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 WEECHS STUDY
RESULTS........................................................................14
2.5.1. PORE WATER PRESSURE RESPONSE DURING HELICAL PILE
INSTALLATION..............14 2.5.2. PORE WATER PRESSURE DISSIPATION
AFTER HELICAL PILE INSTALLATION. ...........15 2.6. SUMMARY
........................................................................................................................17
3.0 LITERATURE REVIEW
..................................................................................................30
3.1. INTRODUCTION.
...............................................................................................................30
iii
Table of contents. 3.2. PORE PRESSURE RESPONSE INDUCED BY PILE
INSTALLATION INTO FINE GRAINED SOIL AND ITS INFLUENCE ON PILE
CAPACITY
...........................................................................30
3.2.1. FIELD GENERATION OF EXCESS PORE PRESSURE.
......................................................30 3.2.2.
FIELD DISSIPATION OF EXCESS PORE
PRESSURE........................................................31
3.2.3. OBSERVED AXIAL PILE CAPACITY AS FUNCTION OF DISSIPATION OF
EXCESS PORE
PRESSURE..................................................................................................................33
3.3. PREDICTION OF TIME-DEPENDENT PORE PRESSURE RESPONSE
........................................34 3.3.1. PREDICTION
METHODS..............................................................................................34
3.3.2. BASIC CONCEPTS BEHIND EXISTING PREDICTION SOLUTIONS.
.................................37 3.3.2.1. MODELLING
ANALOGUEFOR
SIMULATION OF PILE OR CONE PENETRATION ....37
3.3.2.2. MODELLING FRAMEWORK
.............................................................................38
3.3.3. OVERVIEW OF EXISTING PREDICTION
SOLUTIONS.....................................................39
3.3.3.1. CAVITY EXPANSION SOLUTIONS
....................................................................39
3.3.3.2. SOLUTIONS BASED ON STRAIN PATH METHOD
..............................................42 3.4. SUMMARY
........................................................................................................................42
4.0. FORMULATION OF MODELLING APPROACH
....................................................49 4.1.
INTRODUCTION.
..............................................................................................................49
4.2. MODELLING APPROACH TO SIMULATION OF HELICAL PILE INSTALLATION
INTO FINE GRAINED SOIL
.................................................................................................................49
4.2.1. MODELLING FRAMEWORK
........................................................................................49
4.2.2. MODELLING PROCEDURE FOR SIMULATION OF HELICAL PILE
INSTALLATION. .........50 4.3. NORSANDBIOT FORMULATION.
......................................................................................52
4.3.1. NORSAND CRITICAL STATE MODEL
.........................................................................52
4.3.1.1. MODEL DESCRIPTION
......................................................................................52
4.3.1.2. MODEL PARAMETERS
......................................................................................55
4.3.1.3. BEYOND SAND
................................................................................................56
4.3.2. BIOT COUPLED CONSOLIDATION THEORY
.................................................................57
4.3.3. FINITE ELEMENT IMPLEMENTATION OF NORSANDBIOT FORMULATION
....................58 4.3.4. FINITE ELEMENT CODE VERIFICATION
......................................................................58
4.4. SUMMARY
.......................................................................................................................59
5.0. SELECTION OF SITE-SPECIFIC SOIL PARAMETERS FOR MODELLING
.....67 5.1. INTRODUCTION.
..............................................................................................................67
5.2. SOIL PARAMETERS FOR MODELLING.
.............................................................................67
5.2.1. ELASTIC PROPERTIES G, .
.......................................................................................67
iv
Table of contents. 5.2.2. OVERCONSOLIDATION RATIO OCR.
.........................................................................69
5.2.3. COEFFICIENT OF LATERAL EARTH PRESSURE K0.
.......................................................70 5.2.4.
HYDRAULIC CONDUCTIVITY DERIVATION.
...............................................................71
5.2.4.1. COEFFICIENT OF CONSOLIDATION
....................................................................71
5.2.4.2. COEFFICIENT OF VOLUME CHANGE, mv
...........................................................73
5.2.4.3. RADIAL HYDRAULIC CONDUCTIVITY,
kr..........................................................74
5.2.5. VERTICAL EFFECTIVE STRESS vo AND EQUILIBRIUM PORE PRESSURE
uo. ..............74 5.2.6. NORSAND MODEL PARAMETERS DERIVATION
..........................................................74
5.2.6.1. CRITICAL STATE COEFFICIENT, Mcrit
...............................................................75
5.2.6.2. STATE DILATANCY PARAMETER,
.................................................................75
5.2.6.3. HARDENING MODULUS, Hmod
..........................................................................75
5.2.6.4. SLOPE OF CRITICAL STATE LINE,
.................................................................75
5.2.6.5. INTERCEPT OF CRITICAL STATE LINE AT 1 KPA STRESS,
.............................77 5.2.6.6. STATE PARAMETER,
.....................................................................................77
5.2.7. NORSAND PARAMETERS ANALYSIS
..........................................................................79
5.3. SUMMARY.
.....................................................................................................................80
6.0. NORSAND-BIOT CODE PARAMETRIC STUDY
.....................................................95 6.1.
INTRODUCTION.
..............................................................................................................95
6.2. MODELLING PARTICULARS.
............................................................................................95
6.3. REFERENCE RESPONSE.
...................................................................................................96
6.4. PARAMETRIC STUDY SCENARIOS.
...................................................................................98
6.5. PARAMETRIC STUDY RESULTS. .
...................................................................................100
6.5.1. INFLUENCE OF COEFFICIENT OF LATERAL EARTH PRESSURE
..................................102 6.5.2. INFLUENCE OF MEASURES
OF SOIL OCR
................................................................103
6.5.3. INFLUENCE OF ELASTIC PROPERTIES
......................................................................106
6.5.4. INFLUENCE OF CRITICAL STATE LINE
PARAMETERS................................................108
6.5.5. INFLUENCE OF HARDENING MODULUS
....................................................................109
6.5.6. INFLUENCE OF STATE DILATANCY PARAMETER
......................................................110 6.5.7.
INFLUENCE OF HYDRAULIC CONDUCTIVITY
............................................................110
6.6. CONCLUDING REMARKS ON PARAMETRIC STUDY RESULTS
..........................................111 6.7. SUMMARY
.....................................................................................................................113
7.0. MODELLING OF PORE PRESSURE CHANGES INDUCED BY PILE
INSTALLATION IN 1-D
..............................................................................................138
v
Table of contents. 7.1. INTRODUCTION.
............................................................................................................138
7.2. 1-D SIMULATIONS.
.......................................................................................................138
7.2.1. STAGE I. MODELLING OF HELICAL PILE INSTALLATION AS SINGLE
CAVITY
EXPANSION..............................................................................................................139
7.2.1.1. COMPARISON OF MODELED AND FIELD PORE PRESSURE RESPONSES
.........139 7.2.1.2. NORSANDBIOT BEST FIT WITH FIELD DATA
..........................................141 7.2.2. STAGE II.
MODELLING OF HELICAL PILE AS SERIES OF CAVITY EXPANSIONS ..146
7.2.2.1. DETAILS OF HELIX MODELLING
.................................................................146
7.2.2.2. EFFECT OF CAVITY EXPANSION/CONTRACTION CYCLING ON PORE
PRESSURE
RESPONSE................................................................................................................147
7.3. IMPLICATIONS FROM 1-D MODELLING.
.........................................................................153
7.3.1. PREDICTED VERSUS MEASURED/INTERPRETED PORE PRESSURE RESPONSE
.......153 7.3.2. FROM PORE PRESSURE RESPONSE PREDICTIONS TO PILE
BEARING CAPACITY ..155 7.4. SUMMARY
.....................................................................................................................156
8.0. CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER STUDY
..........175 8.1. SUMMARY AND CONCLUSIONS.
...................................................................................175
8.2. RECOMMENDATIONS FOR FURTHER RESEARCH.
..........................................................177
8.2.1. LABORATORY STUDY
.........................................................................................177
8.2.2. 2-D NUMERICAL MODELLING
...........................................................................178
REFERENCES
.........................................................................................................................180
NOTATION
..............................................................................................................................187
APPENDIX A. SOURCES OF SUBSURFACE INFORMATION FOR COLEBROOK SITE
.....................189 APPENDIX B. PIEZOMETERS RESPONSE
...................................................................................191
APPENDIX C. VALIDATION OF NORSAND MODEL AGAINST BONNIE SILT
.................................193 APPENDIX D. NORSAND-BIOT
COUPLING
..............................................................................197
APPENDIX E. NORSAND-BIOT CODE VERIFICATION.
..............................................................200
APPENDIX F. COUPLED MODELLING OF OBSERVED PORE PRESSURE DISSIPATION
AFTER HELICAL PILE INSTALLATION (PAPER)
...............................................................209
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, , 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 1.1. 2.1. 2.2. 2.3. 2.4.
2.5. 2.6. 2.7. 2.8. 2.9. Page
Helical piles
..........................................................................................................................
7 Helical pile performance research site
location..................................................................
18 Site subsurface conditions at the research site
..................................................................
18 Approximate locations of subsurface investigations at the
Colebrook site........................ 19 Location of CPT tests and
solid-stem auger holes
............................................................. 19
Variation of field vane shear strength test results with
elevation....................................... 20 Example of cone
penetration test results
(CPT-7)..............................................................
21 Helical piles geometry
.......................................................................................................
22 Helical piles
locations.........................................................................................................
23 Variation of excess pore pressure with pile tip depth, S/D=1.5
......................................... 24
2.10. Variation of excess pore pressure with pile tip depth,
S/D=3 ............................................ 25 2.11. Radial
distribution of excess pore pressure generated by penetration of
pile shaft .......... 26 2.12. Radial distribution of maximum
excess pore pressure after penetration of helices .......... 27
2.13. Radial distribution of excess pore pressure around helical
piles (above level of bottom helix) during dissipation process
.......................................................................................
28 2.14. Radial distribution of excess pore pressure above &
below level of bottom helix during dissipation process
.............................................................................................................
28 2.15. Average dissipation trends for different radial distances
from pile .................................. 29 2.16. Dissipation
curves from piezometers/piezo-ports located at different radial
distances from pile
................................................................................................................................
29 3.1. 3.2. 3.3. 3.4. 3.5. 3.6. 3.7. 3.8. 3.9. 4.1. Effect of pile
installation on soil conditions
......................................................................
44 Measured excess pore pressures due to installation of piles
............................................. 44 Typical pore
pressure dissipation measured during CPTU tests
....................................... 45 Increase in pile bearing
capacity with time
.......................................................................
46 Increase in pile bearing capacity and pore pressure dissipation
........................................ 46 Comparison of variation
of pile bearing capacity with time and theoretical decay of excess
pore pressure
..........................................................................................................
47 Idealized schematics of soil set-up phases
........................................................................
47 Cavity expansion zones along pile
....................................................................................
48 Comparison of measured and theoretical soil displacements due to
pile penetration ....... 48 Schematic representation of 2-D
modelling approach
...................................................... 60viii
List of figures. 4.2. 4.3. 4.4. 4.5. 4.6. 4.7. 4.8. 4.9. 5.1.
5.2. 5.3. 5.4. 5.5. 5.6. 5.7. 5.8. 5.9. Conceptual representation
of modelling of helical pile installation as an expansion of
cylindrical cavity in 2-D
....................................................................................................
61 Conceptual representation of modelling of helical pile
installation as an expansion of cylindrical cavity in 1-D
....................................................................................................
61 Normal compression lines from isotropic compression tests on
Erksak sand ................... 62 Definition of NorSand parameters
, , , and R
........................................................... 62
Definitions of internal cap, pi, pc, Mtc, Mi and L on yield surface
for a very loose sand .. 63 Conventional and NorSand representation
of overconsolidation ratio for soil initially at p = 500 kPa
subject to decreasing mean stress
.....................................................................
63 NorSand fit to Bothkennar Soft clay in CK0U triaxial shear
............................................ 64 NorSand simulation
fit to constant p=80kPa drained triaxial test on Bonnie silt
............. 65 Typical shear modulus reduction with strain level
for plasticity index between 10% and 20% 81 Level of shear strain
for various geotechnical measurements
........................................... 81 Variation of small
strain shear modulus Gmax with elevation
............................................ 82 Inferred variation
of rigidity index with depth
..................................................................
83 Variation of shear modulus G with elevation
....................................................................
84 Range of overconsolidation ratio OCR with elevation
...................................................... 85 Variation
of coefficient of earth pressure K0 with elevation
............................................. 86 Variation in
estimated coefficient of horizontal consolidation with depth
....................... 87 Variation in estimated coefficient of
horizontal consolidation with elevation with corrected CPTU derived
values
........................................................................................
88
4.10. Flow chart for large strain numerical code
........................................................................
66
5.10. Variation of vertical effective stress with elevation
.......................................................... 89 5.11.
Variation of equilibrium pore water pressure with elevation
............................................ 90 5.12. Probable
range of slope of critical state line,
..................................................................
91 5.13. Variation of void ratio with mean effective stress based
on data reported by Crawford & Campanella (1988)
............................................................................................................
92 5.14. Variation of state parameter and overconsolidation ratio
with mean effective stress ....... 92 5.15. Simulation of drained
triaxial test with NorSand model, using base case set of input
parameters
..........................................................................................................................
93 5.16. Simulation of undrained triaxial test with NorSand model,
using base case set of parameters
..........................................................................................................................
94 6.1. 6.2. 6.3. FE Mesh for Parametric Study
........................................................................................
114 Cylindrical cavity expansion from non-zero radius
........................................................ 114 Radial
distribution of generated excess pore water pressure at the end of
cavity expansion for base case scenario
...................................................................................................
115ix
List of figures. 6.4. 6.5. 6.6. 6.7. 6.8. 6.9. Time dependent
pore pressure response at cavity wall for base case scenario
........... 115 Stress path for base case scenario
................................................................................
116 Variation of void ratio, e, with mean effective stress, p for
base case simulation ..... 116 Variation of e with p for base case,
20 & 21 scenarios
............................................... 117 Effect of K0 on
radial distribution of generated excess pore pressure at the end of
cavity expansion
.........................................................................................................................
117 Effect of K0 on time dependent pore water pressure response at
cavity wall .................. 118
6.10. Stress paths for base case, 1 & 2 scenarios
..................................................................
118 6.11. Effect of coupled R & on radial distribution of
excess pore pressure response at the end of cavity expansion
..........................................................................................................
119 6.12. Effect of coupled R & on time dependent pore water
pressure response at cavity wall 119 6.13. Effect of uncoupling R
& on radial distribution of excess pore water pressure response
at the end of cavity expansion, for simulations with positive
...................................... 120 6.14. Effect of
uncoupling R & on time dependent pore water pressure response
at the cavity wall, for simulations with positive
...............................................................................
120 6.15. Effect of uncoupling R & on time dependent pore
pressure response at the cavity wall, for simulations with negative
.
......................................................................................
121 6.16. Generation of excess pore pressure during cavity
expansion for the first mesh element adjacent to the cavity,
presented in terms of pore pressure components
......................... 121 6.17. Effect of uncoupling R & on
radial distribution of excess pore water pressure response at the
end of cavity expansion, for simulations with negative .
.................................... 122 6.18. Radial distribution
of different excess pore pressure components for scenario 5a
......... 122 6.19. Radial distribution of generated pore pressure,
for scenario 5a, at different levels cavity expansion
.........................................................................................................................
123 6.20. Initial conditions in e-ln (p) space for scenarios 3..6
and base case .............................. 123 6.21. Stress paths
for scenarios 36 and base case
..................................................................
124 6.22. Variation of e with p for scenarios 36 and base case
................................................... 124 6.23.
Effect of G on radial distribution of excess pore pressure at the
end of cavity expansion .125 6.24. Effect of G on time dependent
pore pressure response at cavity wall .............................
125 6.25. Stress paths for scenarios base case, 7, 8 & 9
............................................................... 126
6.26. Effect of on radial distribution of excess pore pressure at
the end of cavity expansion .. 126 6.27. Effect of on time
dependent pore water pressure response at cavity wall
.................... 127 6.28. Stress paths for scenarios base
case, 22 & 23.
.............................................................. 127
6.29. Effect of on radial distribution of excess pore water
pressure at the end of cavity expansion
.........................................................................................................................
128 6.30. Effect of on time dependent pore water pressure response
at cavity wall .................... 128x
List of figures. 6.31. Stress paths for scenarios base case, 10
& 11
................................................................
129 6.32. Effect of & on radial distribution of excess pore
pressure at the end of cavity
expansion..........................................................................................................................
129 6.33. Effect of & on time dependent pore water pressure
response at cavity wall ............. 130 6.34. Stress paths for
scenarios base case, 12 & 13
................................................................
130 6.35. Effect of Mcrit on radial distribution of excess pore
pressure at the end of cavity expansion .131 6.36. Effect of Mcrit
on time dependent pore water pressure response at cavity wall.
.............. 131 6.37. Stress paths for scenarios base case, 14
& 15
................................................................
132 6.38. Effect of Hmod on radial distribution of excess pore
pressure at the end of cavity expansion 132 6.39. Effect of Hmod on
time dependent pore water pressure response at cavity wall
............... 133 6.40. Stress paths for scenarios base case, 14
& 15
................................................................
133 6.41. Effect of on radial distribution of excess pore pressure
at the end of cavity expansion...134 6.42. Effect of on time
dependent pore water pressure response at cavity wall
.................... 134 6.43. Stress paths for simulations with
base case, scenario 18 & 19 set of input parameters .. 135
6.44. Effect of permeability, k, on radial distribution of excess
pore pressure at the end of cavity
expansion..........................................................................................................................
135 6.45. Effect of permeability, k, on time dependent pore
pressure response at cavity wall ........ 136 6.46. Stress paths
for scenarios base case, 20 & 21.
.............................................................. 136
6.47. Location of final stress state in q-p space, at the end of
pore pressure dissipation, in relation to critical state line
............................................................................................................
137 7.1. 7.2. 7.3. 7.4. 7.5. 7.6. 7.7. 7.8. Radial pore pressure
distribution at the end of pile installation reported by Levadoux
& Baligh (1980), measured by Weech (2002) and simulated with
base case parameters .. 158 Time-dependent pore pressure response
at the pile shaft/soil interface measured by Weech (2002) and
simulated with base case
parameters..........................................................
158 Comparison of modelled undrained triaxial response for best fit
and base case sets of NorSandBiot input parameters
........................................................................................
159 Radial pore pressure distribution at the end of pile
installation reported by Levadoux & Baligh (1980), measured by
Weech (2002) and simulated with best fit parameters .... 160
Time-dependent pore pressure response at the pile shaft/soil
interface measured by Weech (2002) and simulated with best fit
parameters..............................................................
160 Comparison of u/v0 and v/v0 vs. time for best fit and base case
simulation and the field measurements
...........................................................................................................
161 Stress path plot for central gaussian point of the mesh element
adjacent to the cavity wall (r/Rshaft = 1.08) for simulation of
helical pile shaft installation with best fit parameters. 161 Void
ratio versus mean stress (e-ln(p)) plot for central gaussian point
of the mesh element adjacent to the cavity wall (r/Rshaft = 1.08)
for simulation with best fit parameters
........................................................................................................................
162xi
List of figures. 7.9. Modelling cases considered in the analysis
of the effect of the helices ........................... 163
7.10. Modelling algorithm of helical piles installation in 1-D
................................................. 163 7.11.
Comparison of time dependent pore pressure response during helical
pile installation measured in the field and simulated using
NorSandBiot formulation (Case A). ............ 164 7.12. Comparison
of time dependent pore pressure response during helical pile
installation measured in the field and simulated using NorSandBiot
formulation (Case B). ............. 165 7.13. Comparison of radial
pore distribution for simulations with and without helices and the
field
measurements...........................................................................................................
166 7.14. Radial pore pressure distribution during first helix
expansion (Case A).......................... 166 7.15. Radial pore
pressure distribution during first helix contraction (Case
B)........................ 167 7.16. Radial pore pressure
distribution during expansion/contraction cycles for simulation of
helical pile with 5 helices (Case
A)..................................................................................
167 7.17. Radial pore pressure distribution during
expansion/contraction cycles for simulation of helical pile with 3
helices (Case
A)..................................................................................
168 7.18. Radial pore pressure distribution during
expansion/contraction cycles for simulation of helical pile with 5
helices (Case B).
.................................................................................
168 7.19. Radial pore pressure distribution during
expansion/contraction cycles for simulation of helical pile with 3
helices (Case B).
.................................................................................
169 7.20. Time dependent pore pressure response at the cavity wall
for simulation of helical pile with 5 helices (Case
A).....................................................................................................
170 7.21. Time dependent pore pressure response at the cavity wall
for simulation of helical pile with 3 helices (Case A).
...................................................................................................
170 7.22. Time dependent pore pressure response at the cavity wall
for simulation of helical pile with 5 helices (Case
B).....................................................................................................
171 7.23. Time dependent pore pressure response at the cavity wall
for simulation of helical pile with 3 helices (Case
B).....................................................................................................
171 7.24. Stress path plot for mesh element adjacent to the cavity
wall (r/Rshaft = 1.08) for simulation of helical pile shaft
installation.......................................................................
172 7.25. Void ratio versus mean stress (e ln(p)) plot for mesh
element adjacent to the cavity wall (r/Rshaft =
1.08)..................................................................................................................
172 7.26. Comparison of stress paths for central gaussian point of
the mesh element adjacent to the cavity wall (r/Rshaft = 1.08) for
simulations with different set of input parameters and modelling
schemes
............................................................................................................
173 7.27. Radial pore pressure distribution during
expansion/contraction cycles for simulation of helical pile with 5
helices (Case A. Assumption
2)......................................................... 174
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. Shuttles enthusiasm for this project had never run
out and her pressure, in a good sense, kept me going. My study at
the University of British Columbia was a great learning experience.
I would like to thank Dr. Howie for taking me into the UBC
Geotechnical Group. It was always a great pleasure to work with
him. Thoughtful contributions of Dr. Howie to many discussions
related to this project are sincerely appreciated. I would like to
express my gratitude to Dr. Michael Jefferies for shearing the code
and for his valuable suggestions. Special thanks for the ideas and
helpful information belongs to my fellow graduate students: Sung
Sik Park, Mavi Sanin, Ali Amini and Somasundaram Sriskandakumar. My
deep appreciation goes to my fianc Valeria and my stepson Vadim,
who inspired me all the way through. Their patience and moral
support are greatly acknowledged. Most of all, I would like to
thank my parents Sofia & Mikhail, and my elder brother Alexei.
Their unconditional love has always been there for me. I am indebt
for their steadfast backing of my intellectual and spiritual
growth. This thesis is one of the fruits of their dedication and
love. There will be many more to come. I dedicate this work to my
beloved family.
PER ASPERA AD ASTRA
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 piles capacity was obtained from
compensating errors; a too low side friction capacity1
Chapter 1. Introduction. was balanced by a too high end bearing.
Randolph in his Rankine lecture (2003) also Due to shortcomings in
pile capacity
recognized the lack of accuracy in pile design.
predictions geotechnical engineers have to rely on pile load
tests to refine final piling foundation design. The ability to
accurately predict the variation of stresses and pore pressures in
fine-grained soil due to pile installation is a key to improving
pile capacity prediction capabilities. The problem of predicting
the variation of pile capacity in fine-grained soils is one of
predicting the excess pore pressure and associated stresses at the
pile shaft as a function of time. It appears that development of a
robust technique for evaluation of pore pressure changes due to
pile installation will provide a basis from which a method
accounting for capacity gain with time in design and testing can be
developed. This study is concerned with modelling the
time-dependent pore pressure response due to helical pile
installation in soft fine-grained soil. 1.2. HELICAL PILES.
A helical pile is an assembly of mechanically connected steel
shafts with a series of steel helical plates welded at particular
locations on the lead section, as shown in Fig. 1.1.a. Historically
helical piles have evolved from early foundations known as screw
piles. The screw piles have been in use since the early 19th
century. Early applications of these piles were based on hand
installation. The first power installed screw piles were employed
during construction of a series of lighthouses in England in 1833
(Wilson & Guthlac, 1950). Generally, the screw
piles had a very limited use until the 1960s; when reliable
truck mounted hydraulic torque motors became readily available.
Nowadays the major helical piles manufacturer is a USA based
company - AB Chance Ltd. They manufacture piles with the shaft 3.8
25 cm and helical plates 15 - 36 cm. The diameter of manufactured
piles is quite small and their application is currently restricted
to relatively small jobs. It appears that the potential of helical
piles is not fully exploited to date. A new boost in helical piles
application is expected from recent development of high capacity
torque units, which will make possible installation of helical
piles with larger diameters, installed to greater depths.2
Chapter 1. Introduction. Generally, helical piles can be
employed in any application where driven and jacketed piles are
used, except for the cases where penetration of competent rock is
required. Currently helical piles found application in the
following areas: foundation repairs, upgrades & retrofits;
pump-jacks and compressor stations for oil and gas industry (large
diameter piles); pipelines support; foundations for temporary and
mobile structures.
Experience with conventional (small diameter) helical piles in
soft soils in British Columbia revealed a tendency for buckling of
the slender steel shaft during loading. Aiming to reduce the
buckling effect, placement of grout around the shaft was proposed
and patented by Vickars Developments Co. Ltd, as grouted, or
PULLDOWNTM, pile, shown in Fig. 1.1.b. Normally, helical piles are
installed by sections. The leading section, also called a screw
anchor, is placed into the soil by rotation of the pile shaft using
a hydraulic torque unit. The pile is screwed into the ground in the
same method a wood screw is driven. Helical plates of the
leading section create a significant pulling force that makes
the shaft advance downwards. Following the screw anchor
installation, extension sections are bolted to the top of the screw
anchor shaft. Installation continues by resumed rotation, and
further extension sections are added until the project depth of the
pile is reached. For the grouted helical piles, at each
sections connection, displacement plates are attached to the
shaft. During pile installation they create a cylindrical void,
which is filled by the flowable grout. Helical piles have several
distinctive advantages over traditional driven and jacketed piles:
mobilize soil resistance both in compression and uplift; quick and
easy to install: vibration free, no heavy equipment required,
possible to install inside buildings (for small diameter piles);
reusable.
Helical piles are typically installed in soils that permit the
compressive capacity of the pile to be developed through
end-bearing below each of the helices at the bottom of the pile.
Where the thickness of soft cohesive strata is too extensive to
make it practical to advance helical piles to a competent bearing
stratum, it may be necessary to develop the capacity of the piles
in friction within the soft cohesive soil. However, experience
using helical piles in such soils is limited at this time, as is
the understanding of the complex sensitive fine-grained
soil-helical pile interaction.3
Chapter 1. Introduction. 1.3. PURPOSES AND OBJECTIVES OF
RESEARCH. Helical piles are gaining popularity in North America as
an alternative foundation solution to traditional driven and jacked
piles. To date the major research efforts in the field of helical
piles have concentrated on their lateral and uplift capacity.
However, limited knowledge of the timedependent effect of helical
pile installation on soil behaviour remains a significant drawback
to their widespread application in soft fine-grained soils. Pore
pressure response due to helical pile installation has not been
studied until very recently. Field studies of helical pile
performance in soft silty clay, carried out by Weech (2002) in
Surrey, British Columbia, provide quality data on the pore pressure
regime during and after helical pile installation. Given natural
constraints of the field studies, such as a limited number of
measuring points and measurements accuracy, numerical simulation
provides an effective tool for improving our understanding of
complex response of soft fine-grained soil due to helical pile
installation. The main objectives of this research are: Develop a
modelling approach that will realistically simulate the pore
pressure response during helical pile installation and the
subsequent excess pore water pressure dissipation with time.
Numerically model helical pile installation into the soft
fine-grained soil at the Colebrook helical pile research site and
investigate pore water pressure response during and after helical
pile installation. Compare and contrast the modelled response with
the field measurements and the field interpretations performed by
Weech (2002). The ability to understanding and predict the impact
of pile installation on soft fine-grained soil will contribute to
improving existing pile bearing capacity calculation methods. In
addition the conducted research will be a major step towards
development of an independent geotechnical software tool, that will
be able to help practicing engineers to estimate variation of
bearing capacity with time after pile installation. The developed
numerical approach should be extendable to other than helical types
of piles, which is to be confirmed by additional research. 1.4.
SCOPE AND LIMITATION OF STUDY. The conducted study is mainly
focused on soil pore water pressure response due to pile
penetration, as it is believed to be an important factor affecting
the variation of pile bearing4
Chapter 1. Introduction. capacity with time. Adequate simulation
of the pore water pressure response in the soft finegrained soil
requires a realistic soil model and a fully coupled modelling
approach. NorSandBiot formulation adopted in the current study
incorporates the NorSand soil model (Jefferies, 1993; Jefferies
& Shuttle, 2002) to represent the fine-grained soil
stress-strain behaviour and the Biot (Biot, 1941) consolidation
theory to account for the effect of coupling the pore pressure
response to behaviour to the soil stress-strain behaviour. All
numerical simulations conducted in the current study were based on
the finite element implementation of the NorSandBiot formulation
developed by Shuttle (2003). Pore pressure and stress predictions
of the NorSandBiot code were successfully verified against a number
of available analytical solutions. Given the complexity of helical
pile installation process, numerical simulation of excess pore
pressure generated due to helical pile installation poses many
challenges. It appears that the most realistic simulation of
helical pile installation will require a 3-D approach, which is
hard to implement and widely apply. The focus of the current
research was on developing simple, yet realistic representation of
pore pressure response. It was necessary to neglect some features
of helical pile-soil interaction while simplifying the analysis. In
the present study helical pile installation was analyzed in 1-D
employing the cylindrical cavity expansion analogue. A better
insight in pore pressure response induced due to helical pile
installation may be achieved when the effect of soil remoulding and
2-D effects of soil response are considered. Due to the large
volume of the conducted study these issues were left for future
research. Laboratory study was also beyond the scope of this work.
Modelling input parameters were derived from three previous
investigations of Colebrook silty clay properties. They explicitly
provided many, but not all, of the input parameters required for
the NorSandBiot formulation. Some of the input parameters were
taken as a best estimate, believed and shown to be reasonable based
on all information available. Another challenge in establishing
input
parameters resulted from differences between laboratory and
in-situ derived values of soil properties. This is not unusual in a
silty site where soil disturbance during sampling is a major issue.
Local spatial property variation, as seen in the in situ
measurements, added to parameter uncertainty. It appears that
detailed laboratory study is required to refine the modelling input
parameters taken in the current study.
5
Chapter 1. Introduction. 1.5. THESIS ORGANIZATION. In Chapter 1
of this thesis helical piles are introduced, research purposes and
objectives are stated, along with the scope and limitations of the
conducted study. An overview of the study of helical pile
performance in soft fine-grained soils, carried out by Weech
(2002), is given in Chapter 2. This comprises a description of the
scope of the work, information on site stratigraphy and basic soil
properties, geometry of the tested piles and measuring equipment. A
brief outline of the results of the Weechs study relevant to the
current research is also presented. Chapter 3 reviews the
literature to provide information leading to the formulation of the
modelling approach. Modelling approach adopted in this study is
formulated in Chapter 4. NorSand critical state soil model and Biot
consolidation theory are presented along with their finite-element
implementation. Formulation input parameters are explained. Chapter
5 describes the selection of site-specific soil parameters for
modelling. Overview of all available subsurface information is
given. Selection process for all model input parameters is
individually analyzed. Best estimates of the soil properties for
modelling are presented. In Chapter 6, the description and results
of the NorSand-Biot formulation parametric study are presented. An
accent is put on highlighting the input parameters that have the
most profound influence on the modelling results. Chapter 7
presents modelling results and their analysis. A comparison of
modelling with the available field data, including Weech (2002)
measurements, is provided and discussed. Effects of the pile shaft
and the helices on pore pressure response are separately analysed.
from the modelling are presented. Chapter 8 provides conclusions
from the current study and recommendations for further research.
Implications
6
Chapter 1. Introduction.
a
b
Fig. 1.1. Helical piles: a conventional pile; b grouted
(PULLDOWNTM) pile.
7
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. 2.0 OVERVIEW OF FIELD STUDY OF
HELICAL PILE PERFORMANCE IN SOFT SENSITIVE SOIL. 2.1. INTRODUCTION.
This study develops a numerical formulation to analyze pore
pressure response due to helical pile installation. As a basis for
development of a robust numerical approach to modelling of time
dependent pore pressure response, induced by helical pile
installation, high quality field data is essential. Information
obtained in the field provides an initial framework of expected
soil response and can serve as a reference point for modelling
results verification. A comprehensive field study of helical pile
performance in sensitive fine-grained soils, conducted at Surrey,
British Columbia, by Weech (2002), was chosen as a source of
necessary background information for numerical analysis in a
current research. Weechs study was mainly oriented towards
improving understanding of the effects that the installation of
helical piles has on the strength characteristics of sensitive
fine-grained soils. Current research is focused on time-dependent
pore water pressure response due to helical pile installation. In
this chapter a brief overview of Weechs work is given and Weechs
key findings relevant to the current study are presented. In
addition a review of available information on site subsurface
conditions is provided. 2.2. SCOPE OF WEECHS STUDY. Six
instrumented full-scale helical piles were installed in soft
sensitive marine deposits. Prior to pile installation, an in-situ
testing program was carried out, that consisted of: two profiles of
vane shear tests; five piezocone penetration soundings, with pore
pressure dissipation tests carried out at two soundings and shear
wave measurements at three soundings. The excess pore pressures
within the soil surrounding the piles were monitored during and
after pile installation by means of piezometers located at various
depths and radial distances from the pile shaft, and using
piezo-ports, which were mounted on the pile shaft. After allowing a
recovery period following installation, which varied between 19
hours, 7 days and 6 weeks, piles with two different helix plate
spacing were loaded to failure under axial8
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. compressive loads. Strain
gauges mounted on the pile shaft were monitored during load testing
to determine the distribution of loading throughout the pile at the
various load levels up to and including failure. Load-settlement
curves were generated for different pile sections at different
times after installation. The piezometers and piezo-ports were also
monitored during load testing and the distribution of excess pore
pressures 2.3. SITE SUBSURFACE CONDITIONS. The test site, also
referred to as the Colebrook site, is located under the King George
Highway (99A) overpass over Colebrook Road and the adjacent BC
Railway line, South Surrey, BC; approximately 25 km southwest of
downtown Vancouver, as shown in Fig. 2.1. 2.3.1. SITE STRATIGRAPHY.
The subsoils found in this area belong to so called Salish
Sediments. According to Armstrong (1984): Salish sediments include
all postglacial terrestrial sediments and postglacial marine
sediments that were deposited when the sea was within 15 m of
present sea level. These deposits were likely laid down during the
Quaternary period between 10,000 and 5,000 years ago. Cross-section
of site stratigraphy is shown on Fig. 2.2. From the surface there
is a layer of fill, about 0.6 m thick, which was placed during 99A
Highway construction. The fill is underlain by a layer of firm to
stiff peat, possibly bog and swamp deposit, that formed the
original ground surface; the thickness of this peat layer is about
0.3 m. Below the peat there is a layer of firm clayey silt of
deltaic origin, with some sand inclusions. The thickness of this
layer is about 1 m. The layer of clayey silt is underlain by layer
of soft silty clay with organic inclusions (peat, plant stalks).
Given the proximity of the Serpentine river, this deposit likely
has a tidal origin: it was deposited within the inter-tidal zone
between the Serpentine river delta and Semiahmoo Bay. Below the
silty clay layer there is a thick (around 27 m) layer of soft
clayey silt to silty clay of marine origin. The marine deposits are
underlain by a stiff layer of sand and gravels of glacial origin.
Crawford & Campanella (1991) reported slight artesian pressure
at the interface of the silty clay layer and glacial deposits.
Weech (2002) indicated that the groundwater table can be found at 2
m elevation (0.7m from the surface), with an upward hydraulic
gradient of 5 to 10 %, which is possibly explained by the
groundwater recharge from the upland area north of the site.9
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. 2.3.2. SOIL PROPERTIES. Three
subsurface investigations were performed at, or close to, the
helical piles performance research site. Site plan and locations of
all subsurface investigations are presented in Fig. 2.3. A brief
description of each investigation and their reviews reported in the
literature are presented below in chronological order. 2.3.2.1.
FIELD INVESTIGATION BY MINISTRY OF TRANSPORTATION AND HIGHWAYS.
Prior to construction of the Colebrook Road overpass (Highway 99),
the Ministry of Transportation and Highways (MoTH) performed an
extensive geotechnical study of the soil conditions along the
alignment of a planned overpass (in 1969). The MoTH investigation
included dynamic cone penetration tests and drilling with diamond
drill to establish the depth and profile of the competent stratum
underlying the soft sediments. Field vane shear tests were
performed at selected depths. Undisturbed samples of the soft soils
were recovered with a Shelby tube sampler. A number of laboratory
tests were carried out on the MoTH samples, including index tests,
consolidated and unconsolidated triaxial tests and laboratory vane
shear tests. Crawford & deBoer (1987) studied the long-term
consolidation settlements underneath the approach embankments,
located in the vicinity of the helical piles performance research
site. They reported some of the data obtained during the MoTH
investigation - typical for the Colebrook site soil properties and
results of three unidirectional consolidation tests performed in a
triaxial cell, with radial drainage. Crawford & deBoer (1987)
report, based on laboratory testing, an average coefficient of
consolidation in the horizontal direction, ch = 1.510-3 cm2/s, an
average coefficient of secondary consolidation, C = 0.014 and an
initial void ratio, for all three tests, e0 = 1.25. A summary of
typical soil properties from MoTH investigation given by Crawford
& deBoer (1987) are presented in Table A.1 (Appendix A).
2.3.2.2. RESEARCH BY UNIVERSITY OF BRITISH COLUMBIA (1). Crawford
& Campanella (1991) reported the results of a study of the
deformation characteristics of the subsoil, using a range of
in-situ methods and laboratory tests to predict soil settlements
underneath the embankment, and compare them with the actual
settlements. In-situ tests included field vane shear tests,
piezocone penetration test (CPTU) and a flat dilatometer test
(DMT). Laboratory tests were limited to constant rate of strain
odometer consolidation tests on10
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. specimens obtained with a
piston sampler. Results of a series of the CRS consolidation tests
are presented in Table A.2 (Appendix A). As a continuation of
previous works by Crawford & deBoer (1987) and Crawford &
Campanella (1991), Crawford et al. (1994) studied the possible
reasons for the difference between predicted and measured
consolidation settlements underneath the embankment using the
finite-element consolidation analysis with CONOIL computer program
(by Byrne & Srithar, 1989). The soil properties employed in the
numerical analysis are shown in Table A.3 (Appendix A). 2.3.2.3.
RESEARCH BY UNIVERSITY OF BRITISH COLUMBIA (2). As a part of his
study of helical pile performance in soft soils, a comprehensive
investigation of site soil conditions was carried out by Dolan
(2001) and Weech (2002). Dolan (2001) obtained continuous piston
tube samples from ground level to 8.6 m depth and performed index
testing to determine natural moisture content, Atterberg limits,
grain-size distribution, organic and salt content. Results of index
tests carried out by Dolan (2001) on samples obtained with the
piston tube sampler are summarized in Table 2.1 Table 2.1. Average
index properties of clayey silt/silty clay layer (elevation -4.1 m
and below). Soil Property natural moisture content (wn) liquid
limit (wL) plasticity index (PI) unit weight () in-situ void ratio
(eo) Average Value 42%+/-3% 40%+/-4% 13.5%+/-4.5%, 17.8+/-0.3 kN/m3
1.16+/-0.09 Comments below 8m in elevation PI is up to 21% derived
from moisture content data, assuming specific gravity of 2.75
Weech (2002) carried out a detailed in-situ site
characterization program, which included field vane shear tests;
cone penetration tests with pore pressure (CPTU) and shear wave
travel time measurements (SCPT). Locations of sampling and in-situ
soundings are presented in Fig. 2.4. presented in Table A.4
(Appendix A).11
A summary of
engineering parameters for the silty clay layer, estimated from
in-situ tests by Weech, are
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. Field vane shear strength
profiles for the Colebrook site measured by Weech (2002) and
Crawford & Campanella (1991) are shown in Fig. 2.5. In Fig.
2.5a the peak undrained shear strength is plotted with depth. For
the clayey silt/silty clay layer it varies from 15 to 30 kPa. The
profile of the remoulded shear strengths, (su)rem, is also plotted
on Fig. 2.5a, showing a variation from 2 to 0.7 kPa within the
clayey silt/silty clay layer. Due to such low remoulded strengths,
the sensitivity, St = (su)peak/(su)rem, determined from the field
vane measurements is very high. Profiles of sensitivity are shown
on Fig. 2.5b. The sensitivity appears to increase approximately
linearly with depth from a minimum of 6 at surface to about 40 at
12 m elevation. Even higher sensitivity, in the range of 50 to 75,
was measured by Crawford & Campanella (1991) between 12 and 17
m, who state that the high sensitivity of the marine deposits is
likely caused by leaching of pore-water salts due to the slight
artesian conditions, particularly at the lower depth. The ratio of
su to the effective overburden pressure, vo, is presented in Fig.
2.5c. In the upper part of the marine deposits (from 4.1 to 4.4 m
in elevation) the su/vo ratio is quite high around 0.7, which
indicates moderately overconsolidated soil. At lower depths the
deposit is lightly overconsolidated, with the su/vo ratio around
0.4. A typical CPT cone test result for Colebrook site, including
profiles of corrected tip resistance, qT, sleeve friction, fs, and
excess penetration pore pressure, u, measured behind the shoulder
of the cone (u2 filter position), are presented on Fig. 2.6. A
detailed overview of the soil properties, relevant to the current
study, is given in Chapter 5. 2.4. HELICAL PILES AND PORE PRESSURE
MEASURING EQUIPMENT. 2.4.1. TEST PILES GEOMETRY AND INSTALLATION
DETAILS. For the purpose of studying different failure mechanisms,
piles with two different lead sections were used. The largest
helical piles manufacturer, Chance Anchors, commonly uses helical
plates attached to the lead section such that the distance between
successive plates (S) is 3 times the diameter (D) of the lower
plate. In this case, current thinking based on small scale model
tests (Narasimho Rao et al., 1991) is that during loading to
failure, failure occurs at individual helices. For the closer
spacing of the helical plates, the failure mechanism is believed to
be different - all helices fail simultaneously, so that a
cylindrical failure surface is generated12
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. coinciding with the outside
edge of the helical plates. To investigate such a possibility the
testing was carried out on piles which had either 3 plates at S/D =
3, or 5 plates at S/D = 1.5, so that the total length from the top
to bottom helix was equal for the two pile types (2.1 m). The pitch
of the helix plates was 7.5 to 8 cm, which is the standard pitch on
helical piles manufactured by Chance Anchors. The geometry of both
types of lead sections is shown in the Fig. 2.7. In total six
helical piles - three for each leading section type were installed,
their locations are shown in Fig. 2.8. Two piles, TP-1 - with three
helices (S/D = 3) and TP-2 with five helices (S/D = 1.5), were
chosen for the detailed monitoring. The other piles served as a
source of additional information. All piles were installed to a tip
depth of 8.5 m (-9.8 in elevation). Installation of a single pile,
including breaks for section mounting and adjustments to maintain
pile verticality, usually took about 2 hours. Deducting
interruptions, the average rate of soil penetration by helical pile
was about 1.5 cm/s. 2.4.2. MEASURING EQUIPMENT. A total of 26 UBC
push-in piezometers were installed at different depths and radial
distances from the 6 test piles, and a total of 10 piezo-ports were
located at 3 different positions on the shaft of the piles, as
indicated in Table B.1 (Appendix B). Piezo-ports, which contained
an electric pore pressure transducer with a porous filter, were
installed within the wall of the pile shaft on the lead sections.
The piezometers were pushed into the soil at least one week prior
to pile installation so that full dissipation of the excess pore
pressures generated during piezometer installation could occur.
These piezometers were then used to monitor the variation in pore
pressures caused by pile installation and their subsequent
dissipation. During pile installation piezometers were continuously
monitored using the multi-channel data acquisition system. After
the end of pile installation piezoports located on the pile shaft
were also connected to the data acquisition system and were
continuously monitored in conjunction with the piezometers. Two
types of electronic pore pressure transducers were employed for the
piezometers and the piezoports, with measuring capacity 345 and 690
kPa. The resolution of the automatic acquisition system used to
monitor the piezometers was 0.035 to 0.07 kPa (for 345 and 690 kPa
transducers, respectively). The rated accuracy of the pressure
transducer measurements was 0.1% of full scale. Even though every
attempt was made to carefully assemble and install measuring
equipment, the response of many piezometers and piezoports was less
than perfect, as shown in Table B.1.13
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. 2.5. SUMMARY OF WEECHS STUDY
RESULTS. This summary is based on Weechs interpretations of pore
pressure response measured during and after helical pile
installation. Only key points are presented here, more details can
be found in Weech (2002). 2.5.1. PORE WATER PRESSURE RESPONSE
DURING HELICAL PILE INSTALLATION. Pore pressure profiles measured
at different radial distances during installation for piles TP-1
and TP-2 are shown in Fig. 2.9 and Fig. 2.10. In these figures
profiles of normalized peak pore pressure ui/vo are plotted against
the depth of the pile tip below the elevation of the piezometer
filter (zpile zpiezo). For reference, the locations of the
different parts of the pile relative to the tip are also shown on
the right side of these figures. Based on Fig. 2.9 and 2.10 Weech
(2002) made the following observations: There is a very sudden
increase in ui as the tip of the pile shaft approaches and then
passes the elevation of the piezometer filters. This increase is
particularly abrupt at the piezometers located closer to the pile.
The magnitude of excess pore pressure generated within the soil by
the pile installation decreases with radial distance from the pile.
Negative pore pressures were observed just before the pile tip
passes the piezometers locations. Baligh & Levadoux (1980)
linked such behaviour with vertical displacement of soil in advance
of a penetrating pile or probe, which is initially downward.
According to Weech (2002), downward soil movement relative to the
static piezo-cell induces a short lived tensile pore pressure
response which is observed just before the response becomes
compressive with a primarily radial displacement vector. Each
helical plate passing the piezometers generates a pulse in pore
pressure. The first pulse generated by a leading helical plate is
the strongest, all subsequent helical plates generate less
definitive pore pressure pulses. Such an effect is noticeable only
at piezometers located within one helix radius from the helix edge
(r/Rshaft1 = 7 and 8) . Only the soil located very close to the
outside edge of the helix plates (within about 10 to 12 times the
helix plate thickness - thx) appears to respond directly to the
penetration ofIn this overview, radial distance is represented by
the r/Rshaft ratio, where Rshaft is the radius of the pile shaft
(in the current study, identical for all piles), r radial distance
from the pile centre. 141
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. the helix plates. Within this
zone, distinctly different responses are observed for the S/D = 1.5
and S/D = 3 piles. At radial distances larger than about 10-12 thx
beyond the edge of the helices, the pore pressure response to the
penetration of the S/D = 1.5 and S/D = 3 piles is very similar.
Weech (2002) attempted to quantify separately pore pressures
generated by pile shaft and the helices, where the pore pressures
generated by the pile shaft were inferred from the piezometers
response to penetration of the pile tip. In Fig. 2.11 is shown a
radial distribution of normalized pore pressures induced by the
pile tips of all test piles. According to Fig. 2.11, for r/Rshaft =
5 to 17, ushaft/vo decreases steeply and almost linearly. After
r/Rshaft = 17, ushaft becomes quite small (< 0.1vo) and the
slope of the pore pressure decay with distance flattens.
practically negligible. In Fig. 2.12 is shown radial distribution
of peak pore pressures generated, during installation, by helical
pile shaft and the helices, and, the best estimate of pore
pressures generated by helical pile shaft alone, so that the effect
of the helical plates can be studied. Weech (2002) made the
following observations from this figure: The contribution of the
helical plates to the magnitude of generated pore pressures, during
helical pile installation, appears to be quite significant. At
distances up to r/Rshaft = 6, the pore pressures generated by the
helices make up to 20% of the total pore pressures and at distances
greater than r/Rshaft = 17 make up to 75% . Penetration of the
helices extends the radial distance of generated pore pressures
from r/Rshaft about 60, estimated for penetration of pile shaft
alone, to r/Rshaft about 90. Weech (2002) argued that there appears
to be a gradual outward propagation of the pore pressure induced by
the helices, during continuing pile penetration, attributed to
total stress redistribution caused by soil destructuring. 2.5.2.
PORE WATER PRESSURE DISSIPATION AFTER HELICAL PILE INSTALLATION.
Weech (2002) compiled a combined dataset of all (for piles with
both S/D = 1.5 and 3) normalized piezometric measurements, taken at
different times, at the locations above the bottom helical plate as
presented in Fig. 2.13. Despite some scatter in the data there is a
trend in the observed pore pressure dissipation behaviour,
represented by the fitted curves. According to Fig.15
For r/Rshaft 60 generated pore pressures are
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. 2.13, excess pore pressure, u,
decreases monotonically throughout the soil around the pile, out to
a radial distance of at least 30 shaft radii. The rate of
dissipation at different radial distances appears to vary such that
the u(r)/vo-log(r) curve becomes more and more linear as the
dissipation process progresses. Fig. 2.14 shows curves fitted to
all the available data of normalized excess pore pressure measured
at the location above and below the level of the bottom helical
plate (where the influence of plate penetration is minimal). Weech
(2002) made the following observations from this figure: No
residual uhx is observed in the soil (from r/Rshaft = 5 to at least
17) below the level of the bottom helix within 10 minutes after
stopping penetration Dissipation of u within the soil close to the
helices (r/Rshaft < about 10) is much more rapid below the level
of the bottom helix than above, at least during the first 17 - 20
hours of dissipation. The elevated pore pressures at the tail of
the distribution (r/Rshaft > 17), which are due to the
penetration of the helix plates, remain above the initial level
generated by the pile shaft until about 20 hours. Average
dissipation curves at different radial distances from the piles are
shown in Fig. 2.15. Shown dissipation curves do not exhibit a
unified dissipation trend at bigger times, Weech (2002) attributed
this to the higher rate of dissipation at larger radial distances.
In Fig. 2.16 shows the dissipation curves based on u(t)/vo data
from individual piezometers/piezo-ports located at different radial
distances from the test piles (above the bottom helix). Based on
this figure Weech (2002) made the following observations: The
dissipation occurs much more quickly below the bottom helix than
above, at radial distances close to the pile. Even though greater
proportions of dissipation occur sooner at larger radial distances,
all of the curves tend to converge at the end of the dissipation
process. For all monitored piles 100% dissipation occurred at about
7 days for most locations around the piles. The dissipation process
appears to be essentially independent of the number or spacing of
the helix plates.
16
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil. 2.6. SUMMARY. A comprehensive
study of helical pile performance carried out by Weech (2002) was
an important step towards better understanding of a complex helical
pile fine-grained soil interaction. Weech reported details of the
pore pressure response observed during and after installation of
helical piles at the Colebrook site and attempted to interpret
them. However, the presented problem analysis cannot be considered
complete. The applicability of the observations made during Weechs
study on sites with different soil conditions and different helical
piles geometries is questionable. According to Terzaghi2: Theory is
the language by means of which lessons of experience can be clearly
expressed. It appears that the lessons of experience gained during
Weechs study may be effectively utilized using numerical modelling.
In the current study the field measurement of the pore water
pressure response measured by Weech (2002) is employed as a
reference point for analysing the results of numerical
modelling.
2
Quote from Karl Terzaghi biography by Goodman (1999). 17
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil.
Test Site
N Surrey, BC
Fig. 2.1. Helical pile performance research site location.
Fig. 2.2. Site subsurface conditions at the research site
(modified after Weech, 2002).
18
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soil.
scale - metres
Fig. 2.3. Approximate locations of subsurface investigations at
the Colebrook site (modified after Crawford & Campanella,
1991).
Fig. 2.4. Location of CPT tests and solid-stem auger holes
(after Weech, 2002)19
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.a)0 -3 Field Vane Shear
Strength (s u)FV (kPa) 10 20 30 40
b)0 10
Sensitivity St = (su )peak/(su )rem 20 30 40 50
c)0.0 0.2
Strength Ratio s u/ 'vo 0.4 0.6 0.8
-4
-5
-6
Possibly affected by sandy silt
Elevation (m)
-7 -8
-9
-10
-11
-12Peak Strength (VH-1&2)Remoulded Strength (VH-1&2)Peak
(from Craw ford & Campanella, 1991)Rem (from Craw ford &
Campanella, 1991)VH-1&2Craw ford & Campanella (1991)
Fig. 2.5.
Variation of field vane shear strength test results with
elevation (after Weech, 2002).20
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.
a)0 -3 1 2
Tip Resistance QT (bar)3 4 5 6 7
b)0 1
Sleeve Friction fs (kPa) 2 3 4 5 6
c)-50
Excess Pore Pressure at U2 - u (kPa) 0 50 100 150 200 250
-4
-5
-6
Elevation (m)
-7
-8
-9
-10
-11
Note: Breaks in profile correspond to data recorded upon
resuming penetration after seismic tests
-12
Fig. 2.6. Example of cone penetration test results (CPT-7)
(after Weech, 2002).21
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.
Fig. 2.7. Helical piles geometry (modified after Weech,
2002).
22
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.
3rd bridge pier from South abutmentpile cap
2nd bridge pier from South abutment
300 mm wide hexagonal RC piles
Helical piles
Fig. 2.8. Helical piles locations (modified after Weech,
2002).
23
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.Excess Pore Pressure during
Pile Installation - ui/'vo-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
1.8 2.0
5Note: Dissipation during breaks in installation removed.
4
Grout Column
Depth of Pile Tip Below Piezo Filter Elev. (m)
3
Grout Disc
2Line of Max Pore Pressure
Helix Plates
1
0PZ-TP4-1 (r/R = 4.8) PZ-TP2-5 (r/R = 7.3)
-1
PZ-TP2-1 (r/R = 8.0) PZ-TP2-7 (r/R = 11) r = radial distance
from pile center R = radius of pile shaft PZ-TP2-3 (r/R = 17)
PZ-TP2-4 (r/R = 30)
-2
Fig. 2.9. Variation of excess pore pressure with pile tip depth,
S/D=1.5. (after Weech, 2002)24
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.Excess Pore Pressure during
Pile Installation - ui/'vo-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
1.8 2.0
5Note: Dissipation during breaks in installation removed. Grout
Column
4
Depth of Pile Tip Below Piezo Filter Elev. (m)
3
Grout Disc
2Line of Max Pore Pressure Helix Plates
1
0
PZ-TP3-1 (r/R = 5.8)PZ-TP3-2 (r/R = 8.1)
-1PZ-TP1-7 (r/R = 12)r = radial distance from pile center R =
radius of pile shaft
PZ-TP1-3 (r/R = 14)PZ-TP1-4 (r/R = 25)
-2
Fig. 2.10. Variation of excess pore pressure with pile tip
depth, S/D=3. (after Weech, 2002).25
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.
1.8 1.7 1.6 1.5 1.4Linear Trend Line Pile Piezos (due to pile
tip penetration) Pile Piezo-Ports (End of Installation)
Excess Pore Pressure during Installation - ui/'vo
1.3 1.2TP4-1
Logarithmic Trend Line
1.1 1.0 0.9
TP1-9 TP1-6
TP3-1 TP5-1 TP4-2 TP6-2 TP6-1 TP3-2
TP2-6
TP2-2
0.8 0.7
TP2-5
TP2-1 TP1-5
Linear Trend Line
0.6 0.5 0.4 0.3 0.2 0.1 0.0 1 10 100TP1-3 TP2-9
Edge of Helices
TP2-7
TP1-7
TP2-3
TP1-4
TP2-4
Radial Distance from Pile Center (shaft radii) - r/Rshaft
Fig. 2.11. Radial distribution of excess pore pressure generated
by penetration of pile shaft (modified after Weech, 2002).26
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.1.8Peak u at Piezos after
Passing of Pile Tip
1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0
Max u at Piezo-Ports (End of Installation) Shaft Penetration
(best fit of data from Fig. 2.11) Shaft Penetration (best estimate
for r < 5R)
uhx(best estimate)
u/ 'vo
0.9 0.8 0.7 0.6 0.5 0.4 0.3Edge of Helices
uhx0.2 0.1 0.0
1
10 Radial Distance from Pile Center (shaft radii) - r/R
shaft
100
Fig. 2.12. Radial distribution of maximum excess pore pressure
after penetration of helices(after Weech, 2002).27
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.1.6 1.4 1.2 1.00.1 min (Ushaft
= 0%) 10 min (Ushaft = 4%) 1 hr (Ushaft = 16%) 5 hrs (Ushaft = 35%)
17-20 hrs (Ushaft = 57%) 2 days (Ushaft = 76%) Edge of Helices
u/'vo
0.1 min after stopping 10 min after stopping 1 hr after stopping
5 hrs after installation 17-20 hrs after installation 2 days after
installation Initial Shaft Penetration
0.8 0.6 0.4 0.2 0.0 1
10 Radial Distance from Pile Center (shaft radii) - r/Rshaft
100
Fig. 2.13. Radial distribution of excess pore pressure around
helical piles (above level of bottom helix) during dissipation
process (after Weech, 2002).1.6 1.410 min Edge of Helices
1.2 1.0u/'vo1 hr
0.85 hrs
0.6 0.4 0.2 0.0 1 1017-20 hrs
10 min (Below Helices) 1 hr (Below Helices) 5 hrs (Below
Helices) 17-20 hrs (Below Helices) 2 days (Below Helices) 10 min
(Above Bottom Helix) 1 hr (Above Bottom Helix) 5 hrs (Above Bottom
Helix) 17-20 hrs (Above Bottom Helix) 2 days (Above Bottom
Helix)
2 days
100
Radial Distance from Pile Center (shaft radii) - r/Rshaft
Fig. 2.14. Radial distribution of excess pore pressure above
& below level of bottom helix during dissipation process (after
Weech, 2002).28
Chapter 2. Overview of the field study of helical pile
performance in soft sensitive soils.1.0 0.9 0.8 0.7u(t)/uouo = u at
0.1 min after stopping installation
0.6 0.5 0.4 0.3 0.2 0.1 0.0 1 10 100 1000 10000 Time after
Stopping Installation (min)r/R = 1(Pile Shaft)
r/R = 4 (Edge of Helices)
r/R = 6
r/R = 8
r/R = 12
r/R = 16.5
r/R = 25
Fig. 2.15. Average dissipation trends for different radial
distances from pile (after Weech, 2002)1.6 1.4 1.2 1.0Between
Helices, r/R = 1 (TP1-PP1) Below Helices, r/R = 1 (TP4-PP3)
Opposite Helices, r/R = 6.3 (PZ-TP4-2) Below Helices, r/R = 5.5
(PZ-TP1-9) Opposite Helices, r/R = 8.1 (PZ-TP3-2) Opposite Helices,
r/R = 12 (PZ-TP1-7) Below Helices, r/R = 16 (PZ-TP2-9)
u/'vo
0.8 0.6 0.4 0.2 0.0 1 10 100 1000 10000
Time (min) from End of Installation
Fig. 2.16. Dissipation curves from piezometers/piezo-ports
located at different radial distances from pile (after Weech,
2002).
29
Chapter 3. Literature review. 3.0. LITERATURE REVIEW. 3.1.
INTRODUCTION. Pore water pressure response, including pore pressure
generation and subsequent dissipation, due to helical pile
installation into fine-grained soil has not been addressed until
very recently. A field study by Weech (2002) provided the necessary
factual information. However it is rather difficult to explain
complex soil response based solely on interpretation of the field
measurement. Prediction of pore water pressure response during and
after pile installation into fine-grained soils has been the
subject of a number of theoretical studies. Moreover, an extensive
body of work exists in the field of cone penetration testing, where
dissipation solutions were employed for the prediction of soil
consolidation characteristics. Essentially, the CPT cone is a
scaled instrumented pile and the pore pressure prediction solutions
developed for cones may be applicable for prediction of the pore
water response due to installation of driven and jacked piles. The
main objective of this chapter is to establish a theoretical
background upon which a numerical formulation for the analysis of
pore pressure response due to helical pile penetration can be
developed. To meet this objective, the existing state of knowledge
on field observation of time dependent pore pressure response due
to penetration of piles and piezocones is summarized, and a brief
review of well known methodologies for pore pressure predictions is
provided. 3.2. PORE PRESSURE RESPONSE INDUCED BY PILE INSTALLATION
INTO FINE GRAINED SOILAND ITS INFLUENCE ON PILE CAPACITY.
3.2.1. FIELD GENERATION OF EXCESS PORE PRESSURE. Pile
installation causes disturbance in the soil adjacent to the pile.
Flaate (1972) studied impact of timber pile installation on
fine-grained soils. It was observed that installation of a circular
timber pile 0.33m in diameter formed a zone of up to 0.10 0.15 m
from the pile shaft where the soil was completely remoulded.
severely diminished. Stiffness and undrained strength in this zone
were found It was also observed that outside the remoulded zone
exists a zone of
reduced stiffness and undrained strength, or transition zone.
According to Flaate (1972) the extent of the transition zone
largely depends on natural soil properties, pile dimensions and the
mechanism of penetration. The concept described by Flaate (1972) is
shown in Fig. 3.1.
30
Chapter 3. Literature review. Soil deformations cause high pore
pressures in excess of equilibrium hydrostatic values. The
magnitude of generated excess pore pressures will depend on the
type of soil and its properties. A number of accounts (Bjerrum
& Johannessen, 1961; Lo & Stermac, 1965; Orrje & Broms,
1967; Koizumi & Ito, 1967; Bozozuk et al., 1978; Roy et al.,
1981 and Pestana et al., 2002) report generation of significant
positive excess pore pressures due to pile driving in fine-grained
soils. Baligh & Levadoux (1980) compiled data from a number of
sites where pore pressures were measured during pile installation
(Fig. 3.2). It was found that, for most of the cases, the excess
pore pressures at the pile shaft were about twice the vertical
effective stress and that the extent of the generated pore
pressures, having any significance (u/ v > 0.1), was about 20-30
pile radii. For penetration under undrained conditions, generated
excess pore pressure can be represented as a sum of pore pressure
generated due to change in the mean stress, and deviator shear
stress, as show in Eq. 3.1.
u = umean + ushear
(3.1