MASc thesis: NUMERICAL MODELLING OF TIME DEPENDENT PORE PRESSURE RESPONSE INDUCED BY HELICAL PILE INSTALLATION

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.
ii

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 WEECH’S 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 ANALOGUE FOR 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, ν. .......................................................................................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 σ΄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, χ .................................................................75
5.2.6.3. HARDENING MODULUS, Hmod..........................................................................75
5.2.6.4. SLOPE OF CRITICAL STATE LINE, λ .................................................................75
5.2.6.5. INTERCEPT OF CRITICAL STATE LINE AT 1 KPA STRESS, Γ .............................77
5.2.6.6. STATE PARAMETER, ψ.....................................................................................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
vi

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, λ, 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
vii

List of figures.
LIST OF FIGURES
Figure Page
1.1. Helical piles.......................................................................................................................... 7
2.1. Helical pile performance research site location.................................................................. 18
2.2. Site subsurface conditions at the research site .................................................................. 18
2.3. Approximate locations of subsurface investigations at the Colebrook site........................ 19
2.4. Location of CPT tests and solid-stem auger holes ............................................................. 19
2.5. Variation of field vane shear strength test results with elevation....................................... 20
2.6. Example of cone penetration test results (CPT-7).............................................................. 21
2.7. Helical piles geometry ....................................................................................................... 22
2.8. Helical piles locations......................................................................................................... 23
2.9. 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. Effect of pile installation on soil conditions ...................................................................... 44
3.2. Measured excess pore pressures due to installation of piles ............................................. 44
3.3. Typical pore pressure dissipation measured during CPTU tests ....................................... 45
3.4. Increase in pile bearing capacity with time ....................................................................... 46
3.5. Increase in pile bearing capacity and pore pressure dissipation ........................................ 46
3.6. Comparison of variation of pile bearing capacity with time and theoretical decay of
excess pore pressure .......................................................................................................... 47
3.7. Idealized schematics of soil set-up phases ........................................................................ 47
3.8. Cavity expansion zones along pile .................................................................................... 48
3.9. Comparison of measured and theoretical soil displacements due to pile penetration ....... 48
4.1. Schematic representation of 2-D modelling approach ...................................................... 60
viii

List of figures.
4.2. Conceptual representation of modelling of helical pile installation as an expansion of
cylindrical cavity in 2-D .................................................................................................... 61
4.3. Conceptual representation of modelling of helical pile installation as an expansion of
cylindrical cavity in 1-D .................................................................................................... 61
4.4. Normal compression lines from isotropic compression tests on Erksak sand ................... 62
4.5. Definition of NorSand parameters Γ, λ, ψ, and R ........................................................... 62
4.6. Definitions of internal cap, pi, pc, Mtc, Mi and ηL on yield surface for a very loose sand .. 63
4.7. Conventional and NorSand representation of overconsolidation ratio for soil initially at p′
= 500 kPa subject to decreasing mean stress ..................................................................... 63
4.8. NorSand fit to Bothkennar Soft clay in CK0U triaxial shear ............................................ 64
4.9. NorSand simulation fit to constant p=80kPa drained triaxial test on Bonnie silt ............. 65
4.10. Flow chart for large strain numerical code ........................................................................ 66
5.1. Typical shear modulus reduction with strain level for plasticity index between 10% and 20% 81
5.2. Level of shear strain for various geotechnical measurements ........................................... 81
5.3. Variation of small strain shear modulus Gmax with elevation ............................................ 82
5.4. Inferred variation of rigidity index with depth .................................................................. 83
5.5. Variation of shear modulus G with elevation .................................................................... 84
5.6. Range of overconsolidation ratio OCR with elevation ...................................................... 85
5.7. Variation of coefficient of earth pressure K0 with elevation ............................................. 86
5.8. Variation in estimated coefficient of horizontal consolidation with depth ....................... 87
5.9. Variation in estimated coefficient of horizontal consolidation with elevation with
corrected CPTU derived values ........................................................................................ 88
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, λ .................................................................. 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. FE Mesh for Parametric Study ........................................................................................ 114
6.2. Cylindrical cavity expansion from non-zero radius ........................................................ 114
6.3. Radial distribution of generated excess pore water pressure at the end of cavity expansion
for “base case” scenario ................................................................................................... 115
ix

List of figures.
6.4. Time dependent pore pressure response at cavity wall for “base case” scenario ........... 115
6.5. Stress path for “base case” scenario ................................................................................ 116
6.6. Variation of void ratio, e, with mean effective stress, p΄ for “base case” simulation ..... 116
6.7. Variation of e with p΄ for “base case”, 20 & 21 scenarios ............................................... 117
6.8. Effect of K0 on radial distribution of generated excess pore pressure at the end of cavity
expansion ......................................................................................................................... 117
6.9. 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 & ψ on radial distribution of excess pore pressure response at the end
of cavity expansion .......................................................................................................... 119
6.12. Effect of coupled R & ψ on time dependent pore water pressure response at cavity wall 119
6.13. Effect of uncoupling R & ψ on radial distribution of excess pore water pressure response
at the end of cavity expansion, for simulations with positive ψ ...................................... 120
6.14. Effect of uncoupling R & ψ on time dependent pore water pressure response at the cavity
wall, for simulations with positive ψ ............................................................................... 120
6.15. Effect of uncoupling R & ψ on time dependent pore pressure response at the cavity wall,
for simulations with negative ψ. ...................................................................................... 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 & ψ on radial distribution of excess pore water pressure response
at the end of cavity expansion, for simulations with negative ψ. .................................... 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 3…6 and base case .................................................................. 124
6.22. Variation of e with p΄ for scenarios 3…6 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 ν on radial distribution of excess pore pressure at the end of cavity expansion .. 126
6.27. Effect of ν 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 Γ on radial distribution of excess pore water pressure at the end of cavity
expansion ......................................................................................................................... 128
6.30. Effect of Γ on time dependent pore water pressure response at cavity wall .................... 128
x

List of figures.
6.31. Stress paths for scenarios “base case”, 10 & 11 ................................................................ 129
6.32. Effect of Γ & λ on radial distribution of excess pore pressure at the end of cavity
expansion.......................................................................................................................... 129
6.33. Effect of Γ & λ on time dependent pore water pressure response at cavity wall............. 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
6.38. 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
6.41. Effect of χ on radial distribution of excess pore pressure at the end of cavity expansion...134
6.42. Effect of χ 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. 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
7.2. Time-dependent pore pressure response at the pile shaft/soil interface measured by Weech
(2002) and simulated with “base case” parameters.......................................................... 158
7.3. Comparison of modelled undrained triaxial response for ”best fit” and “base case” sets of
NorSandBiot input parameters ........................................................................................ 159
7.4. 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
7.5. Time-dependent pore pressure response at the pile shaft/soil interface measured by Weech
(2002) and simulated with “best fit” parameters.............................................................. 160
7.6. Comparison of ∆u/σ′v0 and σ′v/σ′v0 vs. time for “best fit” and “base case” simulation and the
field measurements ........................................................................................................... 161
7.7. 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
7.8. 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 ........................................................................................................................ 162
xi

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
helical pile with 3 helices (Case A).................................................................................. 168
helical pile with 5 helices (Case B).................................................................................. 168
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
with 3 helices (Case A). ................................................................................................... 170
with 5 helices (Case B)..................................................................................................... 171
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
helical pile with 5 helices (Case A. Assumption 2)......................................................... 174
xii

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. Shuttle’s 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 fiancé 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
xiii

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 pile’s capacity was obtained from compensating errors; a too low side friction capacity
1

was balanced by a too high end bearing. Randolph in his Rankine lecture (2003) also
recognized the lack of accuracy in pile design. Due to shortcomings in pile capacity
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 1960’s; 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 Ø 3.8 – 25 cm and helical plates Ø 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 pile’s 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

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
section’s 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

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 time-
dependent 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 bearing
4

capacity with time. Adequate simulation of the pore water pressure response in the soft fine-
grained 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

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 Weech’s 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. Implications
from the modelling are presented.
Chapter 8 provides conclusions from the current study and recommendations for further research.
6

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.
Weech’s 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 Weech’s work is given and Weech’s 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 WEECH’S 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 axial
8

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

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 = 1.5·10-3
cm2
/s, an
average coefficient of secondary consolidation, Cα = 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 on
10

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 Average Value Comments
natural moisture content (wn) 42%+/-3% -
liquid limit (wL) 40%+/-4% -
plasticity index (PI) 13.5%+/-4.5%,
below –8m in elevation PI is up to
21%
unit weight (γ) 17.8+/-0.3 kN/m3
-
in-situ void ratio (eo) 1.16+/-0.09
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. A summary of
engineering parameters for the silty clay layer, estimated from in-situ tests by Weech, are
presented in Table A.4 (Appendix A).
11

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, σ΄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/σ΄vo ratio is quite high –
around 0.7, which indicates moderately overconsolidated soil. At lower depths the deposit is
lightly overconsolidated, with the su/σ΄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 generated
12

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

2.5. SUMMARY OF WEECH’S STUDY RESULTS.
This summary is based on Weech’s 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/σ΄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/Rshaft
1
= 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 of
1
In 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.
14

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/σ′vo decreases steeply and
almost linearly. After r/Rshaft = 17, ∆ushaft becomes quite small (< 0.1σ′vo) and the slope of the
pore pressure decay with distance flattens. For r/Rshaft ≥ 60 generated pore pressures are
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

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)/σ′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)/σ΄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

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 Weech’s 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 Weech’s 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

Test Site
N
Fig. 2.1. Helical pile
performance research site
location.
Surrey, BC
Fig. 2.2. Site subsurface conditions
at the research site (modified after
Weech, 2002).
18

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.
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
0 10 20 30 40
Field Vane Shear Strength
(su)FV (kPa)
Elevation(m)
Peak Strength (VH-1&2)
Remoulded Strength (VH-1&2)
Peak (from Craw ford & Campanella, 1991)
Rem (from Craw ford & Campanella, 1991)
Possibly
affected by
sandy silt
a)
0.0 0.2 0.4 0.6 0.8
Strength Ratio
su/σ'vo
0 10 20 30 40 50
Sensitivity
St = (su)peak/(su)rem
VH-1&2
Craw ford & Campanella
(1991)
c)b)
Fig. 2.5. Variation of field vane shear strength test results with elevation (after Weech, 2002).
20

Fig. 2.6. Example of cone penetration test results (CPT-7) (after Weech, 2002).
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
0 1 2 3 4 5 6 7
Tip Resistance
QT (bar)
Elevation(m)
a)
0 1 2 3 4 5 6
Sleeve Friction
fs (kPa)
b)
-50 0 50 100 150 200 250
Excess Pore Pressure
at U2 - ∆u (kPa)
c)
Note:
Breaks in profile correspond to
data recorded upon resuming
penetration after seismic tests
21

Fig. 2.7. Helical piles geometry (modified after Weech, 2002).
22

pile cap
Helical piles
300 mm wide
hexagonal
RC piles
3rd
bridge pier
from South
abutment
2nd
bridge pier
from South
abutment
Fig. 2.8. Helical piles locations (modified after Weech, 2002).
23

-2
-1
0
1
2
3
4
5
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Excess Pore Pressure during Pile Installation - ∆ui/σ'vo
DepthofPileTipBelowPiezoFilterElev.(m)
PZ-TP4-1 (r/R = 4.8)
PZ-TP2-5 (r/R = 7.3)
PZ-TP2-1 (r/R = 8.0)
PZ-TP2-7 (r/R = 11)
PZ-TP2-3 (r/R = 17)
PZ-TP2-4 (r/R = 30)
Note:
Dissipation during breaks in
installation removed.
Helix
Plates
Grout
Disc
Grout Column
Line of Max
Pore Pressure
r = radial distance from pile center
R = radius of pile shaft
Fig. 2.9. Variation of excess pore pressure with pile tip depth, S/D=1.5. (after Weech, 2002)
24

-2
-1
0
1
2
3
4
5
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Excess Pore Pressure during Pile Installation - ∆ui/σ'vo
DepthofPileTipBelowPiezoFilterElev.(m)
PZ-TP3-1 (r/R = 5.8)
PZ-TP3-2 (r/R = 8.1)
PZ-TP1-7 (r/R = 12)
PZ-TP1-3 (r/R = 14)
PZ-TP1-4 (r/R = 25)
Note:
Dissipation during breaks in installation removed.
Helix
Plates
Grout
Disc
Grout Column
Line of Max
Pore Pressure
r = radial distance from pile center
R = radius of pile shaft
Fig. 2.10. Variation of excess pore pressure with pile tip depth, S/D=3. (after Weech, 2002).
25

TP1-4
TP2-4
TP5-1
TP1-3
TP1-6
TP2-3
TP1-5
TP2-7
TP6-2
TP2-2
TP2-5
TP2-6
TP1-9
TP4-2
TP4-1
TP3-1
TP6-1
TP3-2
TP2-1
TP1-7
TP2-9
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1 10
Radial Distance from Pile Center (shaft radii) - r/Rshaft
ExcessPorePressureduringInstallation-∆ui/σ'vo
Pile Piezos (due to pile tip penetration)
Pile Piezo-Ports (End of Installation)
EdgeofHelices
Logarithmic Trend Line
Linear Trend Line
Linear Trend Line
100
Fig. 2.11. Radial distribution of excess pore pressure generated by penetration of pile shaft
(modified after Weech, 2002).
26

0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1 10
∆u/σ'vo
Peak u at Piezos after Passing of Pile Tip
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)
EdgeofHelices
∆uhx
(best estimate)
∆uhx
100
Fig. 2.12. Radial distribution of maximum excess pore pressure after penetration of helices
(after Weech, 2002).
27

0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1 10 100
∆u/σ'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
Edge of
Helices
Fig. 2.13. Radial distribution of excess pore pressure around helical piles (above level of bottom
helix) during dissipation process (after Weech, 2002).
10 min (Ushaft = 4%)
10 min
1 hr
5 hrs
17-20 hrs
2 days
1 hr
(Ushaft = 16%)
5 hrs
(Ushaft = 35%)
17-20 hrs
(Ushaft = 57%)
2 days
(Ushaft = 76%)
0.1 min (Ushaft = 0%)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1 10
∆u/σ'vo
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)
Edge of Helices
100
Fig. 2.14. Radial distribution of excess pore pressure above & below level of bottom helix
during dissipation process (after Weech, 2002).
Fig. 2.14. Radial distribution of excess pore pressure above & below level of bottom helix
during dissipation process (after Weech, 2002).
28

0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1 10 100 1000 10000
Time after Stopping Installation (min)
∆u(t)/∆uo
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
∆uo = ∆u at 0.1 min after stopping installation
Fig. 2.15. Average dissipation trends for different radial distances from pile (after Weech, 2002)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1 10 100 1000 10000
Time (min) from End of Installation
∆u/σ'vo
Between 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)
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 SOIL
AND 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. Stiffness and undrained strength in this zone were found
severely diminished. 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

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/ σ΄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)
The components of excess pore pressure from Eq. 3.1 cannot be measured individually in the
field and can only be separated in the laboratory.
The pore pressure generated due to a change in mean stress, ∆σmean, is equal to the magnitude of
∆σmean change (assuming that water is incompressible relative to the soil). The magnitude of the
pore pressure in fine grained soils induced by shear is highly dependent on soil stress history
(OCR). Normally consolidated to lightly overconsolidated clays are contractive when sheared,
hence positive ∆ushear pore pressures are generated. Moderately to heavily overconsolidated
clays demonstrate dilatant behaviour when sheared, hence negative ∆ushear pore pressures are
generated. The magnitude of shear induced pore pressure is usually small for soft normally to
lightly overconsolidated clays, whereas more structured highly overconsolidated clays exhibit
larger magnitude of shear induced pore pressure.
3.2.2. FIELD DISSIPATION OF EXCESS PORE PRESSURE.
When pile installation into fine-grained soil is complete, the induced excess pore pressure will
gradually dissipate to the equilibrium value in time.
Water flow naturally takes the path of lowest resistance and due to the complex soil stratigraphy
and layering, accurate estimation of in-situ drainage characteristics is quite difficult. Field
studies by Bjerrum & Johannessen (1961), Koizumi & Ito (1967) and Roy et al. (1981), where
31

pore pressures were monitored during and after pile penetration into soft-fine grained soils,
indicate that over most of the pile length horizontal flow of water is predominant.
Gillespie & Campanella (1981) compared pore pressures measured at the different locations on
the CPT cone shaft. They conducted dissipation tests at the same depth in holes 1-2 meters apart,
with four different measurements locations: on the cone shoulder (standard u2 position, shown in
Fig. 3.3), 12.5, 25 and 38 cm from the cone shoulder. They found that the dissipation rate for u2
is only slightly higher than for the other tested locations. This implies that horizontal drainage
dominates the consolidation process.
Similar conclusions were reached from the theoretical studies of the effect of linear anisotropy in
soil consolidation characteristics on pore pressure dissipation behaviour by Levadoux & Baligh
(1980), Tumay et al (1982) and Houlsby & Teh (1988).
The rate of pore pressure dissipation largely depends on the soil hydraulic conductivity and its
consolidation characteristics. Immediately after pile installation the rate of pore pressure
dissipation may not be constant due to highly disturbed state of soil. However, after some initial
consolidation, it becomes constant (Komurka et al., 2003).
Dissipation behaviour varies depending on soil stress history. Dissipation response in normally
consolidated or lightly-overconsolidated clays is usually monotonic, with the pore pressure
magnitude gradually decreasing with time, as shown in Fig. 3.3a. Whereas dissipation behaviour
of overconsolidated clays is quite different. Coop & Wroth (1989) document pore pressures which
increase and then decrease after the driving of cylindrical steel piles in the heavily
overconsolidated Gault clay. Similar observations were made by Lehane & Jardine (1994), while
studying pore pressure response due to penetration of closed-ended pipe piles in the stiff glacial
clay deposit at Cowden, England. Coop & Wroth (1989) have suggested that the maximum
penetration pore pressure in overconsolidated soils is located at some distance away from the shaft.
This causes a rise of pore pressure at the shaft at early dissipation times due to redistribution effect.
Pore pressure measured at a standard monitoring location (u2) during CPTU dissipation tests in
overconsolidated clays also shows an initial increase followed by a subsequent decrease in
excess pore pressure with time, as shown in Fig. 3.3b (Davidson, 1985; Campanella et al., 1986;
Lutenegger & Kabir, 1988 and Sully & Campanella, 1994). Sully & Campanella (1994)
suggested that this phenomenon is related to the inflow of pore pressure from the zone of higher
gradients at the tip to the zone of lower gradients behind the tip.
32

3.2.3. OBSERVED AXIAL PILE CAPACITY AS FUNCTION OF DISSIPATION OF EXCESS PORE
PRESSURE.
Typically, when a pile is installed into fine-grained soil, high excess pore water pressures are
generated in the vicinity of pile. Over time the pore pressures induced by pile installation begin
to dissipate, primarily in a radial direction. Consequently the soil in the vicinity of the pile
consolidates. As the water content of the soil gradually decreases during the dissipation process,
the soil strength and stiffness recover and may increase. A number of studies linked pore
pressure dissipation, induced by pile installation, with the increase in pile bearing capacity.
One of the first documented accounts of such behaviour belongs to Seed & Reese (1957). They
studied the effect of pile driving on soil properties and pile bearing capacity on an instrumented
pipe pile, 0.15 m in diameter installed into sensitive soft clay at the San-Francisco – Oakland
bridge site, in California. Pore pressure measurements were taken in the vicinity of the pile after
installation. The pile was loaded seven times in a time span from 3 hours after installation to 33
days (800 hours). A dramatic increase in pile capacity (5.4 times) was reported, as shown in Fig.
3.4. The pore pressure measurements indicated full dissipation of the excess pore pressures due
to pile installation about 20 days after installation, the same period over which the pile acquired
most of its bearing capacity.
Konrad & Roy (1987) performed a comprehensive analysis of bearing capacity of friction piles
in the marine clays at St.Alban, Quebec. Soil-pile interaction was studied on two closed ended
instrumented pipe piles. Combined results of pile loading tests and pore pressure measurements,
shown in Fig. 3.5, indicate an increase in pile bearing capacity with dissipation of the excess pore
pressures, so that after full dissipation of the excess pore pressures in about 25 days, pile bearing
capacity had increased by about 97% of the total capacity observed in two years.
Other field studies of pile capacity in fine-grained soils, including Eide et al. (1961), Flaate
(1972) and Chen et al. (1999), confirm the increase in pile bearing capacity with dissipation of
excess pore pressures generated during pile installation.
Randolph & Wroth (1979) compared the theoretical decay of pore pressure with time with the
measured bearing capacity of driven piles, reported by Seed & Reese (1957) and Eide et al
(1961), as a percentage of their long term bearing capacity, as shown in Fig. 3.6. The main
implication of this figure is that the pile bearing capacity is strongly dependent on the degree of
excess pore pressure dissipation.
33

Komurka et al. (2003) studied the effect of soil/pile set up (increase of pile capacity with time).
They idealized the mechanism of set up as follows:
• Phase 1 - Logarithmically Nonlinear Rate of Excess Porewater Pressure Dissipation.
• Phase 2 - Logarithmically Linear Rate of Excess Porewater Pressure Dissipation.
• Phase 3 – Aging/Thixotropy.
The first two phases are associated with the dissipation of excess pore pressure induced by pile
installation. During the third stage, increase in pile capacity occurs with no change in pore
pressure (constant effective stress). The phenomenon of aging is related to the particle frictional
interlocking and the thixotropy related to chemical bonding or cementation between the particles.
The concept of soil/pile set up proposed by Komurka et al. (2003) is schematically represented in
Fig. 3.7. It can be seen that the majority of the pile capacity increase is related to the pore
pressure dissipation and the effect of aging and thixotropy on pile capacity increase may not be
very significant. Here we should recognize that in fine grained soils it is likely that aging and
thixotropy may begin to occur before complete pore pressure dissipation takes place. However,
due to the slow rate of these processes they are expected to take place over a much longer time
span than the excess pore pressure dissipation.
As such, the treatment of thixotropic and aging effects is impractical in most piling analysis.
Based on the works of Soderberg (1962) and Randolph & Wroth (1979), Guo (2000) suggested
that the problem of predicting the variation of capacity is one of predicting the excess pore
pressure at the pile shaft as a function of time.
3.3. PREDICTION OF TIME-DEPENDENT PORE PRESSURE RESPONSE.
3.3.1. PREDICTION METHODS.
Prediction of pore water pressure response is quite complex. A number of factors complicate the
analysis: vertical drainage, soil remoulding in the vicinity of penetrating body, soil non-linearity
and anisotropy, boundary effect of soil layering, soil stress and strain history (Campanella &
Robertson, 1988).
There is no method available, among those published to date, which can account for the full
complexity of the pore water pressure response. However, a reasonable approximation of the
problem is possible. Discussed herein are well known prediction solutions, varying in their
degree of complexity and comprehensiveness, that provide some capabilities for estimation of
34

pore water pressure response generated due to pile (or cone) penetration and subsequent pore
pressure dissipation.
A selection of such solutions is shown in chronological order in Table 3.1. It should be noted that
the majority of these solutions were specifically developed for prediction of the pore pressure
dissipation around piezocones. Due to observed similarities between pile and piezocone
penetration, all of these solutions are generally assumed applicable to pore prediction around
piles.
The following sections will present basic concepts behind the prediction methods and address
their predictive capabilities.
35

MASc thesis: NUMERICAL MODELLING OF TIME DEPENDENT PORE PRESSURE RESPONSE INDUCED BY HELICAL PILE INSTALLATION

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