helical plates 15 - 36 cm. The diameter of manufactured piles is quite small and their application is currently restricted to relatively small jobs. It appears that the potential of helical piles is not fully exploited to date. A new boost in helical piles application is expected from recent development of high capacity torque units, which will make possible installation of helical piles with larger diameters, installed to greater depths.2

Chapter 1. Introduction. Generally, helical piles can be employed in any application where driven and jacketed piles are used, except for the cases where penetration of competent rock is required. Currently helical piles found application in the following areas: foundation repairs, upgrades & retrofits; pump-jacks and compressor stations for oil and gas industry (large diameter piles); pipelines support; foundations for temporary and mobile structures.

Experience with conventional (small diameter) helical piles in soft soils in British Columbia revealed a tendency for buckling of the slender steel shaft during loading. Aiming to reduce the buckling effect, placement of grout around the shaft was proposed and patented by Vickars Developments Co. Ltd, as grouted, or PULLDOWNTM, pile, shown in Fig. 1.1.b. Normally, helical piles are installed by sections. The leading section, also called a screw anchor, is placed into the soil by rotation of the pile shaft using a hydraulic torque unit. The pile is screwed into the ground in the same method a wood screw is driven. Helical plates of the

leading section create a significant pulling force that makes the shaft advance downwards. Following the screw anchor installation, extension sections are bolted to the top of the screw anchor shaft. Installation continues by resumed rotation, and further extension sections are added until the project depth of the pile is reached. For the grouted helical piles, at each

sections connection, displacement plates are attached to the shaft. During pile installation they create a cylindrical void, which is filled by the flowable grout. Helical piles have several distinctive advantages over traditional driven and jacketed piles: mobilize soil resistance both in compression and uplift; quick and easy to install: vibration free, no heavy equipment required, possible to install inside buildings (for small diameter piles); reusable.

Helical piles are typically installed in soils that permit the compressive capacity of the pile to be developed through end-bearing below each of the helices at the bottom of the pile. Where the thickness of soft cohesive strata is too extensive to make it practical to advance helical piles to a competent bearing stratum, it may be necessary to develop the capacity of the piles in friction within the soft cohesive soil. However, experience using helical piles in such soils is limited at this time, as is the understanding of the complex sensitive fine-grained soil-helical pile interaction.3

Chapter 1. Introduction. 1.3. PURPOSES AND OBJECTIVES OF RESEARCH. Helical piles are gaining popularity in North America as an alternative foundation solution to traditional driven and jacked piles. To date the major research efforts in the field of helical piles have concentrated on their lateral and uplift capacity. However, limited knowledge of the timedependent effect of helical pile installation on soil behaviour remains a significant drawback to their widespread application in soft fine-grained soils. Pore pressure response due to helical pile installation has not been studied until very recently. Field studies of helical pile performance in soft silty clay, carried out by Weech (2002) in Surrey, British Columbia, provide quality data on the pore pressure regime during and after helical pile installation. Given natural constraints of the field studies, such as a limited number of measuring points and measurements accuracy, numerical simulation provides an effective tool for improving our understanding of complex response of soft fine-grained soil due to helical pile installation. The main objectives of this research are: Develop a modelling approach that will realistically simulate the pore pressure response during helical pile installation and the subsequent excess pore water pressure dissipation with time. Numerically model helical pile installation into the soft fine-grained soil at the Colebrook helical pile research site and investigate pore water pressure response during and after helical pile installation. Compare and contrast the modelled response with the field measurements and the field interpretations performed by Weech (2002). The ability to understanding and predict the impact of pile installation on soft fine-grained soil will contribute to improving existing pile bearing capacity calculation methods. In addition the conducted research will be a major step towards development of an independent geotechnical software tool, that will be able to help practicing engineers to estimate variation of bearing capacity with time after pile installation. The developed numerical approach should be extendable to other than helical types of piles, which is to be confirmed by additional research. 1.4. SCOPE AND LIMITATION OF STUDY. The conducted study is mainly focused on soil pore water pressure response due to pile penetration, as it is believed to be an important factor affecting the variation of pile bearing4

Chapter 1. Introduction. capacity with time. Adequate simulation of the pore water pressure response in the soft finegrained soil requires a realistic soil model and a fully coupled modelling approach. NorSandBiot formulation adopted in the current study incorporates the NorSand soil model (Jefferies, 1993; Jefferies & Shuttle, 2002) to represent the fine-grained soil stress-strain behaviour and the Biot (Biot, 1941) consolidation theory to account for the effect of coupling the pore pressure response to behaviour to the soil stress-strain behaviour. All numerical simulations conducted in the current study were based on the finite element implementation of the NorSandBiot formulation developed by Shuttle (2003). Pore pressure and stress predictions of the NorSandBiot code were successfully verified against a number of available analytical solutions. Given the complexity of helical pile installation process, numerical simulation of excess pore pressure generated due to helical pile installation poses many challenges. It appears that the most realistic simulation of helical pile installation will require a 3-D approach, which is hard to implement and widely apply. The focus of the current research was on developing simple, yet realistic representation of pore pressure response. It was necessary to neglect some features of helical pile-soil interaction while simplifying the analysis. In the present study helical pile installation was analyzed in 1-D employing the cylindrical cavity expansion analogue. A better insight in pore pressure response induced due to helical pile installation may be achieved when the effect of soil remoulding and 2-D effects of soil response are considered. Due to the large volume of the conducted study these issues were left for future research. Laboratory study was also beyond the scope of this work. Modelling input parameters were derived from three previous investigations of Colebrook silty clay properties. They explicitly provided many, but not all, of the input parameters required for the NorSandBiot formulation. Some of the input parameters were taken as a best estimate, believed and shown to be reasonable based on all information available. Another challenge in establishing input

parameters resulted from differences between laboratory and in-situ derived values of soil properties. This is not unusual in a silty site where soil disturbance during sampling is a major issue. Local spatial property variation, as seen in the in situ measurements, added to parameter uncertainty. It appears that detailed laboratory study is required to refine the modelling input parameters taken in the current study.

5

Chapter 1. Introduction. 1.5. THESIS ORGANIZATION. In Chapter 1 of this thesis helical piles are introduced, research purposes and objectives are stated, along with the scope and limitations of the conducted study. An overview of the study of helical pile performance in soft fine-grained soils, carried out by Weech (2002), is given in Chapter 2. This comprises a description of the scope of the work, information on site stratigraphy and basic soil properties, geometry of the tested piles and measuring equipment. A brief outline of the results of the Weechs study relevant to the current research is also presented. Chapter 3 reviews the literature to provide information leading to the formulation of the modelling approach. Modelling approach adopted in this study is formulated in Chapter 4. NorSand critical state soil model and Biot consolidation theory are presented along with their finite-element implementation. Formulation input parameters are explained. Chapter 5 describes the selection of site-specific soil parameters for modelling. Overview of all available subsurface information is given. Selection process for all model input parameters is individually analyzed. Best estimates of the soil properties for modelling are presented. In Chapter 6, the description and results of the NorSand-Biot formulation parametric study are presented. An accent is put on highlighting the input parameters that have the most profound influence on the modelling results. Chapter 7 presents modelling results and their analysis. A comparison of modelling with the available field data, including Weech (2002) measurements, is provided and discussed. Effects of the pile shaft and the helices on pore pressure response are separately analysed. from the modelling are presented. Chapter 8 provides conclusions from the current study and recommendations for further research. Implications

6

Chapter 1. Introduction.

a

b

Fig. 1.1. Helical piles: a conventional pile; b grouted (PULLDOWNTM) pile.

7

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. 2.0 OVERVIEW OF FIELD STUDY OF HELICAL PILE PERFORMANCE IN SOFT SENSITIVE SOIL. 2.1. INTRODUCTION. This study develops a numerical formulation to analyze pore pressure response due to helical pile installation. As a basis for development of a robust numerical approach to modelling of time dependent pore pressure response, induced by helical pile installation, high quality field data is essential. Information obtained in the field provides an initial framework of expected soil response and can serve as a reference point for modelling results verification. A comprehensive field study of helical pile performance in sensitive fine-grained soils, conducted at Surrey, British Columbia, by Weech (2002), was chosen as a source of necessary background information for numerical analysis in a current research. Weechs study was mainly oriented towards improving understanding of the effects that the installation of helical piles has on the strength characteristics of sensitive fine-grained soils. Current research is focused on time-dependent pore water pressure response due to helical pile installation. In this chapter a brief overview of Weechs work is given and Weechs key findings relevant to the current study are presented. In addition a review of available information on site subsurface conditions is provided. 2.2. SCOPE OF WEECHS STUDY. Six instrumented full-scale helical piles were installed in soft sensitive marine deposits. Prior to pile installation, an in-situ testing program was carried out, that consisted of: two profiles of vane shear tests; five piezocone penetration soundings, with pore pressure dissipation tests carried out at two soundings and shear wave measurements at three soundings. The excess pore pressures within the soil surrounding the piles were monitored during and after pile installation by means of piezometers located at various depths and radial distances from the pile shaft, and using piezo-ports, which were mounted on the pile shaft. After allowing a recovery period following installation, which varied between 19 hours, 7 days and 6 weeks, piles with two different helix plate spacing were loaded to failure under axial8

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. compressive loads. Strain gauges mounted on the pile shaft were monitored during load testing to determine the distribution of loading throughout the pile at the various load levels up to and including failure. Load-settlement curves were generated for different pile sections at different times after installation. The piezometers and piezo-ports were also monitored during load testing and the distribution of excess pore pressures 2.3. SITE SUBSURFACE CONDITIONS. The test site, also referred to as the Colebrook site, is located under the King George Highway (99A) overpass over Colebrook Road and the adjacent BC Railway line, South Surrey, BC; approximately 25 km southwest of downtown Vancouver, as shown in Fig. 2.1. 2.3.1. SITE STRATIGRAPHY. The subsoils found in this area belong to so called Salish Sediments. According to Armstrong (1984): Salish sediments include all postglacial terrestrial sediments and postglacial marine sediments that were deposited when the sea was within 15 m of present sea level. These deposits were likely laid down during the Quaternary period between 10,000 and 5,000 years ago. Cross-section of site stratigraphy is shown on Fig. 2.2. From the surface there is a layer of fill, about 0.6 m thick, which was placed during 99A Highway construction. The fill is underlain by a layer of firm to stiff peat, possibly bog and swamp deposit, that formed the original ground surface; the thickness of this peat layer is about 0.3 m. Below the peat there is a layer of firm clayey silt of deltaic origin, with some sand inclusions. The thickness of this layer is about 1 m. The layer of clayey silt is underlain by layer of soft silty clay with organic inclusions (peat, plant stalks). Given the proximity of the Serpentine river, this deposit likely has a tidal origin: it was deposited within the inter-tidal zone between the Serpentine river delta and Semiahmoo Bay. Below the silty clay layer there is a thick (around 27 m) layer of soft clayey silt to silty clay of marine origin. The marine deposits are underlain by a stiff layer of sand and gravels of glacial origin. Crawford & Campanella (1991) reported slight artesian pressure at the interface of the silty clay layer and glacial deposits. Weech (2002) indicated that the groundwater table can be found at 2 m elevation (0.7m from the surface), with an upward hydraulic gradient of 5 to 10 %, which is possibly explained by the groundwater recharge from the upland area north of the site.9

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. 2.3.2. SOIL PROPERTIES. Three subsurface investigations were performed at, or close to, the helical piles performance research site. Site plan and locations of all subsurface investigations are presented in Fig. 2.3. A brief description of each investigation and their reviews reported in the literature are presented below in chronological order. 2.3.2.1. FIELD INVESTIGATION BY MINISTRY OF TRANSPORTATION AND HIGHWAYS. Prior to construction of the Colebrook Road overpass (Highway 99), the Ministry of Transportation and Highways (MoTH) performed an extensive geotechnical study of the soil conditions along the alignment of a planned overpass (in 1969). The MoTH investigation included dynamic cone penetration tests and drilling with diamond drill to establish the depth and profile of the competent stratum underlying the soft sediments. Field vane shear tests were performed at selected depths. Undisturbed samples of the soft soils were recovered with a Shelby tube sampler. A number of laboratory tests were carried out on the MoTH samples, including index tests, consolidated and unconsolidated triaxial tests and laboratory vane shear tests. Crawford & deBoer (1987) studied the long-term consolidation settlements underneath the approach embankments, located in the vicinity of the helical piles performance research site. They reported some of the data obtained during the MoTH investigation - typical for the Colebrook site soil properties and results of three unidirectional consolidation tests performed in a triaxial cell, with radial drainage. Crawford & deBoer (1987) report, based on laboratory testing, an average coefficient of consolidation in the horizontal direction, ch = 1.510-3 cm2/s, an average coefficient of secondary consolidation, C = 0.014 and an initial void ratio, for all three tests, e0 = 1.25. A summary of typical soil properties from MoTH investigation given by Crawford & deBoer (1987) are presented in Table A.1 (Appendix A). 2.3.2.2. RESEARCH BY UNIVERSITY OF BRITISH COLUMBIA (1). Crawford & Campanella (1991) reported the results of a study of the deformation characteristics of the subsoil, using a range of in-situ methods and laboratory tests to predict soil settlements underneath the embankment, and compare them with the actual settlements. In-situ tests included field vane shear tests, piezocone penetration test (CPTU) and a flat dilatometer test (DMT). Laboratory tests were limited to constant rate of strain odometer consolidation tests on10

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. specimens obtained with a piston sampler. Results of a series of the CRS consolidation tests are presented in Table A.2 (Appendix A). As a continuation of previous works by Crawford & deBoer (1987) and Crawford & Campanella (1991), Crawford et al. (1994) studied the possible reasons for the difference between predicted and measured consolidation settlements underneath the embankment using the finite-element consolidation analysis with CONOIL computer program (by Byrne & Srithar, 1989). The soil properties employed in the numerical analysis are shown in Table A.3 (Appendix A). 2.3.2.3. RESEARCH BY UNIVERSITY OF BRITISH COLUMBIA (2). As a part of his study of helical pile performance in soft soils, a comprehensive investigation of site soil conditions was carried out by Dolan (2001) and Weech (2002). Dolan (2001) obtained continuous piston tube samples from ground level to 8.6 m depth and performed index testing to determine natural moisture content, Atterberg limits, grain-size distribution, organic and salt content. Results of index tests carried out by Dolan (2001) on samples obtained with the piston tube sampler are summarized in Table 2.1 Table 2.1. Average index properties of clayey silt/silty clay layer (elevation -4.1 m and below). Soil Property natural moisture content (wn) liquid limit (wL) plasticity index (PI) unit weight () in-situ void ratio (eo) Average Value 42%+/-3% 40%+/-4% 13.5%+/-4.5%, 17.8+/-0.3 kN/m3 1.16+/-0.09 Comments below 8m in elevation PI is up to 21% derived from moisture content data, assuming specific gravity of 2.75

Weech (2002) carried out a detailed in-situ site characterization program, which included field vane shear tests; cone penetration tests with pore pressure (CPTU) and shear wave travel time measurements (SCPT). Locations of sampling and in-situ soundings are presented in Fig. 2.4. presented in Table A.4 (Appendix A).11

A summary of

engineering parameters for the silty clay layer, estimated from in-situ tests by Weech, are

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. Field vane shear strength profiles for the Colebrook site measured by Weech (2002) and Crawford & Campanella (1991) are shown in Fig. 2.5. In Fig. 2.5a the peak undrained shear strength is plotted with depth. For the clayey silt/silty clay layer it varies from 15 to 30 kPa. The profile of the remoulded shear strengths, (su)rem, is also plotted on Fig. 2.5a, showing a variation from 2 to 0.7 kPa within the clayey silt/silty clay layer. Due to such low remoulded strengths, the sensitivity, St = (su)peak/(su)rem, determined from the field vane measurements is very high. Profiles of sensitivity are shown on Fig. 2.5b. The sensitivity appears to increase approximately linearly with depth from a minimum of 6 at surface to about 40 at 12 m elevation. Even higher sensitivity, in the range of 50 to 75, was measured by Crawford & Campanella (1991) between 12 and 17 m, who state that the high sensitivity of the marine deposits is likely caused by leaching of pore-water salts due to the slight artesian conditions, particularly at the lower depth. The ratio of su to the effective overburden pressure, vo, is presented in Fig. 2.5c. In the upper part of the marine deposits (from 4.1 to 4.4 m in elevation) the su/vo ratio is quite high around 0.7, which indicates moderately overconsolidated soil. At lower depths the deposit is lightly overconsolidated, with the su/vo ratio around 0.4. A typical CPT cone test result for Colebrook site, including profiles of corrected tip resistance, qT, sleeve friction, fs, and excess penetration pore pressure, u, measured behind the shoulder of the cone (u2 filter position), are presented on Fig. 2.6. A detailed overview of the soil properties, relevant to the current study, is given in Chapter 5. 2.4. HELICAL PILES AND PORE PRESSURE MEASURING EQUIPMENT. 2.4.1. TEST PILES GEOMETRY AND INSTALLATION DETAILS. For the purpose of studying different failure mechanisms, piles with two different lead sections were used. The largest helical piles manufacturer, Chance Anchors, commonly uses helical plates attached to the lead section such that the distance between successive plates (S) is 3 times the diameter (D) of the lower plate. In this case, current thinking based on small scale model tests (Narasimho Rao et al., 1991) is that during loading to failure, failure occurs at individual helices. For the closer spacing of the helical plates, the failure mechanism is believed to be different - all helices fail simultaneously, so that a cylindrical failure surface is generated12

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. coinciding with the outside edge of the helical plates. To investigate such a possibility the testing was carried out on piles which had either 3 plates at S/D = 3, or 5 plates at S/D = 1.5, so that the total length from the top to bottom helix was equal for the two pile types (2.1 m). The pitch of the helix plates was 7.5 to 8 cm, which is the standard pitch on helical piles manufactured by Chance Anchors. The geometry of both types of lead sections is shown in the Fig. 2.7. In total six helical piles - three for each leading section type were installed, their locations are shown in Fig. 2.8. Two piles, TP-1 - with three helices (S/D = 3) and TP-2 with five helices (S/D = 1.5), were chosen for the detailed monitoring. The other piles served as a source of additional information. All piles were installed to a tip depth of 8.5 m (-9.8 in elevation). Installation of a single pile, including breaks for section mounting and adjustments to maintain pile verticality, usually took about 2 hours. Deducting interruptions, the average rate of soil penetration by helical pile was about 1.5 cm/s. 2.4.2. MEASURING EQUIPMENT. A total of 26 UBC push-in piezometers were installed at different depths and radial distances from the 6 test piles, and a total of 10 piezo-ports were located at 3 different positions on the shaft of the piles, as indicated in Table B.1 (Appendix B). Piezo-ports, which contained an electric pore pressure transducer with a porous filter, were installed within the wall of the pile shaft on the lead sections. The piezometers were pushed into the soil at least one week prior to pile installation so that full dissipation of the excess pore pressures generated during piezometer installation could occur. These piezometers were then used to monitor the variation in pore pressures caused by pile installation and their subsequent dissipation. During pile installation piezometers were continuously monitored using the multi-channel data acquisition system. After the end of pile installation piezoports located on the pile shaft were also connected to the data acquisition system and were continuously monitored in conjunction with the piezometers. Two types of electronic pore pressure transducers were employed for the piezometers and the piezoports, with measuring capacity 345 and 690 kPa. The resolution of the automatic acquisition system used to monitor the piezometers was 0.035 to 0.07 kPa (for 345 and 690 kPa transducers, respectively). The rated accuracy of the pressure transducer measurements was 0.1% of full scale. Even though every attempt was made to carefully assemble and install measuring equipment, the response of many piezometers and piezoports was less than perfect, as shown in Table B.1.13

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. 2.5. SUMMARY OF WEECHS STUDY RESULTS. This summary is based on Weechs interpretations of pore pressure response measured during and after helical pile installation. Only key points are presented here, more details can be found in Weech (2002). 2.5.1. PORE WATER PRESSURE RESPONSE DURING HELICAL PILE INSTALLATION. Pore pressure profiles measured at different radial distances during installation for piles TP-1 and TP-2 are shown in Fig. 2.9 and Fig. 2.10. In these figures profiles of normalized peak pore pressure ui/vo are plotted against the depth of the pile tip below the elevation of the piezometer filter (zpile zpiezo). For reference, the locations of the different parts of the pile relative to the tip are also shown on the right side of these figures. Based on Fig. 2.9 and 2.10 Weech (2002) made the following observations: There is a very sudden increase in ui as the tip of the pile shaft approaches and then passes the elevation of the piezometer filters. This increase is particularly abrupt at the piezometers located closer to the pile. The magnitude of excess pore pressure generated within the soil by the pile installation decreases with radial distance from the pile. Negative pore pressures were observed just before the pile tip passes the piezometers locations. Baligh & Levadoux (1980) linked such behaviour with vertical displacement of soil in advance of a penetrating pile or probe, which is initially downward. According to Weech (2002), downward soil movement relative to the static piezo-cell induces a short lived tensile pore pressure response which is observed just before the response becomes compressive with a primarily radial displacement vector. Each helical plate passing the piezometers generates a pulse in pore pressure. The first pulse generated by a leading helical plate is the strongest, all subsequent helical plates generate less definitive pore pressure pulses. Such an effect is noticeable only at piezometers located within one helix radius from the helix edge (r/Rshaft1 = 7 and 8) . Only the soil located very close to the outside edge of the helix plates (within about 10 to 12 times the helix plate thickness - thx) appears to respond directly to the penetration ofIn this overview, radial distance is represented by the r/Rshaft ratio, where Rshaft is the radius of the pile shaft (in the current study, identical for all piles), r radial distance from the pile centre. 141

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. the helix plates. Within this zone, distinctly different responses are observed for the S/D = 1.5 and S/D = 3 piles. At radial distances larger than about 10-12 thx beyond the edge of the helices, the pore pressure response to the penetration of the S/D = 1.5 and S/D = 3 piles is very similar. Weech (2002) attempted to quantify separately pore pressures generated by pile shaft and the helices, where the pore pressures generated by the pile shaft were inferred from the piezometers response to penetration of the pile tip. In Fig. 2.11 is shown a radial distribution of normalized pore pressures induced by the pile tips of all test piles. According to Fig. 2.11, for r/Rshaft = 5 to 17, ushaft/vo decreases steeply and almost linearly. After r/Rshaft = 17, ushaft becomes quite small (< 0.1vo) and the slope of the pore pressure decay with distance flattens. practically negligible. In Fig. 2.12 is shown radial distribution of peak pore pressures generated, during installation, by helical pile shaft and the helices, and, the best estimate of pore pressures generated by helical pile shaft alone, so that the effect of the helical plates can be studied. Weech (2002) made the following observations from this figure: The contribution of the helical plates to the magnitude of generated pore pressures, during helical pile installation, appears to be quite significant. At distances up to r/Rshaft = 6, the pore pressures generated by the helices make up to 20% of the total pore pressures and at distances greater than r/Rshaft = 17 make up to 75% . Penetration of the helices extends the radial distance of generated pore pressures from r/Rshaft about 60, estimated for penetration of pile shaft alone, to r/Rshaft about 90. Weech (2002) argued that there appears to be a gradual outward propagation of the pore pressure induced by the helices, during continuing pile penetration, attributed to total stress redistribution caused by soil destructuring. 2.5.2. PORE WATER PRESSURE DISSIPATION AFTER HELICAL PILE INSTALLATION. Weech (2002) compiled a combined dataset of all (for piles with both S/D = 1.5 and 3) normalized piezometric measurements, taken at different times, at the locations above the bottom helical plate as presented in Fig. 2.13. Despite some scatter in the data there is a trend in the observed pore pressure dissipation behaviour, represented by the fitted curves. According to Fig.15

For r/Rshaft 60 generated pore pressures are

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. 2.13, excess pore pressure, u, decreases monotonically throughout the soil around the pile, out to a radial distance of at least 30 shaft radii. The rate of dissipation at different radial distances appears to vary such that the u(r)/vo-log(r) curve becomes more and more linear as the dissipation process progresses. Fig. 2.14 shows curves fitted to all the available data of normalized excess pore pressure measured at the location above and below the level of the bottom helical plate (where the influence of plate penetration is minimal). Weech (2002) made the following observations from this figure: No residual uhx is observed in the soil (from r/Rshaft = 5 to at least 17) below the level of the bottom helix within 10 minutes after stopping penetration Dissipation of u within the soil close to the helices (r/Rshaft < about 10) is much more rapid below the level of the bottom helix than above, at least during the first 17 - 20 hours of dissipation. The elevated pore pressures at the tail of the distribution (r/Rshaft > 17), which are due to the penetration of the helix plates, remain above the initial level generated by the pile shaft until about 20 hours. Average dissipation curves at different radial distances from the piles are shown in Fig. 2.15. Shown dissipation curves do not exhibit a unified dissipation trend at bigger times, Weech (2002) attributed this to the higher rate of dissipation at larger radial distances. In Fig. 2.16 shows the dissipation curves based on u(t)/vo data from individual piezometers/piezo-ports located at different radial distances from the test piles (above the bottom helix). Based on this figure Weech (2002) made the following observations: The dissipation occurs much more quickly below the bottom helix than above, at radial distances close to the pile. Even though greater proportions of dissipation occur sooner at larger radial distances, all of the curves tend to converge at the end of the dissipation process. For all monitored piles 100% dissipation occurred at about 7 days for most locations around the piles. The dissipation process appears to be essentially independent of the number or spacing of the helix plates.

16

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil. 2.6. SUMMARY. A comprehensive study of helical pile performance carried out by Weech (2002) was an important step towards better understanding of a complex helical pile fine-grained soil interaction. Weech reported details of the pore pressure response observed during and after installation of helical piles at the Colebrook site and attempted to interpret them. However, the presented problem analysis cannot be considered complete. The applicability of the observations made during Weechs study on sites with different soil conditions and different helical piles geometries is questionable. According to Terzaghi2: Theory is the language by means of which lessons of experience can be clearly expressed. It appears that the lessons of experience gained during Weechs study may be effectively utilized using numerical modelling. In the current study the field measurement of the pore water pressure response measured by Weech (2002) is employed as a reference point for analysing the results of numerical modelling.

2

Quote from Karl Terzaghi biography by Goodman (1999). 17

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil.

Test Site

N Surrey, BC

Fig. 2.1. Helical pile performance research site location.

Fig. 2.2. Site subsurface conditions at the research site (modified after Weech, 2002).

18

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soil.

scale - metres

Fig. 2.3. Approximate locations of subsurface investigations at the Colebrook site (modified after Crawford & Campanella, 1991).

Fig. 2.4. Location of CPT tests and solid-stem auger holes (after Weech, 2002)19

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.a)0 -3 Field Vane Shear Strength (s u)FV (kPa) 10 20 30 40

b)0 10

Sensitivity St = (su )peak/(su )rem 20 30 40 50

c)0.0 0.2

Strength Ratio s u/ 'vo 0.4 0.6 0.8

-4

-5

-6

Possibly affected by sandy silt

Elevation (m)

-7 -8

-9

-10

-11

-12Peak Strength (VH-1&2)Remoulded Strength (VH-1&2)Peak (from Craw ford & Campanella, 1991)Rem (from Craw ford & Campanella, 1991)VH-1&2Craw ford & Campanella (1991)

Fig. 2.5.

Variation of field vane shear strength test results with elevation (after Weech, 2002).20

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.

a)0 -3 1 2

Tip Resistance QT (bar)3 4 5 6 7

b)0 1

Sleeve Friction fs (kPa) 2 3 4 5 6

c)-50

Excess Pore Pressure at U2 - u (kPa) 0 50 100 150 200 250

-4

-5

-6

Elevation (m)

-7

-8

-9

-10

-11

Note: Breaks in profile correspond to data recorded upon resuming penetration after seismic tests

-12

Fig. 2.6. Example of cone penetration test results (CPT-7) (after Weech, 2002).21

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.

Fig. 2.7. Helical piles geometry (modified after Weech, 2002).

22

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.

3rd bridge pier from South abutmentpile cap

2nd bridge pier from South abutment

300 mm wide hexagonal RC piles

Helical piles

Fig. 2.8. Helical piles locations (modified after Weech, 2002).

23

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.Excess Pore Pressure during Pile Installation - ui/'vo-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

5Note: Dissipation during breaks in installation removed.

4

Grout Column

Depth of Pile Tip Below Piezo Filter Elev. (m)

3

Grout Disc

2Line of Max Pore Pressure

Helix Plates

1

0PZ-TP4-1 (r/R = 4.8) PZ-TP2-5 (r/R = 7.3)

-1

PZ-TP2-1 (r/R = 8.0) PZ-TP2-7 (r/R = 11) r = radial distance from pile center R = radius of pile shaft PZ-TP2-3 (r/R = 17) PZ-TP2-4 (r/R = 30)

-2

Fig. 2.9. Variation of excess pore pressure with pile tip depth, S/D=1.5. (after Weech, 2002)24

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.Excess Pore Pressure during Pile Installation - ui/'vo-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

5Note: Dissipation during breaks in installation removed. Grout Column

4

Depth of Pile Tip Below Piezo Filter Elev. (m)

3

Grout Disc

2Line of Max Pore Pressure Helix Plates

1

0

PZ-TP3-1 (r/R = 5.8)PZ-TP3-2 (r/R = 8.1)

-1PZ-TP1-7 (r/R = 12)r = radial distance from pile center R = radius of pile shaft

PZ-TP1-3 (r/R = 14)PZ-TP1-4 (r/R = 25)

-2

Fig. 2.10. Variation of excess pore pressure with pile tip depth, S/D=3. (after Weech, 2002).25

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.

1.8 1.7 1.6 1.5 1.4Linear Trend Line Pile Piezos (due to pile tip penetration) Pile Piezo-Ports (End of Installation)

Excess Pore Pressure during Installation - ui/'vo

1.3 1.2TP4-1

Logarithmic Trend Line

1.1 1.0 0.9

TP1-9 TP1-6

TP3-1 TP5-1 TP4-2 TP6-2 TP6-1 TP3-2

TP2-6

TP2-2

0.8 0.7

TP2-5

TP2-1 TP1-5

Linear Trend Line

0.6 0.5 0.4 0.3 0.2 0.1 0.0 1 10 100TP1-3 TP2-9

Edge of Helices

TP2-7

TP1-7

TP2-3

TP1-4

TP2-4

Radial Distance from Pile Center (shaft radii) - r/Rshaft

Fig. 2.11. Radial distribution of excess pore pressure generated by penetration of pile shaft (modified after Weech, 2002).26

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.1.8Peak u at Piezos after Passing of Pile Tip

1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0

Max u at Piezo-Ports (End of Installation) Shaft Penetration (best fit of data from Fig. 2.11) Shaft Penetration (best estimate for r < 5R)

uhx(best estimate)

u/ 'vo

0.9 0.8 0.7 0.6 0.5 0.4 0.3Edge of Helices

uhx0.2 0.1 0.0

1

10 Radial Distance from Pile Center (shaft radii) - r/R shaft

100

Fig. 2.12. Radial distribution of maximum excess pore pressure after penetration of helices(after Weech, 2002).27

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.1.6 1.4 1.2 1.00.1 min (Ushaft = 0%) 10 min (Ushaft = 4%) 1 hr (Ushaft = 16%) 5 hrs (Ushaft = 35%) 17-20 hrs (Ushaft = 57%) 2 days (Ushaft = 76%) Edge of Helices

u/'vo

0.1 min after stopping 10 min after stopping 1 hr after stopping 5 hrs after installation 17-20 hrs after installation 2 days after installation Initial Shaft Penetration

0.8 0.6 0.4 0.2 0.0 1

10 Radial Distance from Pile Center (shaft radii) - r/Rshaft

100

Fig. 2.13. Radial distribution of excess pore pressure around helical piles (above level of bottom helix) during dissipation process (after Weech, 2002).1.6 1.410 min Edge of Helices

1.2 1.0u/'vo1 hr

0.85 hrs

0.6 0.4 0.2 0.0 1 1017-20 hrs

10 min (Below Helices) 1 hr (Below Helices) 5 hrs (Below Helices) 17-20 hrs (Below Helices) 2 days (Below Helices) 10 min (Above Bottom Helix) 1 hr (Above Bottom Helix) 5 hrs (Above Bottom Helix) 17-20 hrs (Above Bottom Helix) 2 days (Above Bottom Helix)

2 days

100

Radial Distance from Pile Center (shaft radii) - r/Rshaft

Fig. 2.14. Radial distribution of excess pore pressure above & below level of bottom helix during dissipation process (after Weech, 2002).28

Chapter 2. Overview of the field study of helical pile performance in soft sensitive soils.1.0 0.9 0.8 0.7u(t)/uouo = u at 0.1 min after stopping installation

0.6 0.5 0.4 0.3 0.2 0.1 0.0 1 10 100 1000 10000 Time after Stopping Installation (min)r/R = 1(Pile Shaft)

r/R = 4 (Edge of Helices)

r/R = 6

r/R = 8

r/R = 12

r/R = 16.5

r/R = 25

Fig. 2.15. Average dissipation trends for different radial distances from pile (after Weech, 2002)1.6 1.4 1.2 1.0Between Helices, r/R = 1 (TP1-PP1) Below Helices, r/R = 1 (TP4-PP3) Opposite Helices, r/R = 6.3 (PZ-TP4-2) Below Helices, r/R = 5.5 (PZ-TP1-9) Opposite Helices, r/R = 8.1 (PZ-TP3-2) Opposite Helices, r/R = 12 (PZ-TP1-7) Below Helices, r/R = 16 (PZ-TP2-9)

u/'vo

0.8 0.6 0.4 0.2 0.0 1 10 100 1000 10000

Time (min) from End of Installation

Fig. 2.16. Dissipation curves from piezometers/piezo-ports located at different radial distances from pile (after Weech, 2002).

29

Chapter 3. Literature review. 3.0. LITERATURE REVIEW. 3.1. INTRODUCTION. Pore water pressure response, including pore pressure generation and subsequent dissipation, due to helical pile installation into fine-grained soil has not been addressed until very recently. A field study by Weech (2002) provided the necessary factual information. However it is rather difficult to explain complex soil response based solely on interpretation of the field measurement. Prediction of pore water pressure response during and after pile installation into fine-grained soils has been the subject of a number of theoretical studies. Moreover, an extensive body of work exists in the field of cone penetration testing, where dissipation solutions were employed for the prediction of soil consolidation characteristics. Essentially, the CPT cone is a scaled instrumented pile and the pore pressure prediction solutions developed for cones may be applicable for prediction of the pore water response due to installation of driven and jacked piles. The main objective of this chapter is to establish a theoretical background upon which a numerical formulation for the analysis of pore pressure response due to helical pile penetration can be developed. To meet this objective, the existing state of knowledge on field observation of time dependent pore pressure response due to penetration of piles and piezocones is summarized, and a brief review of well known methodologies for pore pressure predictions is provided. 3.2. PORE PRESSURE RESPONSE INDUCED BY PILE INSTALLATION INTO FINE GRAINED SOILAND ITS INFLUENCE ON PILE CAPACITY.

3.2.1. FIELD GENERATION OF EXCESS PORE PRESSURE. Pile installation causes disturbance in the soil adjacent to the pile. Flaate (1972) studied impact of timber pile installation on fine-grained soils. It was observed that installation of a circular timber pile 0.33m in diameter formed a zone of up to 0.10 0.15 m from the pile shaft where the soil was completely remoulded. severely diminished. Stiffness and undrained strength in this zone were found It was also observed that outside the remoulded zone exists a zone of

reduced stiffness and undrained strength, or transition zone. According to Flaate (1972) the extent of the transition zone largely depends on natural soil properties, pile dimensions and the mechanism of penetration. The concept described by Flaate (1972) is shown in Fig. 3.1.

30

Chapter 3. Literature review. Soil deformations cause high pore pressures in excess of equilibrium hydrostatic values. The magnitude of generated excess pore pressures will depend on the type of soil and its properties. A number of accounts (Bjerrum & Johannessen, 1961; Lo & Stermac, 1965; Orrje & Broms, 1967; Koizumi & Ito, 1967; Bozozuk et al., 1978; Roy et al., 1981 and Pestana et al., 2002) report generation of significant positive excess pore pressures due to pile driving in fine-grained soils. Baligh & Levadoux (1980) compiled data from a number of sites where pore pressures were measured during pile installation (Fig. 3.2). It was found that, for most of the cases, the excess pore pressures at the pile shaft were about twice the vertical effective stress and that the extent of the generated pore pressures, having any significance (u/ v > 0.1), was about 20-30 pile radii. For penetration under undrained conditions, generated excess pore pressure can be represented as a sum of pore pressure generated due to change in the mean stress, and deviator shear stress, as show in Eq. 3.1.

u = umean + ushear

(3.1