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1999 Determination of Consolidation Characteristics in Fine Soils Evaluated by Piezocone Tests. Beyongseock Lim Louisiana State University and Agricultural & Mechanical College

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Bell & Howell Information and Learning 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 800-521-0600

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DETERMINATION OF CONSOLIDATION CHARACTERISTICS IN FINE SOILS EVALUATED BY PIEZOCONE TESTS

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agriculture and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy

in

The Department of Civil & Environmental Engineering

by Beyongseock Lim B.S., Korea University, Seoul, Korea, 1990 M.S., Korea University, Seoul, Korea, 1993 December 1999

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 9960074

UMI

UMI Microform 9960074 Copyright 2000 by Bell & Howell Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code.

Bell & Howell Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS

Financial support for this research was provided by the National Science

Foundation under grant (CMS-9531782). The miniature piezo/friction cone and

standard reference (10 cm2) penetrometer were provided by Fugro-McClelland

Engineers B.V., The Netherlands, and the miniature piezocone penetrometer was

provided by Fugro-Geoscience. inc.. U.S.A. Their generous collaboration is gratefully

acknowledged. Appreciation is expressed to Mr. Van Auker of Feldspar Corporation.

Edgar, Florida, for donating the sand and kaolinite used in this research.

The author expresses sincere gratitude to his advisor, Professor Mehmet T.

Tumay and co-adviser. Professor Pradeep U. Kurup, for their guidance and constant

inspiration throughout this research. The author is greatly indebted to them for their

advice, encouragement, and good will.

The author is thankful to Dr. George Z. Voyiadjis. Dr. Roger K. Seals.

Dr. Kenneth E Jr. Damann for serving on the examining committee and for their helpful

suggestions. Special thanks are extended to Dr. Kaare Senneset of the Norwegian

University of Science & Tchnology, Trondheim, for many valuable technical

discussions.

The author is thankful to all members of LTRC’s soil lab, concrete lab, and

asphalt lab for their assistance in operation of LSU/CALCHAS. Sincere thanks are

expressed to Mr. Bill Tiemey for his valuable help during the entire experimental

research phase. Appreciation is expressed to Dr. Dario C. de Lima for developing the

LSU Calibration Chamber System under the supervision of Dr. Mehmet T. Tumay. The

help of Daekyu Kim, Danny J. Folly, Jinwook Shin, Seongyong Kim while preparing

ii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the soil specimens is gratefully acknowledged. I would like to thank Dr. Sewhan Paik

for his friendship and continuous care like a real brother during my Ph.D. studies.

I dedicate this dissertation to my mother, the late Kyongsook Lee, for her

sacrifice to her children. I have felt her love and caring throughout my life.

Very special thanks are due to my lovely wife Mikyung, for the patience,

encouragement and love demonstrated during this endeavor. I am also indebted to

Jonguk and Jeffrey, my two sons, for not being there where they needed me.

Finally, I would like to extend my sincere thanks to my parents, Jangkun Yu and

Jeongseop Jeong, Heewon Lim for their understanding, support, and constant

inspiration during the course of this research.

iii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ACKNOWLEDGEMENTS...... »i

LIST OF TABLES...... v»

LIST OF FIGURES...... viii

ABSTRACT...... xv

CHAPTER 1 INTRODUCTION...... 1 1.1 Research Objectives...... 3 1.2 Organization of Dissertation ...... 4

2 LITERATURE REVIEW...... 6 2.1 Background ...... 6 2.2 Interpretation Methods ...... 10 2.2.1 Cavity Expansion Method ...... 11 2.2.2 Strain Path Method ...... 13 2.2.3 Semi-Empirical Method ...... 13 2.2.4 Finite Element Analysis ...... 14 2.2.5 General Interpretation Procedure ...... 16 2.2.6 Initial Excess Pore Pressure Variation ...... 19 2.2.7 Excess Pore Pressure Drop Due to Normal Stress Release...... 24 2.3 Piezocone Penetration Test(PCPT) ...... 25 2.3.1 Piezocone Penetrometer ...... 25 2.3.2 Test Procedure ...... 31 2.3.2.1 Verticality ...... 31 2.3.2.2 Rate of Penetration ...... 31 2.3.2.3 Saturation of Piezocone ...... 32 2.3.2.4 Dissipation Test ...... 37 2.3.2.5 Correction of Measurements from Unequal End Area Effect ...... 37 2.4 Calibration Chamber Testing ...... 39 2.4.1 History ...... 40 2.4.2 Chamber Size and Boundary Condition Effects ...... 43 2.4.3 Development of the Cohesive Soil in Calibration Chamber ...... 45

3 DESCRIPTION OF THE TESTING EQUIPMENT...... 47 3.1 Cone Penetrometers ...... 47 3.1.1 Miniature Piezocone ...... 47 3.1.2 Reference Cone ...... 50 3.2 Slurry Consolidometer ...... 50

iv

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2.1 Loading System ...... 52 3.2.2 Instrumentation ...... 54 3.3 The LSU Calibration Chamber System ...... 57 3.3.1 Control Panel ...... 63 3.3.2 Hydraulic and Chucking System ...... 63 3.3.3 Data Acquisition and Monitoring System ...... 65

4 TEST PROCEDURE...... 71 4.1 Specimen Preparation ...... 71 4.1.1 Slurry Consolidation ...... 71 4.1.2 Reconsolidation in a Calibration Chamber ...... 76 4.1.2.1 Specimen Transfer and Placement ...... 76 4.1.2.2 Reconsolidation ...... 81 4.2 Piezocone Penetration Tests ...... 89

5 TEST RESULTS...... 97 5.1 General Results ...... 97 5.2 Results...... 108 5.2.1 Specimen No. 1 ...... 108 5.2.2 Specimen No. 2 ...... 110 5.2.3 Specimen No. 3 ...... 112 5.2.4 Specimen No. 4 ...... 113 5.2.5 Specimen No. 5 ...... 114 5.3 Specimen Homogeneity ...... 115

6 ANALYSIS OF TEST RESULTS...... 117 6.1 Initial Excess Pore Pressure Distribution ...... 117 6.1.1 Recording Excess Pore Pressure and Tip Resistance During Initial phase of Dissipation Using Oscilloscope .... 118 6.1.2 Test Results...... 120 6.2 Comparison Experimental and Predicted Initial Excess Pore Pressure Distribution ...... 121 6.2.1 Dissipation Phase ...... 135 6.2.2 Limitations ...... 177 6.3 Application of the Interpretation Models to Chamber Penetration D ata ...... 178 6.4 Undrained Shear Strength ...... 192 6.4.1 Interpretation Methods ...... 193 6.4.1.1 Bearing Capacity Models ...... 193 6.4.1.2 Capacity Expansion Models ...... 194 6.4.1.3 Strain Path Method ...... 196 6.4.1.4 Steady Penetration Approach...... 197 6.4.1.5 Empirical and Semi-Empirical Methods ...... 199 6.4.2 Application of the Interpretation Methods to the Chamber Specimens ...... 200 6.5 Lateral Stress Coefficient ...... 203

V

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.5.1 Interpretation Methods ...... 203 6.5.2 Evaluation of the Interpretation Methods ...... 206 6.6 Overconsolidation Ratio ...... 209 6.6.1 Interpretation Methods...... 210 6.6.1.1 Schmertmann Method ...... 210 6.6.1.2 Pore Pressure Parameters ...... 211 6.6.1.3 Cavity Expansion/Modified Cam Clay Methods .... 213 6.6.1.5 Kurup Method ...... 216 6.6.2 Evaluation of the Interpretation Methods ...... 217

7 SUMMARY. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH...... 222 7.1 Summary ...... 222 7.2 Accomplishments and Conclusions ...... 223 7.2.1 Accomplishments ...... 223 7.2.2 Conclusions ...... 224 7.3 Recommendations for Future Research ...... 227

REFEFERENCES...... 230

VITA...... 244

vi

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

Table Pa8e

2.1 Modified time factors T ' from consolidation analysis ( Houlsby& Teh. 1988) ...... 18

4.1 Properties of Kaolin and K-33 mixture ...... 75

4.2 Summary of the stress history of the chamber specimens...... 87

4.3 Reference soil parameters ...... 90

4.4 Summary of the cone penetration test locations ...... 95

4.5 Legends for Test ID ...... 96

5.1 Summary of the cone penetration test results ...... 109

6.1 Summary of oscilloscope tests ...... 121

6.2 Location of pore pressure access ducts in the calibration chamber specimen ...... 140

6.3 Comparison between the estimated and reference cr values at 50 % dissipation level using Aup and Auj ...... 191

6.4 Comparison of analytical bearing capacity factors, Nc, with NkT 202

6.5 Interpretation of OCR from PCPT data ...... 218

6.6 Pore pressure ratios and pore pressure difference for the chamber specimens ...... 220

vii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

Figure **a8e

2.1 Typical dissipation test results ( Lunne et. al., 1997) ...... 7

2.2 Normalized dissipation tests curves. (Lunne et. al., 1997) ...... 9

2.3 Expansion of spherical cavity (Vesic, 1972) ...... 12

2.4 Modeling piezocone penetration as successive Spherical cavity expansion (Gupta and Davison, 1986) ...... 15

2.5 Typical dissipation curves for NC soil(Sullv & Campanella, 1994) ...... 21

2.6 Initial excess pore pressure (Kurup & Tumay, 1995) ...... 22

2.7 In-Situ pore pressure measurements during piezocon penetration test in cohesive soils. (Robertson, et al., 1986) ...... 23

2.8 Reference cone (ISSMFE, 1977. 1989) ...... 27

2.9 Schematic showing common location of porous filter element. (Chen and Mayne, 1994) ...... 28

2.10 Effect of filter size and location on penetration pore pressure (Campanella and Robertson. 1988) ...... 30

2.11 Influence of penetration rate on cone resistance (Acar. 1981) ...... 33

2.12 Continuous push device ...... 34

2.13 Influence of penetration rate on measured pore pressures (Roy, et al.. 1982) ...... 35

2.14 Influence of improper saturation on the dissipation profile (Baligh, et al., 1981; Campanella, 1981) ...... 36

2.15 Unequal area effect (Lune et al, 1997) ...... 38

2.16 “ Skippy” calibration chamber ...... 41

2.17 Boundary conditions using flexible wall calibration chamber ...... 42

2.18 Chamber size and boundary effects (Parkin and Lunne, 1982) ...... 44

viii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.1 Cone Penetrometers ( mini-piezocone, mini-piezo/friction cone, and reference cone) ...... 48

3.2 Schematic of the miniature piezocone ...... 49

3.3 Change of filter location ...... 51

3.4 Slurry consolidometer ...... 53

3.5 A schematic of view slurry consolidometer ...... 55

3.6 Base plate with miniature pore pressure transducer ...... 56

3.7 Assembling pore pressure transducer, ducts, and adapter submerged in de-aired water ...... 58

3.8 Vacumm saturation of duct in the Nold DeArerator ...... 59

3.9 LSU Calibration Chamber System (LSU/CHAMBER) (Tumay and de Lima. 1992) ...... 60

3.10 Schematic of the flexible double wall calibration chamber ...... 62

3.11 A schematic of control panel ...... 64

3.12 Hydraulic push jack system ...... 66

3.13 Depth encorder device ...... 67

3.14 Flow chart for data acquisition and monitoring system ...... 68

3.15 General view of data acquisition system set u p ...... 69

4.1 Installing pore pressure transducers inside empty consolidometer 73

4.2 Particle size distribution curves (Kurup, 1993) ...... 74

4.3 Pore pressure dissipation during slurry consolidation ...... 77

4.4 Settlement during slurry consolidation ...... 78

4.5 Collapse of specimen due to sliding in slurry consolidation ...... 79

4.6 Bulging of soil sample ...... 80

4.7 Specimen placement on the chamber base plate ...... 82

ix

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.8 Curing damage of membrane before installing chamber c e ll ...... 83

4.9 Installing chamber cells over the specimen ...... 84

4.10 Final assembly of the specimen ...... 85

4.11 Flushing out the transducer cavity of the cone using funnel ...... 92

4.12 Vacuum saturation in the Nold DeAerator ...... 93

5.1a Penetration profiles in specimen 1 ...... 98

5.2a Penetration profiles in specimen 2 ...... 99

5.3a Penetration profiles in specimen 3 ...... 100

5.4a Penetration profiles in specimen 4 ...... 101

5.5a Penetration profiles in specimen 5 ...... 102

5.1b Dissipation results in specimen 1 ...... 103

5.2b Dissipation results in specimen 2 ...... 104

5.3b Dissipation results in specimen 3 ...... 105

5.4b Dissipation results in specimen 4 ...... 106

5.5b Dissipation results in specimen 5 ...... 107

6.1 Digital Oscilloscope (Nicolet 310) ...... 119

6.2 Immediate initial drop of excess pore pressure and tip resistance of 23/AF2 0 4 ...... 122

6.3 Immediate initial drop of excess pore pressure and tip resistance of 23/ArF203 ...... 123

6.4 Immediate initial drop of excess pore pressure and tip resistance of 3i/iF102 ...... 124

6.5 Immediate initial drop of excess pore pressure and tip resistance of 3,/iF203 ...... 125

6.6 Immediate initial drop of excess pore pressure and tip resistance of 43/AF2N0 ...... 126

X

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.7 Immediate initial drop of excess pore pressure and tip resistance of 43/ArF2N2 ...... 127

6.8 Immediate initial drop of excess pore pressure and tip resistance of 43/aF 1 0 3 ...... 128

6.9 Immediate initial drop of excess pore pressure and tip resistance of 43/aF 2 0 4 ...... 129

6.10 Immediate initial drop of excess pore pressure and tip resistance of 53/aFR10...... 130

6.11 Immediate initial drop of excess pore pressure and tip resistance of 53/aF2N1 ...... 131

6.12 Immediate initial drop of excess pore pressure and tip resistance of 53/ArF103 ...... 132

6.13 Immediate initial drop of excess pore pressure and tip resistance of 53/aF 2 0 4 ...... 133

6.14 Finite difference m esh ...... 137

6.15 Location of pore pressure access ducts in the chamber specimen ...... 139

6.16a Measured and predicted dissipation profiles at the cone base for 23/A204 ...... 141

6.16b Measured and predicted dissipation profiles at the cone base for 23/ArF203 ...... 142

6.16c Measured and predicted dissipation profiles at the cone base for 3 i/iF2N 1 ...... 143

6.16d Measured and predicted dissipation profiles at the cone base for43/AF2N0 ...... 144

6.16e Measured and predicted dissipation profiles at the cone base for43/ArF2N2 ...... 145

6.16f Measured and predicted dissipation profiles at the cone base for 43/AF2 0 4 ...... 146

6.16g Measured and predicted dissipation profiles at the cone base for 53/AF2 N l ...... 147

xi

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.16h Measured and predicted dissipation profiles at the cone base for 53/aF 2 0 4 ...... 148

6.161 Measured and predicted dissipation profiles at the cone surface for 1 i/jFR10 ...... 149

6.16j Measured and predicted dissipation profiles at the cone surface for 23/AFR10 ...... 150

6.16k Measured and predicted dissipation profiles at the cone surface forSwiFRlO ...... 151

6.161 Measured and predicted dissipation profiles at the cone surface for 53/aFRIO ...... 152

6.17a Spatial pore pressure dissipation at level 1 in specimen 2 for the dissipation test of 23/AF204 in level 1 ...... 153

6.17b Spatial pore pressure dissipation at level 2 in specimen 2 for the dissipation test of 23/ aF204 in level 1 ...... 154

6.18a Spatial pore pressure dissipation at level 1 in specimen 2 for the dissipation test of 23/ArF203 in level I ...... 155

6.18b Spatial pore pressure dissipation at level 2 in specimen 2 for the dissipation test of 23/ArF203 in level 1 ...... 156

6.19a Spatial pore pressure dissipation at level 1 in specimen 3 for the dissipation test of 3|/,F2Nl in level 1 ...... 157

6.19b Spatial pore pressure dissipation at level 2 in specimen 3 for the dissipation test of 3|/;F2N1 in level I ...... 158

6.20a Spatial pore pressure dissipation at level 1 in specimen 4 for the dissipation test of 43/AF2N0 in level 1 ...... 159

6.20b Spatial pore pressure dissipation at level 2 in specimen 4 for the dissipation test of 43/AF2N0 in level I ...... 160

6.21a Spatial pore pressure dissipation at level 1 in specimen 4 for the dissipation test of 43/ArF2N2 in level 1 ...... 161

6.21b Spatial pore pressure dissipation at level 2 in specimen 4 for the dissipation test of 43/ArF2N2 in level 1 ...... 162

xii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.22a Spatial pore pressure dissipation at level 1 in specimen 4 tor the dissipation test of 43 /aF204 in level 1 ...... 16j

6.22b Spatial pore pressure dissipation at level 2 in specimen 4 for the dissipation test of 43M.F204 in level 1 ...... 164

6.23a Spatial pore pressure dissipation at level I in specimen 5 for the dissipation test of 53 /aF2NI in level 1 ...... 165

6.23b Spatial pore pressure dissipation at level 2 in specimen 5 for the dissipation test of 53/AF2Nl in level 1 ...... 166

6.24a Spatial pore pressure dissipation at level I in specimen 5 for the dissipation test of 53 /aF204 in level 1 ...... 167

6.24b Spatial pore pressure dissipation at level 2 in specimen 5 for the dissipation test of 53 /aF204 in level 1 ...... 168

6.25a Spatial pore pressure dissipation at level 1 in specimen 1 for the dissipation test of li/jFR.10 in level 1 ...... 169

6.25b Spatial pore pressure dissipation at level 2 in specimen 1 for the dissipation test of 1 i/jFRl0 in level 1 ...... 170

6.26a Spatial pore pressure dissipation at level 1 in specimen 2 for the dissipation test of 23/aFR10 in level 1 ...... 171

6.26b Spatial pore pressure dissipation at level 2 in specimen 2 for the dissipation test of 23/aFR10 in level 1 ...... 172

6.27a Spatial pore pressure dissipation at level I in specimen 3 for the dissipation test of 3|/jFR10 in level I ...... 173

6.27b Spatial pore pressure dissipation at level 2 in specimen 3 for the dissipation test of 3i/jFRl0 in level 1 ...... 174

6.28a Spatial pore pressure dissipation at level 1 in specimen 5 for the dissipation test of 53 /aFRIO in level 1 ...... 175

6.28b Spatial pore pressure dissipation at level 2 in specimen 5 for the dissipation test of 53 /aFR10 in level 1 ...... 176

6.29a Comparison of dissipation results in specimen 1 with the Torstensson’s m odel ...... 179

xiii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.29b Comparison of dissipation results in specimen 2 with the Torstensson's model ...... 180

6.29c Comparison of dissipation results in specimen 3 with the Torstensson’s model ...... 181

6.29d Comparison of dissipation results in specimen 4 with the Torstensson’s model ...... 182

6.29e Comparison of dissipation results in specimen 5 with the Torstensson's model ...... 183

6.30a Comparison of dissipation results in specimen 1 with the The strain path method ...... 186

6.30b Comparison of dissipation results in specimen 2 with the The strain path method ...... 187

6.30c Comparison of dissipation results in specimen 3 with the The strain path method ...... 188

6.30d Comparison of dissipation results in specimen 4 with the The strain path method ...... 189

6.30e Comparison of dissipation results in specimen 5 with the The strain path method ...... 190

6.31 Factors influencing Nc (Houlsby and Teh. 1988) ...... 198

6.32 Interpretation charts for N^u (Massarsch and Broms, (1981) ...... 201

6.33 PPSV-Ko correlation for clays (Sully and Campanella, 1991) ...... 205

6.34 PPSV-K0 comparison in a fixed wall chamber test (Mayne and Kulhawy, 1992) ...... 207

6.35 PPSV-Ko comparison for the four chamber specimens ...... 208

6.36a PPR and PPR1 vs. OCR (after Sully, et al., 1988) ...... 214

6.36b PPD-OCR correlation in clays (after Sully, et al., 1988) ...... 215

xiv

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

There has been an increasing trend to use the piezocone penetration test (PCPT)

as an in situ tool of choice for determining the consolidation and flow characteristics of

cohesive soils. The focus of this research is to study various factors that would affect

the determination of consolidation characteristics in fine soils evaluated by piezocone

tests. Immediate changes in excess pore pressure and tip resistance after penetration

arrest for dissipation were experimentally identified. Undrained shear strength,

influence of stress history, and lateral stress coefficient effects on the penetration pore

pressure were investigated.

By using a two-stage slurry consolidation technique, cohesive soil specimens of

very high quality were prepared in a specially designed calibration chamber

(LSU/CALCHAS). Four standard piezocone penetration tests (reference) and twenty

one miniature piezocone penetration tests were performed at various boundary

conditions, stress level, and stress history. In order to capture the true excess pore

pressure drop after penetration arrest in dissipation tests, data acquired at very close

time intervals (0.01 seconds) using a digital oscilloscope were utilized.

The oscilloscope results indicate a sudden drop in the excess pore pressure,

especially at the tip of the cone, due to the normal stress reduction as soon as the

penetration ceases. Hence, the interpretation of the dissipation results for the

determination of the radial coefficient of consolidation (cr) should be based on the

initial dissipation values of the excess pore pressure and not the penetration pore

pressures. Determination of the initial excess pore pressure distribution for a

dissipation analysis should take into account the dissipation which has already occured

XV

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. during piezocone penetration. The method proposed by Gupta and Davidson (1986),

simulating the piezocone penetration process as successive spherical cavity expansions

and taking into account the dissipation effect gave very good agreement with the

dissipation results at the cone base. The predicted spatial pore pressure distribution

during the dissipation phase showed only a qualitative agreement with the experimental

results due to the limitations and simplifying assumptions in the method. The PCPT

results were comparatively evaluated using state-of-the-art methods to estimate the

undrained shear strength, Su; lateral stress coefficient, Ko; overconsolidation ratio,

OCR.

xvi

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1

INTRODUCTION

Estimation of consolidation and flow characteristics in fine grained soils has

received much attention in modem soil mechanics. The primary consolidation

parameters considered are the coefficient of consolidation and the hydraulic

conductivity for settlement and seepage analysis. Consolidation parameters are

usually estimated from oedometer tests or back analyses of field performance. In

spite of their popularity, the oedometer tests and back analysis have some

limitations in the determination of consolidation and permeability parameters. In

oedometer tests, small soil samples fail to account for soil variability and specimen

boundary conditions and do not appropriately mimic the in-situ conditions. Fissures

and layering of soil are difficult to determine with small intact soil samples and the

sampling disturbance is hard to avoid. Hence, oedometer tests may not truly

represent in-situ conditions. For back analysis, variation in soil stratigraphy make it

difficult to estimate the correct drainage path during consolidation.

Lately, there has been an increasing trend to use the piezocone penetration

test (PCPT) as an in situ tool of choice for determining the consolidation and flow

characteristics of cohesive soils. However, the current interpretation of

consolidation and flow characteristics using piezocone test has not been entirely

satisfactory to eliminate a number of pressing needs in design and testing practice.

The main advantages of using piezocone tests are its simplicity, repeatability, and

speed. Usually, piezocone tests are conducted during the profiling phase of the

l

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. investigation. Thus in addition to determining site stratigraphy and estimating other

soil parameters, pore pressure dissipation tests can offer direct estimates of the

coefficient of consolidation. Use of piezocone dissipation test for determining

consolidation characteristics of cohesive soils were not available until the early

1970’s, since cone resistance, skin friction and penetration generated pore pressure

had been measured separately. Fast developments in transducer technology provided

the incorporation of piezometric elements into the standard electric cone

penetrometers which made possible the simultaneous measurements of pore

pressure, cone resistance and skin friction (Tumay et al, 1981; Baligh et al, 1981;

Campanella and Robertson, 1981; de Ruiter, 1982; Zuidberg et al, 1982; Smits,1982;

Juran and Tumay, 1989).

A satisfactory interpretation of PCPT requires precisely controlled field and

laboratory calibration tests. Field calibration of the piezocone has numerous

limitations and flaws due to soil inhomogeneities and uncertainties regarding the

magnitude of in-situ stresses and stress history of the deposit. It is almost impossible

to obtain real undisturbed samples in the field for determining the '‘true” reference

soil parameters. Moreover, the influence of particular parameters (stress anisotropy,

soil fabric, stress history, stiffness, void ratio, compressibility, etc.) can not be

determined by independent variation in the field. The use of laboratory calibration

chamber to calibrate an in-situ device has definite advantages since homogeneous,

reproducible and instrumented soil specimens, subjected to a known stress history

can be prepared and tested under controlled boundary conditions.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Laboratory calibration chamber tests for cone pentrometers, pressuremeters

and dilatometers in cohesionless soil specimens have been conducted by numerous

researchers. However, there have been only few applications to compacted or

preconsolidated cohesive soils (Huang, 1986; Huang, et al., 1988; Bunting, 1990;

Mcmanus and Kulhway, 1991; Anderson, et al., 1991; de Lima and Tumay, 1991;

Tumay and de Lima, 1992; Kurup, et al., 1993). This can be attributed to the

extremely time consuming and laborious process involved in the preparation of

large cohesive soil specimens in addition to other complexities involving

instrumentation for pore pressure monitoring and the need for maintaining

saturation by back pressure.

This research presents results of four standard 10 cm2 piezocone penetration

tests, twenty one miniature piezocone penetration tests on large instrumented

cohesive soil specimens in a calibration chamber system. By using a two-stage

slurry consolidation technique, cohesive soil specimens of very high quality were

obtained. The time consuming and laborious soil sample preparation limited the

number of tests that were conducted. Mainly, the performance of the piezocone

penetrometer to predict consolidation and flow characteristics are evaluated.

Undrained shear strength, influence of stress history, lateral stress coefficient effects

on the penetration pore pressure, and initial excess pore pressure drop in dissipation

were investigated.

1.1 Research Objectives

The objectives of this research were to:

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4

(1) Study various factors that would affect the determination of consolidation

characteristics in fine grained soils evaluated by piezocone tests.

(1.1) To interpret the immediate initial excess pore pressure drop due to normal

stress reduction and its variation due to redistribution

(1.2) To evaluate the influence of stress history, lateral stress, boundary

condition and filter location on the measured piezocone penetration data.

(1.3) To try to find any new data not used in routinely interpreting

dissipation curves of piezocone tests, but which may be of potential

significance in better understanding of flow mechanisms around

the piezocone.

(1.4) To evaluate the dissipation data using current methodologies, study their

limitations and suggest better interpretation of results.

(2) Estimate soil engineering parameters using the results of piezocone penetration

tests and evaluate the scale effects between standard 10 cm2 cone and

miniature cone from penetration tests in a calibration chamber.

(3) Study the factors affecting the laboratory calibration of piezocone penetrometers

and to make recommendations and changes in the equipment and procedure.

1.2 Organization of Dissertation

In order to meet the objectives previously stated, Chapter 2 of this dissertation

presents an extensive literature review regarding the pore pressure measured by

piezocone, initial excess pore pressure variation, excess pore pressure drop due to

normal stress release, interpretation methods of dissipation curves, aspects of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. piezocone penetrometers, and calibration chamber testing. Chapter 3 and Chapter 4

describe testing equipment, the methodology of preparing large size cohesive

specimens with known stress history, and depict in detail the test procedures. Chapter

5 summarize the results of various size piezocone penetration tests. Chapter 6

analyzes the tests results using some of existing interpretation models and estimate

the effect of boundary conditions, stress history, and cone type. Especially, the

mechanism of immediate initial excess pore pressure drop after arresting piezocone

penetration is described in detail. The undrained shear strength, the influence of

lateral stress, the coefficient of overcosolidation, and the variation of spatial pore

pressure during penetration and dissipation were studied. Chapter 7 embodies the

summary/conclusion of this research and recommendation for future research

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2

LITERATURE REVIEW

2.1 Background

The pore pressure measured by piezocone can be divided into two

components:

- in situ hydrostatic pressure ( uo)

- excess pore pressure generated by piezocone intrusion ( Au )

The excess pore pressure(Au) is a combination of two different stresses:

octahedral excess pore pressure (AuoCt) and shear-induced excess pore pressure

(AUshear), SUch that

Au Alloct AUshear

According to Campanella et al (1988), when saturated soils are subjected to an

increase in octahedral stresses, positive pore pressures are generated. When

subjected to only shear stresses, pore pressures generated can be either positive or

negative depending on the contractive or dilative response of the soil to shear. The

time variation of the excess pore pressure provides information concerning the

coefficient of consolidation. To evaluate the dissipation of the excess pore pressure,

piezocone is stopped and the decay of pore pressure (Au) with time is recorded.

Typical dissipation curves for soft clay plotted on a logarithmic scale is shown in

Figure 2.1. The excess pore pressure can be defined as :

Aup =up-u 0

6

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Pore pressure (MPa) 0.60 0.40 0.80 0.20 0.00 iue21 yia ispto etrsls(Lnee.a. 1997) al.. et. Lunne ( results test dissipation Typical 2.1 Figure 0.01 0.1 Log time (min) time Log 1 10 0 1000 100 7 Up is the measured total excess pore pressure at the time of penetration arrest and uo

is the in-situ hydrostatic pressure. Interpretation of dissipation records is generally

based on a normalized excess pore pressure ratio U, defined as:

u _ A u ^ (u t ~ u o) A up (up-u 0)

Where Au is the excess pore pressure at the depth of interest changing with time in

dissipation tests, Aup is the penetration excess pore pressure at the time of

penetration arrest and u-r is the total excess pore pressure at any time t. Figure 2.1

can be replotted in normalized form as Figure 2.2. Generally, piezocone penetration

involves both vertical and horizontal drainage. For porous elements located behind

the tip, the drain flow is governed predominantly by radial (horizontal) direction.

For radial drainage, the differential equation of Terzaghi (1925) and Rendulic's

(1935) consolidation theory is expressed as

3u f 3: u 1 3 u N To ~ i ~ Ct ^ d r +r Or J

where u = excess pore pressure, r = radial distance from the centerline of the

piezocone, t = time ( t = 0 at arrest of piezocone), cr = coefficient of consolidation

in radial direction (T x r2 / 1), and T = nondimensional time factor. This differential

equation is valid for an axisymmetrical condition with no vertical drainage of the

soil during dissipation test. Piezocone penetration generates excess pore pressure

around the probe in normally consolidated cohesive soils, which reduce the effective

stresses in the surrounding soils. This indicates unloading stage.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 .0 0 Liiili ■L.mitI i i i m i l l I t I M 111 0.01 0.1 1 10 100 1000 Log time min.

Figure 2.2 Normalized dissipation tests curves. (Lunne et. ai., 1997)

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However, during dissipation of excess pore pressure the effective stresses

surrounding piezocone increases, which indicates that the stress state of soil is under

reloading state. ( the overconsolidated , OC, state ). As dissipation continues, the

increasing effective stresses exceed the reloading state and enter into the normally

consolidated (NC) loading state. The degree of dissipation shorter than the 50%

(U 0 .5 ) is generally assumed as the effective stress around piezocone under the

reloading state. Hence, the recording of the dissipation results should continue until

at least half the initial excess pore pressure has dissipated (U=0.5). However, the

above explanation is based on Terzaghi (1925) & Rendulic (1935) uncoupled

consolidation theory. If the effect of coupled consolidation is taken into account,

the total stress is not constant with the increase of generated excess pore pressure

during penetration. It is hard to estimate how much effective stress interacts with

the variation of excess pore pressure.

2.2 Interpretation Method

The accuracy with which the coefficient of consolidation characteristics are

determined will depend on the interpretation method adopted. Many investigators

have proposed various interpretation methods for analyzing the consolidation that

occurs when piezocone penetration is stopped. They can be classified as:

•. Cavity expansion models

•. Strain path method

•. Semi-empirical method

•. Finite element analysis

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2.2.1 Cavity Expansion Method

The cavity expansion theory is based on the assumption of undrained soil

response and disregarding the shear induced excess pore pressure. The general

form of expansion of cavity induced penetration may be visualized from Figure 2.3.

The equation of equilibrium in the radial direction is given by:

where (3 = 2 for spherical expansion, (3 = I for cylindrical expansion Theories for

cylindrical and spherical cavity expansion in soils have been developed by

Soderberg(1962), Ladanyi(1963), and Vesic(1972). Torstensson (1977) assumes

isotropic initial stress distribution, ideal elastic-perfectly plastic material, undrained

one-dimensional cavity expansion and uses a linear, uncoupled finite difference

scheme to analyze the pore pressure dissipation and consolidation. The radius of the

plastic zone (rp) is given by:

Cylindrical cavity rp = r0 Spehrical cavity rp = r 01

where ro = equivalent penetrometer radius, and G/su = Ir = rigidity index. The initial

excess pore pressure distribution at any radius ‘r’ in the plastic zone is given by

Cylindrical cavity Au = su In 21n — s

Spherical cavity

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Figure 2.3 Expansion of Spherical Cavity (Vesic, 1972)

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2.2.2 Strain Path Method

The strain path method (Levadoux and Baligh, 1980) predicts soil velocities

and strain rates using potential theory (for ideal incompressible fluid flow) and a

suitable distribution of sources and sinks to simulate the geometry of the cone. The

strain rates are integrated along streamlines to determine the strain path of the

elements as they move past the cone. Deviatoric stresses and shear-induced pore

pressures are computed using a total stress soil model. The excess pore pressures are

determined considering the equilibrium in the radial direction.

Houlsby and Teh (1988) adopted a large strain finite element analysis to

supplement the strain path method (SPM) for analyzing piezocone test in clays.

They pointed out that the derived stresses by the SPM involved a small error in

equilibrium due to the approximate strain field assumed. Hence, the inequilibrium

of the initial stresses are corrected by applying incremental equal and opposite

forces; with the cone held stationary. They also took into consideration the

influence of Ir (rigidity index = G/su ) on excess pore pressure dissipation and

suggested the following modified time factor ( T* ) for the evaluation of coefficient

of consolidation.

2.2.3 Semi-Empirical Method

The method suggested by Gupta and Davidson(1986) for determining the in

situ coefficient of consolidation consists of matching the field piezocone dissipation

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curve with computer-generated dissipation plots. The computer plots were obtained

by a two dimensional uncoupled axisymmetric dissipation of an assumed initial

excess pore pressure distribution. The method assumes that the advance of the cone

produces in its immediate vicinity a series of successive spherical cavity expansions

(Figure 2.4). Computations are made in an incremental manner to permit pore

pressure dissipation during the advance of the probe. Two-dimensional

axisymmetric consolidation problem was solved with assumed values of in situ

coefficient of consolidation until a good match between the field and computer

generated dissipation curves were obtained.

2.2.4 Finite Element Analysis

Several researchers have analyzed penetration of piezocone using

incremental displacement finite element methods that might be divided into two

groups, small strain models and large strain models. Small strain analyses

introduced the cone as a pre-bored hole and a plastic collapse calculation is carried

out. The collapse load is assumed to be identified as the cone resistance. This

analysis has been carried out in the past (de Brost and Vermeer, 1984;

Sandven,l990; Kurup et al,1994) to analyze PCPT. However, cone penetration is a

large deformation problem since the pushed cone into soil penetrates several times

the diameter of the cone penetrometer.

Kiousis, et al (1988) with a large strain model using an elasto-plastic

formulation developed the basic constitutive relation in a spatial reference frame

which was subsequently transformed in Lagrangian coordinates. The analysis was

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Zone of Excess Pore Pressure Distribution

// / 1

Final Location of Piezocone

Figure 2.4 Modeling piezocone penetration as successive spherical cavity expansions (Gupta and Davidson, 1986)

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based on the assumption of negligible interface friction between the soil and the

penetrometer. The excess pore water pressure distribution was obtained assuming

undrained penetration and was calculated using the relation:

where = time derivative of the pore pressure, undrained bulk modulus of the

soil-water system, and J = material time derivative of the Jacobian of

deformation. However, the above analytical methods are not able to exactly predict

the spatial excess pore pressures and in-situ states and soil parameters. This is

mainly due to many simplifying assumptions and the difficulty in simulating the

continuous, rate dependent, large strain, penetration mechanism.

2.2.5 General Interpretation Procedure

Based on the above introduced interpretation methods, the recommended

general procedure to estimate the coefficient of consolidation is as follows:

(1) Plot the dissipation curve form data which is obtained after arresting piezocone

penetration. The data is plotted at an enlarged scale, either log or square root

time, and the initial pore pressure, Ui is extrapolated.

(2) Define Uo from available data on ground water level or data ffom adjacent

piezocone tests.

(3) Normalize excess pore pressure based on following equation

u = Au = uT-u0 Aup up-u 0

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where U = the normalized excess pore pressure (at time t)

ur = the total excess pore pressure at the depth of interest changing with time in dissipation tests

uo = the static (or in situ) pore pressure

Up = the total penetration excess pore pressure at the time of penetration arrest (just before stopping)

(4) Plot normalized dissipation curves with normalized excess pore pressure

versus time (t) on log or root time (t) scale. In general, the curves

decrease monotonically from 1.0 (at t = 0) to 0 (t -> co )

(5) Compare the normalized dissipation curve to the relevant theoretical curve. If

the observed curve follows the theoretical curve very closely, the correct

ambient pore pressure has been chosen initially. This operation requires

engineering judgement and some experience.

(6) Estimate the coefficient of consolidation at a given degree of consolidation. The

horizontal coefficient of consolidation Ch is obtained from following expression:

where R = the radius of the cone shaft: t = the measured time to reach this degree of

consolidation: and T = the time factor. Houlsby and Teh(1988) suggested using a

modified dimensionless time factor, T* given in Table 2.1, defined as follows:

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Table 2.1 Modified time factors T* from consolidation analysis ( Houlsby & Teh, 1988)

Location

Cylindrical extension Five radii Ten radii Degree of Cone above cone above above consolidation (Ml) base (z/ 2) cone base cone base

20% 0.014 0.038 0.294 0.378 - 30% 0.032 0.078 0.503 0.662 40% 0.063 0.142 0.756 0.995 50% 0.118 0.245 1.11 1.458 60% 0.226 0.439 1.65 2.139 70% 0.463 0.804 2.43 3.238 80% 1.04 1.60 4.10 5.24

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where R = radius of cone, Ir = rigidity index (G/su). Generally, the above

recommended procedure should provide estimates of Ch to within ± half an order of

magnitude (Lunne, et al 1997).

Though there are difficulties in determining the ratio of horizontal to vertical

coefficient of hydraulic conductivity due to the effects of sample size, sample

disturbance, etc., Levadoux & Baligh (1986) suggested using ch of piezocone to

predict vertical coefficient of consolidation, cv from the following expression:

where kv = vertical coefficient of hydraulic conductivity, kh = horizontal coefficient

hydraulic conductivity.

2.2.6 Initial Excess Pore Pressure Variation

The initial excess pore pressure distribution due to piezocone penetration in

clays, is an important factor affecting the interpretation of the coefficient of

consolidation. The methods described above provide fairly good approximations of

the initial distribution of excess pore pressure around a piezocone for normally

consolidated clays and probably also for lightly overconsolidated clays (OCR<5)

(Robertson, 1992).

In soft, normally consolidated clay, a typical set of standard excess pore

pressure dissipation curves resemble those displayed in Figure 2.5 (Sully

and Campanella, 1994). However, in stiff heavily overconsolidated clays the excess

pore pressure above the base of the cone (sometimes, even negative) could

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increase initially due to redistribution around the tip (Figure 2.6 Type II and Type

III). Davidson (1985) explained this increase of excess pore pressure for

overconsolidated soil using Figure 2.7. During the piezocone penetration, the

excess pore pressures behind the tip could become negative, because of the high

shear stresses and the heavily overconsolidated nature of the soils. Upon stopping

penetration, these negative excess pore pressures are quickly swamped by inflow

from the higher positive pore pressure zone in front of the tip and a peak pore

pressure is reached which then dissipates with time back towards zero.

According to Sully and Campanella (1994), when the rate of pore pressure

redistribution is higher than the rate of dissipation, the measured pore pressure may

increase over and above the in situ equilibrium value (Figure 2.6 Type II, III). Also

if the rate of dissipation is faster than the rate of redistribution the pore pressure

does not overshoot but directly arrives at the equilibrium value (Figure 2.6 Type

IV). Kurup and Tumay (1995) also pointed out that such non-standard dissipation

curves, can arise from initial excess pore pressure variation due to normal stress

reduction, redistribution and stress history effects, and dissipation during advance.

Approximate procedures have been outlined by Sully and Campanella(1994)

to correct such non-standard dissipation curves so that they may be interpreted by

existing methods. They used a graphical extrapolation technique using Log-time

plot and Root-time plot methods, and assumed the peak pore pressure to be the

initial pore pressure. However, they neglected the effect of sudden normal stress

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TYPE I

NC Soil

U

u0- (a) t TYPE I 1.0

0 (b)

Log t

Figure 2.5 Typical dissipation curves for NC soil (Sully & Campanella, 1994)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. K Equilibrium pore- p re s s u re T y p e T y p e Type ■Type I Time Time (log scale) i Figure 2.6 Initial excess pore pressure (Kurup & Tumay, 1995) . X (A o (0 O o S (A s a (A s Ul 0 CL

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. / Batow 12 m Abov* * 2 m 2 m * Abov* 12 Batow / p ^.Taiantoctay fj / OCR-20 OCR-30 Atv»w«am/ I i dayi Haaviiy o»ar conaoMaiad ctay conaoMaiad o»ar Haaviiy Batowdm Taramoday-c«

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drop on the cone face and the contribution of octahedral pore pressure and shear-

induced pore pressure to the initial pore pressure. Burns and Mayne (1995) also

tried to interpret to non-standard dissipation curves by decoupling the octahedral

components of excess pore pressure from the shear components. Their method also

have limitations, such as: anisotropic influence, necessity of triaxial tests to obtain

shear strength (su), unreliability in using Cam-Clay model. Both of theses methods

are approximate and require further refinement.

2.2.7 Excess Pore Pressure Drop Due to Normal Stress Release

The excess pore pressure drop due to normal stress release when a cone

penetrometer is stopped has not been given proper attention, because this feature

is not easily identified in field tests due to soil inhomogeneities. If this initial drop of

excess pore pressure is ignored, the interpretation of dissipation results can not be

achieved correctly. It was recommended (Kurup, 1993 ; Kurup et al, 1994;

Voyiadjis et al, 1994; Kurup et al, 1995) that the interpretation of the dissipation

results to evaluate the coefficient of consolidation should be based on the initial

dissipation values of excess pore pressure Au„ and not the penetration excess pore

pressure, Aup. This initial, drop which is primarily due to the normal stress

reduction that occurs when penetration ceases, is influenced by a variety of factors

such as the rate of penetration, stress-strain behavior at very high strain rates, OCR,

Ko, plasticity index and hydraulic conductivity. Improper clamping of the

penetrometer rod, and filter expansion (flexible filters) due to normal stress

reduction at the tip, are other factors that can contribute to the sudden drop in

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excess pore pressures (Kurup & Tumay, 1995). Monitoring tip resistance during

dissipation tests can provide further information regarding this normal stress

reduction. There are essentially two effects that influence the cone resistance and

excess pore pressure during varying rates of penetration, (I) Viscous and dynamic

effects, and (2) Drainage effect. The effect of rate the of penetration on cone

resistance and generated pore pressures and drop in their values when penetration is

stopped for a dissipation analysis, need to be investigated for a proper interpretation

of the coefficient of radial consolidation. The penetration rate changes abruptly

from 2 cm/s to 0 cm/s when the cone is stopped for a dissipation test, resulting in a

steep decrease in cone resistance accompanied by a sudden drop in the excess pore

pressures at the tip. This influence of penetration rate on excess pore pressure

should be considered by a coupled (between total stresses and pore pressure)

consolidation analysis taking into account the dissipation that occurs even during

penetration.

2.3 Piezocone Penetration Test (PCPT)

The main advantages of the cone penetration test are its simplicity,

repeatability, and speed. The piezocone test is often denoted CPTU (Cone

Penetration Test Undrained) or PCPT (PiezoCone Penetration Test). In this study,

the piezocone test is referred to as PCPT.

2.3.1 Piezocone Penetrometer

A standard for piezocone has been developed by the ISSMFE (1977, 1989)

and ASTM (1979) regarding the cone geometry (size and shape), known as the

with permission of the copyright owner. Further reproduction prohibited without permission. 26

reference cone (Figure 2.8). The reference cone has an apex angle of 60° and a

cross- sectional area (base area) of 10 cm2. According to the ISSMFE (1977, 1989),

the recommended filter location (reference location) is on the cylindrical part,

within a distance of 15mm from the conical edge. However, piezocone with filter

elements located at the tip or at the cone base or at some point on the cone face and

sometimes located above the friction sleeve are frequently used as illustrated in

Figure 2.9. In general, the standard location of the filter element is not yet specified.

The preference of filter location is dependent on in-situ soil conditions and the

objective of information expected from piezocone penetration. The location behind

the cone ( U 2 ) has advantage of being subjected to less wear and tear during

penetration, ui location also provides pore pressure correction of cone resistance,

more reliable dissipation test due to less influence by connections of push rods, and

less influence from piezo element compressibility. Placement of the piezo element

on the cone (cone tip or cone face), ui, for measurement of penetration generated

pore pressures can provide better sensitivity to identify thin layers. Pore pressure

measurement behind the friction sleeve, U 3, may also be required for the correction

of sleeve friction due to pore pressure effects. Therefore, sometimes use of dual or

triple element piezocones prove to be valuable in research.

The location of the pore pressure elements has a significant influence on the

magnitude of the measured pore pressure. The soil below the tip of the conical part

is subjected to predominantly octahedral normal stress and the measured pore

pressure is very high. For soil along the conical face, both octahedral normal and

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REFERENCE CONE GEOMETRY

REFERENCE LOCATION (filter element) DIRT SEAL

FRICTION SLEEVE

WLTER- n. «11

WATERTOHT SEAL o n rs c A L I 1 «, < Sum l».,

a , ■ 3S.7nm

Figure 2.8 Reference cone (ISSMFE, 1977.1989)

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C 0) CO CM *- E 3 3 3 Jfi LU■ 01 a 3 CO a> a . .> * □ ] >

CM a.O i >

at

CL element (Chen and Mayne, 1994)

x at a . Figure 2.9 Schematic showing common location of porous filter >* I cn >

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shear stresses dominate and the magnitude of the measured pore pressure will

depend on the stress history of the soil. The filter size may also have an influence on

the magnitude of the recorded pore pressure (Figure 2.10).

In addition to above concern, the frequency response of the pore pressure

element is also considered in the design of piezocone. The requirements for fast

pore pressure frequency are low compressibility and viscosity of the saturating fluid,

small fluid cavity, high porous filter permeability, and large area to wall thickness

ratio of the filter. Adversely, a high permeability of the porous element and low

viscosity of the saturating fluid could cause a loss of saturation of the porous filter

element. Therefore, most people choose the compromising requirements between a

high porous filter permeability for a fast pore pressure frequency response and a low

permeability for maintaining saturation. Some of the materials that have been used

for the porous filter element are stainless steel, sintered bronze, ceramic,

carborundum, cemented quartz sand, stone. Teflon, and polypropylene.

General advantages of PCPT are as follows:

(1) Ability to distinguish drainage conditions during cone penetration.

(2) Correction of measured cone penetration resistance and to some extent sleeve

friction, to account for unbalanced water forces due to unequal end area in

cone designs.

(3) Estimation of equilibrium ground water condition.

(4) Improvement in soil profiling and identification

(5) Improvement in evaluation of geotechnical parameters

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Pore pressure, u (m. of water) Cone resistance. qT(bar) 00

2.5mm 2.5mm

\*T ' 5mm 5- face element

10- 10-

\_2.5mm thick element *B" 5mm thick element *C" 5mm 5mm

Figure 2.10 Effect of filter size and location on penetration pore pressure (Campanella and Robertson. 1988)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31

(6) Ability to estimate flow and consolidation characteristics.

2.3.2 Test Procedure

There are many factors that influence the reliability of test results in

piezocone test procedures. The sensitivity of PCPT requires meticulous preparation

and performance of tests during the whole procedure of piezocone test to avoid

errors. Generally, the test procedure follows closely the recommendations of

ISSMFE. The following sections cover tests procedures as well as above mentioned

factors.

2.3.2.1 Verticality

The thrust machine and pushing rods should be checked for verticality

before penetration. Deviation from verticality should not exceed 2°. Most

piezocones today have inclinometers installed within the probe shaft to detect

verticality. This sensor also is very useful to avoid damage from sudden deflections

and maintain the straightness in deep sounding. However, if the sounding does not

exceed 15 m, a piezocone without slope sensor can be used. Usually, the deflection

of less than 1° per meter length of push rods is acceptable. This deflection is

unlikely to be caught by operator unless the piezocone has its own slope sensor.

2.3.2.2 Rate of Penetration

The standard rate of penetration is defined as 20 mm/sec ± 5 mm/sec by

ISSMFE. At higher rates of penetration, the cone resistance may increase by viscous

and dynamic effects. The cone resistance usually is likely to decrease on penetration

rate less than 20 m/sec. The effects of penetration rate on cone resistance is shown

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in Figure 2.11 (Acar, 1981). In the field , penetration is conducted in one meter

strokes, because push rods are generally one meter long. This pause in the

penetration process can allow excess pore pressures to dissipate. To avoid this

problem, pushing systems have recently been developed to provide continuous

penetration without pause. Among them, the Continuous-Intrusion Miniature Cone

Penetration System (CIMCPT) improved the minimization of intermittent pushing

(Tumay, et al.,1998). This system uses a caterpillar-type continuous chucking

device for advancing the cone penetrometer. A coiled thrust rod eliminates threaded

connections and simplifies cabling (Figure 2.12). The rate of penetration may also

affect the generated pore pressures. In fine grained soils, penetration takes place

under predominantly undrained conditions. The influence of the penetration rate on

the excess pore pressure is shown in Figure 2.13.

2.3.2.3 Saturation of Piezocone

Complete saturation of the piezocone is very important during a PCPT.

Incomplete saturation can lead to inaccurate and sluggish pore pressure response. Errors

may occur in both the maximum values of the excess pore pressure and dissipation time

for an improperly saturated piezocone. The influence of improper saturation on the

dissipation profile (Campanella, et al., 1981) is shown in Figure 2.14. Current practice is

to saturate the filter elements under vacuum keeping it submerged in the saturating fluid.

The fluids used for saturation are usually either de-aired water, silicone oil or glycerin.

The cavity in the cone is de-aired by flushing with the saturating fluid using a

hypodermic needle.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

q (/q,(2cm/sec) iue21 Ifuneo eerto aeo oerssac Aa, 1981) (Acar, resistance cone on rate penetration of Influence 2.11 Figure - 02 . 1. 20 . 1. 20.0 10.0 5.0 2.0 1 .0 0.S 0.2 0-1 aihe a. 1979\ 9 7 9 1 al..Baligh et i i i i i i i i i i i i / J v ' ' v J / / ' ' / / eerto rate, Penetration ' Sal'9h et al- et Sal'9h ' eeul 1‘ Jezequel, .... Mur . i i i i i i i i i i 1 i i r V 969— mci 1974 )machi, 1974 iar, (cm/sec) ^ y / / . / 4 / / . 33 Figure 2.12 Continuous push device

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Pore pressure, u (kPa) 100 0 zoo 3 0 0

roi* I cm

6 0 120 2 2 4 0

3

4

3

MTt o n m 00 T

Figure 2.13 Influence of penetration rate on measured pore pressures (Roy, etai.. 1982)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U o> 4 T Equi1ibrium pore p ressu re uo =200kPa r PIEZOMETER ELEMENT ELEMENT PIEZOMETER WITH ENTRAPPED AIR ENTRAPPED WITH (after Campanella et 1.(1981)> a ------Time (m inutes) i i 1 1 2 3 i (Baligh, et al., Campanella, 1981; 1981) from from 16.7m to 17.7m Penetration VERY SOFT SILTY CLAY SOFT SILTY CLAY VERY qj=4kPa, k=10‘ 7cm/sec 0 T~ Figure 2.14 Influence ofimproper saturation on the dissipation profile Penetrationstopped a t 17.7m 800 700 500 «o L. a> £ 600 i. 01 a. ttl Q.

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In the field, a hole is predrilled down to the water table using a dummy cone

to ascertain performance of piezocone in a saturated media. This is

especially essential when penetrating unsaturated clays which have high soil

suction.

2.3.2.4 Dissipation Test

A dissipation test can be performed at any required depth by stopping the

penetration and measuring the decay of generated pore water pressure with time.

The piezocone may continue to move slightly as the elastic strain energy in the rods

is released. This movement alters the total stresses in the soil around the cone and

may influence the measured decay of pore water pressure with time. It is in often

recommended that the dissipation be continued to at least degree of dissipation (U)

= 50 %. If equilibrium pore water pressure is to be determined, the dissipation test

should not stop until no further dissipation is observed. Since dissipation is

generally more rapid initially, it is preferable to collect data more frequently in the

early part of the dissipation test.

2.3.2.5 Correction of Measurements from Unequal End Area Effect

Many researchers recommended the correction of measured cone resistance

for pore pressure acting in the groove behind the cone tip (Campanella, et al., 1982;

Tumay and Acar, 1985). The corrected cone resistance qr (Figure 2.15) is given by:

1 T = Clc + ^ u2

, A — A area of groove where X. = —5----- = ------Ac projected area of cone

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Cross sectional area (top) Ast

Friction sleeve surface area As

Cross sectional area (bottom) Aso 7777? Ac Cross sectional area

Figure 2.15 Unequal area effect (Lune et al, 1997)

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qT = corrected cone resistance

qc = measured cone resistance

U2 = pore pressure at the base (groove) of the cone

A similar correction may be made for the measured sleeve friction, fs. The corrected

sleeve friction, fr is given by:

Where, U3 = pore pressure acting at the top and end of the friction sleeve

Ast = top end area of the friction sleeve

ASb = lower end area of the friction sleeve

As = surface area of the friction sleeve

2.4 Calibration Chamber Testing

Laboratory-prepared soil specimens have many advantages over field

deposits for research and calibration purpose. Many of the uncertainties in the field

have been mentioned. The representative flaws of field tests are soil inhomogeneity

and uncertainties regarding the magnitude of in-situ stresses and stress history of the

deposit. Laboratory calibration tests on the other hand have been noted as a solution

to eliminate such disadvantage of field tests since homogeneous, reproducible and

instrumented soil specimens, subjected to a known stress history can be prepared

and tested under controlled boundary conditions.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40

2.4.1 History

In general, two types of calibration chambers are used: chambers with rigid

or flexible walls. The former type chamber imposes a boundary condition of zero

lateral strain on the specimen, the latter type allows lateral movement. Early

calibration chambers were mostly rigid wall systems (Tcheng, 1966; Melzer, 1968)

which could not control lateral movements. This meant that very large size

specimens with high diameter ratio (calibration chamber diameter/in-situ device

diameter) would be required to minimize influence of rigid boundary effects on the

test results. This disadvantage was overcome by introduction of an advanced

flexible wall calibration chamber by Holden (1971) at Country Roads Board,

Australia. It was designed to test specimens 0.76 m in diameter and 0.91m in height

(i.e. area ratio with respect to standard cone = 21). The chamber had a double wall

cylinder. The inner wall can be maintained vertical and the Ko condition ascertained

(zero lateral strain) by keeping the pressure in the outer cell (gap between the inner

wall and the outer wall) equal to that in the inner cell gap between the inner wall

and the specimen). This mechanism allows accurate control and measurement of

vertical and horizontal stresses and strains. Using the concept of flexible wall

chamber, a large calibration chamber at the University of Florida, U.S.A. was

designed and erected by Holden (1971). This chamber was affectionately known as

the “Skippy” calibration chamber (Figure 2.16) and had an area ratio of about 35

with respect to standard cone penetrometer. The flexible wall chamber can simulate

four types of boundary conditions (Figure 2.17).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TOAIR REGULATOR FRAME PISTON CAVITY WALL JACK i n VALVE n ' = = i ...... 1 r " _ f 1-REACTION J v a lv e CONE PENETROMETER-Q LATERAL LATERAL PRESSURE DIAL MERCURV1 MANOMETER Figure 2.16 “ Skippy” calibration chamber. calibration 2.16 “ Skippy” Figure CHAMBER r * o DIAL VERTICAL VERTICAL IO REGULATOR AIR PRE

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42

o v

i i

a ,

BCl g v = constant CTh= constant

BC2 A8v = 0 Aeh = 0

BC3 G y = constant Aeh = 0

BC4 Aev =0 a n - constant

Figure 2.17 Boundary conditions using flexible wall calibration chamber

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43

Holden (1971) proposed that the field boundary conditions would lie somewhere

between BC1 and BC3. However, for larger diameter ratios, both boundary

conditions should give identical results.

2.4.2 Chamber Size and Boundary Condition Effects

The chamber size and boundary condition effects is represented by the ratio

of the chamber diameter to the penetrometer diameter. These effects in laboratory

calibration chamber have been extensively studied by a number of

investigators (Holden, 1971; Parkin and Lunne, 1982; Parkin, 1988; Jamiolkoski, et

al., 1985; Bellotti, 1985; Schnaid and Houlsby, 1991). In general, with respect to

standard cone penetrometer, no significant influence of a classical chamber size

(120 cm) occurs for loose to medium sands, but in dense to very dense sands and

overconsolidated sands the measured cone resistance is lower than the one which

would be observed in an infinite soil mass. The cone resistance, qc, was higher for

the B3 boundary condition than the B1 condition for smaller values of diameter

ratio due to the increase of lateral stress during cone penetration. Parkin and Lunne

(1982), Belloti et al., (1985) and many others believe that the problems of the

boundary conditions used in die chamber and the related chamber size effect require

further intensive experimental and theoretical investigations. Figure 2.18 depicts the

effect of the chamber size and boundary conditions on the cone penetration for

Hokksund sand (Parkin and Lunne, 1982).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44

81 NC 83 NC S o u Z 0C.BU83 • 83 OC NC.B3 E NC.B1 2 z

0r s 90 V.

e t 1 u

100 Oiamtcr Ratio R,

Figure 2.18 Chamber size and boundary effects (Parkin and Lunne, 1982)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45

2.4.3 Development of the Cohesive Soil in Calibration Chamber

Almost all calibration chamber tests were performed on cohesionless soils

until the first calibration chamber application to cohesive soils at Purdue university

(Huang, 1986). To produce uniform homogeneous and repeatable cohesive soil

samples in a calibration chamber, to simulate naturally sedimented soils, a slurry is

consolidated under Ko conditions. This concept originated from ’stress-free’

reference state which is postulated to be that state at which the slurry transforms

from a fluid-like material (wherein particle interactions are negligible) to a material

whose behavior is influenced by particle interactions (Monte and Krizek, 1976). A

number of researchers have prepared undisturbed cohesive specimens in the

laboratory by consolidation from slurry.

The same concept was adopted for large-size laboratory cohesive soil

samples to be used in calibration chamber tests (Huang, 1986; Bunting, 1990; de

Lima, 1990; de Lima and Tumay. 1991; McManus and Kulhawy. 1991; Anderson, et

al.. 1991; Kurup. et al., 1993). Krizek and Sheeran (1970) indicated that an initial

slurry water content of 2 to 2.5 times the liquid limit is appropriate for ease of de­

airing and providing uniform and reproducible specimens. However, such a water

content results in an initial slurry height of approximately two times the desired

specimen height. Therefore, a two stage consolidation process is required. During

the first stage, the slurry is consolidated inside a slurry consolidometer. Following

this stage, the specimen is removed from the consolidometer and transferred to the

calibration chamber for the second stage consolidation (Huang, et al., 1988). This

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46

two stage consolidation also reduces the rigid boundary effect inherent to

consolidation in a rigid-wall consolidometer. The main hardship of cohesive soil

specimen preparation for calibration chamber tests comes from the difficulties in

mixing, handling, and the time consuming and laborious process involved in the

handling of large size specimens. Additionally, the instrumentation for the

measurements of soil parameters related to pore water pressure require careful

operation. Considerable volume changes occur during initial consolidation of a

cohesive slurry, and several researchers overcame these volume changes using big

rigid wall container (Anderson, et al., 1991; Kurup, et al., 1993). After slurry

consolidation, the specimen is transformed into calibration chamber which is

controlled like a triaxial cell. At this stage the soil sample is reconsolidated under

either an isotropic or anisotropic stress regime.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47

CHAPTER 3

DESCRIPTION OF THE TESTING EQUIPMENT

3.1 Cone Penetrometers

Two kinds of cone penetrometers were used in this study (Figure 3.1). They

are:

(1) Miniature piezocone

(piezocone, piezo/friction cone)

(2) Standard 10 cm2 piezocone (Reference cone)

3.1.1 Miniature Piezocone

Two kinds of miniature piezocones were utilized for this research. A

schematic view is shown in Figure 3.2. Both piezocones are equipped to measure

cone (tip) resistance. One has no friction sleeve, the other has a sleeve 43 mm long

friction sleeve behind cone base. The latter type (piezo/fficition cone) push rod has a

reduced diameter 9.53 mm compared to the friction sleeve which is a 11.28 mm in

diameter. Piezo/friction cone has its own preamplifier housed in the connector

which routes tip resistance, friction resistance and pore pressure signals to the Data

Acquisition System. The miniature piezocone (no friction sleeve) needs a separate

amplifier to connect to the Data Acquisition System. This cone is also fully

compatible with the standard instrumentation equipment of the reference cone

penetrometer. Miniature piezocone (no friction sleeve) penetrometer used in this

study was manufactured by Fugro Geosciences, Inc., Houston, TX, U.S.A. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.1 Cone Penetrometers ( mini-piezocone, mini-piezo/friction cone, and reference cone)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49

iS> Z o 2 - 2 55 3

Figure 3.2 Schematic of the miniature piezocone

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50

manufacturer of piezo/friction cone was Fugro-McClelland Engineers B.V.,

The Netherlands. Both miniature piezocones have a projected cone area of 100 mm2

and a cone apex angle of 60°. The maximum normal load capacity is 5 KN. The

filter location can be changed from ui configuration (the lowest % of the cone at

the very tip, Figure 3.3a) to U 2 configuration ( 0.5 mm above the base of the cone

and 2 mm vertical height, Figure 3.3b). The maximum pore water pressure

transducer capacity is 3.5 Mpa. The area ratio (X) for the correction of measured

cone resistance, qc, of miniature piezocone is 0.62.

3.1.2 Reference Cone

The Reference quasi-static electric cone penetrometer (RQSEC) is a standard

35.6 mm nominal diameter Fugro piezocone penetrometer (Figure 3.1). This cone is

generally accepted as a standard cone penetrometer in the United States and in

Europe. It has a 10 cm2 cone tip (35.6 mm in diameter) with an cone apex angle of

60°. It has also a friction sleeve base with a surface area of 150 cm2 located behind

cone and pore water pressure piezometric element located on cone face. The area

ratio (X) is 0.64. Therefore it can measure cone tip resistance, sleeve friction, and

pore water pressure (ui at cone face) simultaneously.

3.2 Slurry Consolidometer

The slurry consolidometer system was designed by Dr. Pradeep U. Kurup

(1993). The device is designed to produce uniform and repeatable cohesive samples

which simulate naturally sedimented soils by consolidating a slurry under Ko

conditions. The consolidometer body consists of two PVC tubes, 525 mm inside

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (a)

Figure 3.3 Change of filter location : (a) ui configuration (filter at the tip); and (b) U2 configuration (filter, 0.5 mm above the cone base).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52

diameter, 15 mm thick, and 812 mm high (Figure 3.4). It is split longitudinally

into two halves and bolted together. The reason for split design is to avoid any

mechanical extrusion, disturbance and man handling the specimen while transferring

it into the calibration chamber after consolidation. The inside surface of the lower

tube is lined with sand paper to offer friction and avoid slippage of the membrane

caused by the consolidating slurry. The two tubes, upper collar and lower split tube,

are held together using six steel rods connecting an aluminum top lid to the bottom

base frame. Four rollers support the base frame in order to move the whole

consolidometer easily. Double drainage is allowed for the slurry to consolidate. For

this drainage, a porous stone is attached to both the upper surface of the base plate

and the bottom surface of the piston rod which provide vertical consolidation

pressure to specimen.

3.2.1 Loading System

The consolidometer is designed to consolidate the specimen up to a

maximum vertical stress of 552 Kpa. The consolidation pressure is furnished by a

single acting hydraulic cylinder (push jack) with power provided by an air-hydraulic

pump. This pressure is transferred through the push jack to a steel piston rod, an

aluminum piston plate . and finally to the slurry specimen. The pump has an

automatic pressure make-up feature. This feature enables it to keep a constant

consolidation pressure until the end of consolidation. The pressure transducer is

installed on a hub which connects the servo controlled air - hydraulic pump and the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.4 Slurry consolidometer (Kurup, 1993)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54

push jack in order to monitor by the computer the consolidation stress during

consolidation. The vertical settlement of the slurry specimen is recorded by a linear

varying displacement (LVDT) connected to the piston rod.

3.2.2 Instrumentation

Miniature pore water pressure transducers are installed inside the slurry

specimen. The leads for these instruments come through the bottom of the

aluminum base plate, which has eight access holes at fixed positions. There are

also three more access holes for drainage/back pressure in the base plate. The

miniature pore pressure transducer consists of a stainless adapter, ducts consisting of

stainless hypodermic needles with a 1.2 mm inside diameter and a thickness of 0.23

mm. The length of the ducts are designed to monitor pore water pressure at two

different elevation and varying radial distances, above the base plate (Figure 3.5 and

Figure 3.6). The tips of the ducts are sealed with a porous plastic filter material to

prevent soil migration and clogging of the tubes. The other end of the tip is

connected to the pressure transducer ports. The accuracy of pore pressure

measurements depends on the response time and this in turn is mainly dependent

upon the degree of saturation. The miniature pressure transducers are saturated

through a dual stage saturation technique to keep a high quality response time. In

the first stage of saturation, the ducts attached to the adapter are saturated by

flushing with de-aired water, using a special CPV 1000 closed circuit pump. The

ducts with attached adapter are joined with the pressure transducer submerged in a

tub of de-aired water. Secondly, the tips of the ducts are immersed in de-aired water

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55

wrwuinic cnjNocw

ROCICN riUlK

HVPMUUC PC UMC

PHTOM BOO coinenwc «ow H am m immoucw

\!!M > 5SSI \afflO JliS!S -CS-a i aaa—i: I mimsmsjsBst m m --J RUMtW Mfymwc m w saw fouuiow 5AMO PtfC II

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p o k m is p m s i c kmuctmw* l*«4j Mia pwrnm iiumoucf m r u g

ESfiSiSl

A SCHEMATIC VIEW O f THE SLURRY CONSOLIDOMETER

Figure 3.5 A schematic of view slurry consolidometer (Kurup, 1993)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.6 Base plate with miniature pore pressure transducer.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57

and then subjected to vacuum in the Noid Deaerator (Figure 3.7 and Figure 3.8).

The response of the pore pressure transducer on initially pressurizing the slurry is

instaneous and is equals to the applied pressure. The data acquisition software

acquires and appends pore water pressure, consolidation stress, and settlement into a

file and displays the data on computer screen in graphical form plotted in real time.

A schematic view of the slurry consolidometer set up is shown in Figure 3.5.

3.3 The LSU Calibration Chamber System

The calibration chamber system LSU/CALCHAS was designed by Dario C.

De Lima (1990) under the supervision of Dr. M.T. Tumay. The LSU/CALCHAS

(Tumay and de Lima, 1992) consists of a calibration chamber, a panel of controls, a

data acquisition and control system, a hydraulics and chucking system, and a

penetration depth measurement system (Figure 3.9). This chamber allows testing

of different size of cone penetrometers under controlled boundary conditions. The

diameter ratio with respect to the miniature piezocone used in this investigation is

41, and the diameter ratio with respect to the reference cone is 15. The double wall

chamber is flexible, and can house specimens 525mm in diameter and 815mm high.

Its operation is servo-controlled and is capable of consolidating soil specimens at

a variety of stress paths including Ko ( zero lateral strain) consolidation. The

chamber is divided into two sections; namely the piston cell and the chamber cell

unit (Figure 3.3). The chamber cell rests on a bottom plate of 640 mm in diameter

and 38.1 mm in thickness. The piston pushes the bottom plate upwards thereby

applying a vertical stress on the specimen.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.7 Assembling pore pressure transducer, ducts, and adapter submerged in de-aired water

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.8 Vacuum saturation of duct in the Nold DeArerator

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.9 LSU Calibration Chamber System (LSU/CHAMBER) (Tumay and de Lima, 1992)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61

As shown Figure 3.10, the two cylindrical shells made of stainless steel 304 plates

are 6.35 mm thick. The internal diameter of the inner and outer shells are 560 mm

and 580 mm, respectively, and 910 mm high. The shells are designed to withstand a

maximum pressure of 1440 kN/m2. The sample top and bottom plates are made of

6061 T-6 aluminum and are of 530 mm in diameter. The sample bottom plate rests

on the piston cell unit. The sample top plate is bolted to the chamber top plate (top

lid) which is 640 mm in diameter and 31.1 mm in height. The chamber top plate

(top lid), sample cell inner and outer walls, and the piston cell ring are kept together

via twelve stainless steel rods. These rods are tightened up to 65 N m torque to

ensure that the whole assembly does not have any leaks during the testing. The

chamber top plate (top lid) and top plate have provisions for tests to be conducted at

five locations ( insertion of cone ) in the specimen (Figure 3.10). These holes are

sealed by adapters during specimen consolidation against back pressure.

The annular space between the sample encased in a rubber membrane and

inner shell, and between the inner and outer shell are filled with de-aired water via

two water lines connected to the top plate during testing. The pressurizing of these

double shell (wall) systems provide horizontal pressure to the specimen. The

chamber can simulate the four traditional boundary conditions of stress and strains

(Figure 2.17):

BCl : Constant vertical stress and constant lateral stress

BC2 : Zero vertical strain and zero lateral strain

BC3 : Constant vertical stress and zero lateral strain

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 62

SPECIMEN. PLAN

TEST LOCATIONS: 1.2.3

525m m

635mi

l CONE PENETROMETER

DRAINAGE/ ~OP LID SACK PRESSURE

TOP PLATE BOLT RODS

POROUS PLASTIC

OUTER CELL

MEMBRANE

PORE PRESSURE ACCESS DUCTS

POROUS PLASTIC

PISTON

DRAINAGE/ PRESSURE BACK PRESSURE TRANSDUCER

PISTON GUIDE

RASE ERAMF

PISTON SHAFT

Figure 3.10 Schematic of the flexible double wall calibration chamber (Kurup,l993)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63

BC4 : Zero vertical strain and constant lateral stress

3.3.1 Control Panel

The control panel instruments used for regulating the operation of the

chamber are grouped together within reach of the operator on a vertical wooden

panel of 1.22 m x 1.96 m. Copper tubing is used for all control lines to minimize

volume changes and hence the compressibility in the system. Three quick-

connectors link the three water pressure lines from the panel of controls to the piston

and sample cells. A schematic drawing of panel is shown in Figure 3.11. The main

unit of the panel of controls have four components: pressure regulators, electro­

pneumatic transducers, pressure transducers, pressure gauges. They consist of five

Sen Sym ST2000 pressure transducers, four Marsh process gauges, two Fairchild

back pressure regulator, and four Fairchild electro-pneumatic transducers. The

electro-pneumatic transducer converts an electric signal to a linear pneumatic signal.

A DC signal of 0 to 10 V is generated. Two of the transducers are used in the piston

cell operation for applying the vertical stress to the sample. The other two are used

for the pressure compensation between the sample inner and outer shells during Ko

consolidation and penetration phase. The panel is equipped with an air-water

systems that apply the vertical and horizontal pressure, and saturate specimens

under back pressure.

3.3.2 Hydraulic and Chucking System

Hydraulic and push jack system allows for penetrating the cone into the

sample in the chamber in one single stroke. The maximum stroke is 790 mm. This

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. AIR IVAUN © < a o t/l 1/1 til f t n AIR AIR FI! TER 4 IB4 IS wAltF ? WATER WATER IB CALI ON E i 1 B20 1 BALL VALVE . VALVE BALL % i Figure 3.11 A schematic ofcontrol panel BACK PRESSURE FAIRCHILO (B REGULATOR P I.2 2 - 1 5 0 PSI) F: fAIR CHILD E /P 1RANS0UCER (F1.3. S: 6 (SI.2.4, SENSYM - VRANSOUCER 1 0 PSI. 0-30 F2.4. PSI. 3 - 1 SJ.S. 2 0 0-100 PSt) PSI) C: C: MARSH PROCESS PRESSURE GAUGE (G l.2.3.4.5. 0 - 1 0 0 PSI) B: B: ON-Orr C: QUICK-CONNECTOR BP A; A; 3-WAY VALVE BALL V: BlCEDER-PLUG VALVE g *ICOM PRESSED AIR C l< I ® c <

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65

hydraulic system consists of a dual piston, and a double acting hydraulic jack mounted

on a collapsible type ( Figure 3.12). The chucking system for grabbing the cone

penetrometers during penetration and extraction of the pushed shaft at the end of the

experiment is installed on the push jack. In this study, two types of chucking systems are

used for penetration. One of them is for miniature piezocone, the other is for the

reference cone. When the hydraulic system is extended, the full height is 2140 mm. The

penetration depth is measured by the displacement transducer that works via an optical

increment shaft encoder which is friction coupled to the rod ( Figure 3.13 ).

3.3.3 Data Acquisition and Monitoring System

The hardware of the data acquisition process consists of Gateway 2000

Pentium 200 Mhz microcomputer with 32 MB RAM, 4GB hard drive, 17 inch color

monitor with 0.26 mm fine pitch CRT. a data translation DT 2801 A/D board and a

digital oscilloscope (Nicolet model 310). The flow chart for data acquisition and

monitoring system is depicted in Figure 3.14. A digital oscilloscope was used to

capture the tip resistance and excess pore pressure of piezocone immediately after

stopping penetration for the dissipation test. The general view of data acquisition

system set up for calibration chamber test is shown in Figure 3.15.

The data acquisition software used in this study consists of five computer

programs: SLURRY.PAS, ISOCON.PAS, KOCON.PAS, PCPTBC1.PAS,

PCPTBC3.PAS (Kurup. 1993). All of them are written in Turbo Pascal version 4.0

environment. SLURRY.PAS has been developed for data acquisition during the

slurry consolidation phase. Program ISOCON.PA and KOCON.PAS are for data

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.12 Hydraulic push jack system.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.13 Depth encoder device

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. COMPUTER o o OSCILLOSCOPE (NICOLET310) OSCILLOSCOPE

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69

Figure 3.15 General view of data acquisition system set up

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70

acquisition/feed-back control during the reconsolidation of the specimen after

transferring sample from slurry consolidometer to calibration chamber. ISOCON.PAS

was used in isotropic reconsolidation and KOCON.PAS was for Ko (zero lateral strain)

reconsolidation in the calibration chamber. PCPTBC1.PAS and PCPTBC3.PAS are used

in penetration phase. They are also for isotropic condition and Ko condition stress state

during sounding of cone, respectively.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4

TEST PROCEDURE

Two main phases are used in the test procedure. They are the specimen

preparation phase and the penetration phase. The stage of specimen preparation

consists of two steps: slurry consolidation in consolidometer, reconsolidation in

calibration chamber. Each procedure is involved with heavy instrumentation which

provide detailed monitoring of the specimen environment.

4.1 Specimen Preparation

By using the slurry method, many of the uncertainties of field testing can be

eliminated, including magnitude of in-situ stresses, stress history, and soil

inhomogeneity. The technique of two stage consolidation for preparation cohesive

specimens is known to produce cohesive soil specimens of very high quality (Krizek

and Sheeran, 1970; Huang, et al., 1988). The triaxial pressurizing in the calibration

chamber (i.e. reconsolidation) provides the stress condition of the specimen which is

required for the testing scheme. The reconsolidation also reduces the rigid boundary

effect resulting from first slurry consolidation stage, during which one dimensional

loading was exercised with appreciable sliding of specimen along consolidometer

walls.

4.1.1 Slurry Consolidation

The slurry mixing operation is prepared by employing the following

procedure:

71

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72

1. The required quantity of de-ionized water is transferred into the mixing drum

from the graduated water tank. The quantity of water is determined as the water

content necessary to bring the slurry to a consistency of twice the liquid limit.

2. Soil slurry is stirred by thoroughly mixing the kaolin, fine sand and water.

3. Mixing is done in two large 40 gallon polyethylene tanks using a heavy duty

chemical mixer. Mixing is continued for 20 min until the slurry is completely

smooth and free from lumps.

4. Eight pore pressure transducers are installed inside empty consolidometer before

pouring slurry. These transducers are situated at two different elevations and

varying radial distances as depicted in Figure 4.1 (see also Figure 6.15).

5. Mixed slurry is placed very care fully inside consolidometer by pouring through

a 50 mm diameter hollow tube with its lower end immersed in the slurry.

6. A vertical consolidation stress is applied to the slurry. This vertical stress is

selected so as to obtain an initial soil specimen of sufficient strength to

withstand its self weight.

In this study, two vertical consolidation stresses were applied to the slurry:

138 kPa, 193 kPa. The grain size distribution of the kaolin and fine sand is shown in

Figure 4.2. A mixture of 33 % kaolin and 67 % Edgar fine sand by weight was used

to prepare the K.-33 specimen. The Atterberg limits of the virgin kaolin and K-33

mixture are shown in Table 4.1. During consolidation, pore water pressure and

displacement of slurry were monitored continuously. As an example the results of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.1 Installing pore pressure transducers inside empty consolidometer

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

percent finer 100 90 40 60 70 50 80 20 30 10 0 iue42Pril iedsrbto uvs(uu, 1993) (Kurup, curves distribution size Particle 4.2 Figure 100 % Sand Sand % 100 Kaolin % 0 0.1 grain size size grain lxue3%koi,6% ies ) d n sa fine 67% kaolin, VlLxiure(33% (mm) 100 100 % Sand S % 0 0.01 % Kaolin* Kaolin* 0.001 75

Table 4.1 Properties of Kaolin and K-33 mixture

Liquid Plastic Plasticity Specific Soil Limit Limit Index Gravity (%) (%) (%) (Gs)

Kaolinite 54 28 26 2.66

Fine Sand 2.67

Kaolinite and Sand (K33) 20 14 6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76

pore pressure monitored for specimen No.2 during consolidation is shown in Figure

4.3. Figure 4.4 depicts the settlement of the same specimen. The pore water

pressure measurement is used to check if the real effective consolidation vertical

stress is achieved. The amount of time required for full consolidation of the

specimen is generally about 5.5 weeks.

4.1.2 Reconsolidation in a Calibration Chamber

The procedure of reconsolidation in a calibration chamber consists of two

major operations: (1) Transfer and placement of specimen from slurry

consolidometer to calibration chamber, (2) Reconsolidation of specimen. Specimen

transfer is an important procedure that reqiures care and experience. The specimen

may fail for two reasons: 1. the specimen may slide down the slurry consolidometer

during transfer if the grabbing force holding the specimen to the wall of

consolidometer is not enough to support the weight of soil sample (Figure 4.5). 2.

the specimen may fail by bulging after being placed on the piston of chamber. The

strength of specimen for self standing can be destroyed by even a delicate vibration,

when there is no confinement with consolidometer, and the specimen may liquefy

and bulge immediately (Figure 4.6).

4.1.2.1 Specimen Transfer and Placement

The following steps are exercised to prevent collapse and disturbance of soil sample

during transfer and placement of specimen to calibration chamber:

1. Remove the upper tube o f slurry consolidometer. Trim extra height of sample

from the top of lower tube. Place the top plate of chamber over the specimen

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. i 1000 100 Time (hour) Time Specimen no.2 Specimen r Figure 4.3 Pore pressure dissipation during slurry consolidation 10 80 160 120 CO o a? * Q_ a.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78

o a o

a

o Figure 4.4 Figure 4.4 Settlement during slurry consolidation

a o o o o o CM

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79

Figure 4.5 Collapse of specimen due to sliding in slurry consolidation mold.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.6 Bulging of soil sample

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81

and slip O-ring around the top plate covering the membrane.

2. Tighten the bolts while assembling consolidometer to provide enough force to

hold specimen enclosed in the membrane.

3. Using the electronic crane lift the specimen with the consolidometer and

carefully place it on the piston of calibration chamber (Figure 4.7)

4. Apply suction to the specimen through top plate using vacuum pump. This

suction increases the strength of specimen to stand by itself without

confinement when the split lower tube is removed.

5. Dismantle the split lower tube of consolidometer very carefully and check for

any damage of membrane. If puncture spots are identified on surface of

membrane, cure it with strong water-proof glue (Figure 4.8)

6. Lower the inner cell around the specimen. This delicate operation has to be done

without any tilt which could strip the O-rings on the top plate (Figure 4.9).

7. Upon placement of inner and outer cells, the top lid is connected to the piston

assembly through twelve equally spaced rods as shown in Figure 4.10.

8. Fill the inner and outer cells by de-aired water by directing the water from the

container by opening the valve on the control panel.

4.1.2.2 Reconsolidation

Five specimens were prepared by the technique described above. Three of

them were consolidated under Ko conditions, two of them under isotropic stress

conditions. Table 4.2 gives a summary of the stress history for each specimen. Only

one sample was overconsolidated (OCR=l0.9). Before applying stresses on the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.7 Specimen placement on the chamber base plate.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 83

Figure 4.8 Curing damage of membrane before installing chamber cell

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Figure 4.9 Installing chamber cells over the specimen.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.10 Final assembly of the specimen.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 86

specimen, the LVDT was installed on the piston of calibration chamber to monitor

displacement of specimen while consolidating.

The procedure of isotropic consolidation followed a routine similar to triaxial

testing of soils.

1. When the specimen is ready for consolidation, apply ten isotropic stress

increments simultaneous with ten steps of increase in back pressure. The back

pressure should not pressurize over isotropic stress.

2. During No. 1 stage check the B value of the specimen.

3. The isotropic stress is applied over and above the pressure which was used in the

slurry consolidation.

4. Keep the back pressure line closed during the increments of stresses after

verifying saturation of specimen by checking B value. Usually, only isotropic

stress is increased when the effective isotropic stress (isotropic stress minus back

pressure) is over the effective slurry consolidation pressure.

5. When the isotropic stress reaches the final reconsolidation stress level,

commence reconsolidation by opening the back pressure line to drain the

expelled water from the specimen.

For specimen 1 and 3 isotropic consolidation was applied. The stress condition of

specimen 2, 4, and 5 are anisotropic and at Ko condition (zero lateral strain).

The procedure of Ko consolidation is not as simple as the isotropic consolidation.

The anisotropic Ko consolidation followed the procedure suggested by Campanella

and Vaid(1972). Their method was termed as "one increment method" or "single

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 87

Table 4.2 Summary of the stress history of the chamber specimens

Lateral Stress Specimen Chamber Final Effective Stresses (Kpa) OCR Coefficent No. Consolidation Vertical Horizontal (Ko)

1 Isotropic I 207 207 1

2 Anisotropic 1 207 86.2 0.42 (Ko)

3 Isotropic 1 262.2 262.2 I

4 Anisotropic 1 262.2 104.8 0.40 (Ko)

5 Anisotropic 10.9 24.2 40.71 1.70 (Ko)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 88

increment method". The procedure was performed in a rigid wall chamber to

eliminate lateral strain. LSU/CALCHAS is also equipped to provide K0 stress

condition ( no lateral strain) with its double flexible wall system. This system can

behave as a rigid wall chamber by balancing the pressure between the inner cell and

outer cell, and by maintaining constant piston pressure and preventing volume

change in the cell water system. This mechanism simulates Ko condition. The

procedure of accomplishing chamber Ko consolidation is made by following steps

indicated below. The verification of specimen saturation has to be confirmed prior to

this procedure.

1. Close back pressure lines

2. Increase vertical and lateral pressure ( open the connection line between

inner cell and outer cell ) to back pressure + effective consolidation

pressure simultaneously. For instance, if back pressure is 20 psi and

vertical piston pressure in consolidometer is 20 psi , then the increased final

pressure should be 50 psi ( 20 + 20+10). The last 10 psi vertical pressure is

added to consolidate the specimen. This pressure can be selected in

accordance with the testing scheme.

3. Maintain the constant pressure in the system for 1-2 days.

4. Close the supplying inner cell pressure line and disconnect the line between

inner cell and outer cell to make automatic interaction of two cells during

consolidation. Activate Electro-Pneumatic control system.

5. Open the back pressure line and permit drainage against back pressure.

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6. Monitor the inner cell pressure response, axial deformation of specimen,

water level of back pressure chamber which indicate the volume changes of

specimen, and response of eight pore pressure transducers that describe the

state of pore pressure inside the specimen.

7. During the above operation the outer cell and inner cell pressure must be

kept equal.

8. The lateral pressure will initially decrease, however the reduction rate is slow

and will finally converge at some constant level to indicate Ko value. If

reduction in lateral pressure does not cease, some leakage in the chamber system

should be of suspect.

9. When the eight pore pressure transducers match the back pressure, the

primary consolidation is assumed to be complete.

After finishing Ko consolidation of specimen 5, this specimen was unloaded to be

overconsolidated by reducing vertical stresses. Reference soil parameters are shown

in Table 4.3 . These parameters were obtained from laboratory tests conducted on

undisturbed samples.

4.2 Piezocone Penetration Tests

In this study, twenty one miniature piezocone penetration tests, and four

reference piezocone tests were conducted in LSU/CALCHAS. Dissipation tests were

performed at the end of all piezocone penetration tests. Prior to penetration

tests, saturation (de-airing) of the filter elements was performed very carefully. The

saturation of filter elements is very important to obtain accurate and compliant pore

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Table 4.3 Reference soil parameters

Skempton Water Undrained Rigidty Radial Coefficient pore pressure Specimen Content Shear Strength Index of Consolidation parameter at No. (%) Su (Kpa) Ir = G50/S u (cr x 10'3 cm"/sec) failure, At-

1 17.36 80 0.49 100 1.9

2 19.43 85 0.37 333 4.2

3 17.22 98 0.71 167 2.2

4 17.54 121 0.25 400 4.2

5 16.80 35 -0.02 500 1.8

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pressure response. The typical technique of saturation is boiling or filter elements to

vacuum. The following multi-stage saturation method was used in this research:

1. Place the filter elements in the ultra-sonic cleaner and pour de-aired water into

the cleaner container. This device is useful to eliminate any dirt stuck inside

pores of the filter elements.

2. The filter elements were then boiled in water and further saturated by applying

vacuum in the Nold DeAerator (Juran and Tumay. 1989). The Nold DeAerator

consists of a vacuum tight cell, an electric motor, a magnetic clutch, an impeller

and a vacuum pump. The nucleation and cavitation phenomena to remove any

air entrapments from the filter elements explained is in the literature ( Kurup,

1993).

3. A plastic funnel is placed over the cone and a rubber hose used to reduce

leakage. The funnel is filled slowly with de-aired water. By inserting de­

aired water into the transducer cavity using a syringe, air bubbles that might be

present are flushed out ( Figure 4.11).

4. Assemble the saturated filter element and the cone tip submerged in the funnel

full of de -aired water.

5. Finally, the assembled piezocone is once again subjected to vacuum in the

Nold DeAerator ( Figure 4.12).

While the piezocone is positined on the penetration holes of the top plate of

chamber, it is required to keep the filter element in a thin rubber membrane filled

with de-aired water to avoid loss of saturation. After the piezocone is locked in to

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Figure 4.11 Flushing out the transducer cavity of the cone

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.12 Vacuum saturation in the Nold DeAerator

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hydraulic chucking system on the chamber, the penetration test is ready. The

penetration depth is measured using a optical depth encoder that sends an electrical

analog signal to the digital converter. All tests were conducted at the standard

penetration rate of 20 mm/sec. Table 4.4 gives the summary of the cone penetration

test schedule. Each penetration is identified with a Test ID which specifies

pertinent characteristics about : (1) specimen number, (2) boundary condition, (3)

stress condition, (4) test repetition at different location (if applicable), (5) cone

penetrometer manufacturer, (6) cone type - U configuration, (7) piezocone projected

area, and (8) location of penetration. The detailed legend for Test ID is given in

Table 4.5.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SO U i X X X X X 75mm from center (location 4) X X X 160 mm 160 from center (location 3) X 177 mm 177 from center (location 2) Location of Penetration X 160 mm 160 from center (location 1) X X X Center (location 0) XX XX XXX X XXXX XX X XX U| Config at cone Tip X XXXX XXX X XX X Filter Location Ui Config 0 mm5 above above base X X face U, on cone ------1 1 1 1 1 1 1 X X 3 3 3 3 3 3 3 3 3 3 3 3 3 Table 4.4 Summary ofthe cone penetration test locations Boundary Condition

F2NI 3 X F204 F2N0 FI03 FRIO F1N2 3 rF2N2 FI03 F204 FRIO i;204 F2NI FI02 F2NI a a , , FI04 , , m Test ID ia k i i jm I ■/, FIOI ■/, I l,/i FI02 l„, FI03 1,„ t'RIO1,„ 1 X I 5i/a 5i/a 5va 5va 5i/a 5i/a 4],* F1NI4],* 3„, 3„, FIN4 1 X 3|/( FI023|/( 3,;, FRIO 3,;, 3 4„a 5*a 5„a 4„a 4„a 4wa 3i„ F203 1 4*a X 2 2 2),A rF203 2),A 2 2, 1 5 3 4 2 No. Spcimen

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 525 nun mm from Center 75 m m SPI-CTM IN PLAN IN SPI-CTM 1 1 : 160 rnnr from Center 0 : Center 2:177 4 : 75 mm from Center 3 3 : 160 mm from Center l.ocation of Penetration I;b : Fugro b.v. (The Netherlands) Fg : l-ugro Geoscience, Inc (l).S A) * * lest Repetition at Different * Conel.ocation Penetrometer Manufacturer Mini Piezocone h F V f cm - Standard Reference Piezocone with Fricition Measurement No Fricition Measurement * * Piezocone Projected Area 1:10 N : I cm2 - Fg Mini Piezocone O : I cm2 - r r F C A L Table 4.5 Legends for Test ID >b/s Piezoelement at Cone Face Piezoelement at Cone Tip Piezoeicment 0.5 mm above Cone Base I I : Mini Piezocone with R : Reference Piezocone with 2 : Mini Piezocone with Specimen No. 1,3Specimen No. 2,4, : BC1/Isotropic 5 : BC3/Anisotropic V ' ' Cone Type - U configuration * * Specimen Number (I, 2, 3,4, * Boundary5) Condition/Stress condition V

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 5

TEST RESULTS

5.1 General Results

The penetration testing program for this study was achieved with a total 5

large size cohesive specimens on two different stress conditions (isotropic and

anisotropic, Table 4.2). In an attempt to evaluate repeatability and precision,

replicate specimens were prepared. Specimen No. I and 3 were prepared in isotropic

stress condition, and specimen No. 2, 4 and 5 in anisotropic (Ko) condition. The

anisotropic stress conditions specimen No. 5 was highly overconsolidated

(OCR=10.9). All specimens were mixtures of 33% kaolinite, 67% fine sand (by

weight). All specimens have a sand layer on the top of soil sample which does not

reflect the true properties of the layer. The sand layer is for effective upper drainage

in the stage of slurry consolidation and protection of top surface of specimen from

possible damage resulting from transfer of specimen from slurry consolidometer to

calibration chamber.

In the penetration profiles ( Figure 5.1a, 5.2a, 5.3a, 5.4a, 5.5a), the steady

values of corrected net tip resistance (qx - uo) have been obtained after reaching

some depth (approximately 10cm). Although few of the excess pore pressure

profiles (Au = ux-uo) during the penetration exhibited poor response, in general there

was a trend to approach a steady value. In the presentation of dissipation results

(Figure 5.1b, 5.2b, 5.3b, 5.4b, 5.5b), the curves are plotted with normalized excess

pore pressure vs. dissipation time. Normalized excess pore pressure is the ratio of

97

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Depth (cm) 40 20 30 80 70 60 10 50 0 12 3 2 1 0 SAND Cone Resistance Cone qr uo- qr (MPa) Figure 5.1a Penetration profiles in specimen 1 specimen in profiles Penetration 5.1a Figure (See Table 4.5 for Test No. Identification) No. Test for 4.5 Table (See . 05 1. 0.5 0.0 Excess Pore Pressure Pore Excess Au = uj - uo - (MPa) uj Au = poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Depth (cm) 20 10 40 30 60 50 70 80 0 1 2 1 0 SAND Cone Resistance Cone Figure Penetration5.2a 2 profilesin specimen q-r-uo(MPa) (See Table 4.5 forIdentification) TableTest(See 4.5 No. . 05 1.0 0.5 0.0 Excess Pore Pressure Pore Excess Au = ut - uo (MPa) -

99 Cone Resistance Excess Pore Pressure qT - uo (MPa) Au = ut - uo (MPa) 0 1 2 3 0.0 0.5 1.0 0

SAND 10

20

30

40

50

60

70

80

Figure 5.3a Penetration profiles in specimen 3 (See Table 4.5 for Test No. Identification)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cone Resistance Excess Pore Pressure qT- uo (MPa) Au = ut - uo (MPa) 1 0.0 0.5 1. 0

SAND 10

20

30 E u A oo. Q 40

50

60

70

80

Figure 5.4a Penetration profiles in specimen 4 (See Table 4.5 for Test No. Identification)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cone Resistance Excess Pore Pressure qT - uo (MPa) Au = ut - 'Jo (MPa) 0.0 1.0 2 0

SAND

10

20

30

50

60

70

80

Figure 5.5a Penetration profiles in specimen 5 (See Table 4.5 for Test No. Identification)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.8 1.0 0.6 0.4 0.2 0.0 1 10 00 0 0 X 0 1 0 0 X 1 1000 100 10 1 iue51 ispto eut nseie 1 specimen in results 5.1bDissipation Figure (See Table 4.5 for Test No. Identification) No. forTest 4.5 Table (See Time (sec) Time 103 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.8 1.0 0.6 0.4 0.2 0.0 1

Figure 5.2b Dissipation results in specimen 2 specimen in results Dissipation 5.2b Figure (See Table 4.5 for Test No. Identification) No. Test for 4.5 Table (See 10

100 Time(sec)

1000

10000 105

0.8

W

o 0.6

0.4

o— 2 02

0.0 1 10 100 1000 10000 Time (sec)

Figure 5.3b Dissipation results in specimen 3 (See Table 4.5 for Test No. Identification)

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formalized Excess Pore Pressure (Au / Aup) 0.8 0.6 1.0 0.4 0.0 1

Figure 5.4b Dissipation results in specimen 4 specimen in results Dissipation 5.4b Figure (See Table 4.5 for Test No. Identification) No. forTest 4.5 Table (See 10

100

Time (sec) Time 1000

10000 106 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / AUp) 1.2 1.0 0.8 0.6 0.4 0.2 1 Figure 5.5b Dissipation results in specimen 5 specimen in results Dissipation 5.5b Figure (See Table 4.5 for Test No. Identification) No. Test for 4.5 Table (See 10 100 Time (sec) Time 1000 10000 107 108

Au to Aup. Au is the excess pore pressure (uj-uo) at the depth of interest changing

with time in dissipation tests, and Aup is the penetration excess pore pressure (up-uo)

at the time of penetration arrest (just before stopping). From these dissipation

curves, flow characteristics of specimen can be estimated. The initial starting points

the curves exhibit different characteristics depending upon filter location and size of

piezocone. The reason can be attributed to differences in magnitude and mode in

drop of normal stress reduction that occurs when penetration ceases, and the stress

redistribution that takes place around piezocone shaft after stopping penetration.

Comparison of the dissipation curves reveal that shape of the curves are influenced

by a variety of factors such as filter location, stress history, lateral stress coefficient,

Ko, and stress condition.

Summary of the cone penetration test results and dissipation depths are

given in Table 5.1

5.2 Results

5.2.1 Specimen No. 1

Cone penetration tests were performed on a normally consolidated specimen

under boundary condition BC1. The procedure of specimen preparation followed a

two-stage consolidation method. After first consolidating the slurry to 138 kpa, the

specimen was reconsolidated in the calibration chamber to an isotropic stress of 207

kpa. The penetration profiles for this specimen are shown in Figure 5.1a. The

specimen had an approximately 100 mm thick layer of fine sand at the top.

Therefore, the peak values of tip resistance and penetration pore pressure in the top

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 576 59.3 25 25 9 580 55.4 51.7 64 0 64.7 55.5 59.8 70.6 60.1 63 9 51.2 Depth (cm) Dissipation 10.4 55.0 14.4 67 2 10.2 17.5 24.1 263 27.5 110.6 Deviation Standard 643.9 336 61 9 618.5 108 1 665 778.7 6680 490.1 475 7 793 793 6 517.4 4670 200 54 2 527.7 36.9 398.3 288.7 Average Excess Pore (Kpa)Pressure, Au 116.9 418.3 76.4 141.8 6659 267 114 381 381 5 85 85 7 108.3 239 33 33 8 114.4 36.4 23.4 17.6 243.4 57.9 91.4 26.2 266.9 54.1 41.4 33.1 33.2 308.1 7.4 29.6 Deviation Standard 1234.8 1186 1186 1 1192.9 1388 1242.1 1375.8 1136 61136 1182.1 28 1 274.1 356 Average 1434.7 53 1 1121.1 63.9 1194.0 61.8 15399 15599 1406 8 1329.9 1335.4 30.4 535.0 58.3 10395 1373.3 1067.0 1041.0 1097.0 1077.7 Tip(Kpa) Resistance, q,-Uo FIN2 FI03 F204 a a lest va • ns FIOI ns • Its FRIO l,s FI02 FI03us 1 FI04 is 1 2)i* FRIO 2)i* F2NI 2)i* 3,* 3,* F2NI 1514.9 3|* FRIO 2v* F204 2„* rF203 3,* 3,* F203 1512 5 3,*FIN4 4„* F2N0 5u* 5u* FRIO 4U*FIN I 1412.2 4^* rF2N2 4«* FI03 4„* F204 F2N15)/* 5 5« 5» Table 5.1* Summary ofthe cone penetrationTable 5.1* test results Stress History OCR-I OCR-1 2„*FI02 11807 166 OCR-1 3,*FI02 OCR-1 OCR- 10.9 (Ko) Stress (Ko) (Ko) Isotropic Condition Amsotropi Anisotropi Anisotropi 1 2 No. 3 Isotropic 4 S Spccimc

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 110

layer do not reflect the true properties of the clay. As shown in the figure, 5

piezocone penetration tests were performed. One of them was reference piezocone

(li/jFR10), the others were ui type miniature piezocones (1 i/;F101, l|/jFl02,

l t/iF103, 1 i/,F 104). The reference cone was penetrated at center of specimen, the

rest were done at the 4 different test locations (Table 4.4). The cone resistances

show similar penetration profiles along the depth. After a penetration depth of 10

cm, all cone resistances reached a steady value. li/jF101, 1 i/,F 103, and 1 i/-,Fl 04

were penetrated in two strokes, first to a depth of approximately 66 cm, and then

from 66 cm to 72 cm. I i/iFR10 and 1|/-,F102 were penetrated in single continuous

strokes to a depth of 66 cm. When the penetration was resumed following the first

strokes, an initial increase in the cone resistance was observed due to an recovery in

strength from consolidation around the cone. A comparison of the penetration pore

pressure profiles for the specimens tested indicate that reference

piezocone(l i/iFRlO) shows lower penetration pore pressures than the miniature

cones. This is probably due to the difference of filter location (cone face vs cone tip)

and size of piezocone. The dissipation test results for each test are shown in Figure

5.1b. The dissipation of miniature piezocone (U| type at cone tip) is faster than the

one of reference piezocone (U| at cone face). This trend can be attributed to the

influence of different filter location (cone face and tip). The dissipation curve

(li/iF102) was not recorded after stopping penetration since the wire of piezocone

was broken.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ill

5.2.2 Specimen No. 2

Specimen 2 was prepared by consolidation of the slurry to vertical stress of

138 kpa and a second stage chamber reconsolidation under Ko condition to an

effective vertical stress of 207 kpa. A horizontal effective stress of 86.25 kpa was

recorded at the end of the Ko consolidation. This corresponds to a Ko value of 0.42.

The procedure of reconsolidation was performed based on One-Single Increment

method (Campanella and Vaid, 1974). The purpose of Ko consolidation in the

chamber is to simulate the field consolidation condition of zero lateral strain. The

piezocones were penetrated under boundary condition BC3 (Ko condition ; zero

lateral strain). The penetration profiles are shown in Figure 5.2a. The specimen had

a 100 mm thick layer of fine sand at the top. As shown in the figure, a total 5 of

piezocone penetration tests were performed. Two of them were the reference

piezocone ( 23/aFRIO) and U| type miniature piezocone ( 23/aF102). Three of them

were uj type miniature piezocones ( 23/aF2N 1, 23/ArF203 . 23/aF204). The reference

cone was penetrated at center of specimen, the rest were done on the 4 different test

locations (Figure 3.3, Table 4.4). The cone resistances exhibit higher peak strength

than the results of specimen 1, however the steady values attained are not much

different from specimen 1. All penetrations were performed in a single stroke except

the reference cone. The observed penetration profile of the reference cone increased

in strength in the initial part of the second stroke, however the increase was not as

much as second strokes of miniature piezocones of specimen 1. The steady value of

miniature piezocone resistance was observed at depth 18cm. In the penetration pore

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. pressure, miniature piezocone reached higher values than the reference cone. Since

a steady value of penetration pore pressure was not observed for 23/AF2nl, the

penetration pore pressure and dissipation curves were not plotted on Figure 5.2b.

The stable penetration pore pressures (steady value) of specimen 2 indicates lower

range than that of specimen 1. This can be attributed to differences in filter locations

and anisotropic stress conditions applied to the specimen. All miniature piezocones

of specimen 1 were ui type, but those of specimen 2 were ut type except 23/AF102.

The dissipation results are shown in Figure 5.2b. The initial pore pressure drop of ui

type piezocone ( 23/aF102) is more than that of ui. This resulted from typical effect

of the normal stress reduction in U| type.

5.2.3 Specimen No. 3

To obtain replicable penetration results for specimen 1, specimen 3 was

normally consolidated in an isotropic condition like specimen 1. However, the

consolidation vertical stress in the slurry consolidation stage was 193.2 kpa, and the

specimen was reconsolidated in the calibration chamber to an isotropic stress of

262.2 kPa. The penetration profiles for this specimen are shown in Figure 5.3a. As

shown in the figure, a total of 5 piezocone penetration tests were performed. One of

them was reference piezocone (3 i/;FR10), two of them were ui type miniature

piezocones (3 i/jF 102, 3i/;FlN4), and the last two of them were U 2 type miniature

piezocones (3i/jF2Nl. 3[/jF203). The reference cone was penetrated at the center of

specimen, the rest were done on the 4 different test location (Figure 3.3, Table 4.4).

The cone resistances show comparatively same profiles along the depth. The

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reference cone resumed a second stroke at depth 57.6 cm, the others were penetrated

in single continuous strokes. An increase in reference cone resistance is observed

after penetration is resumed at depth 57.6 cm. In general, all cone resistances attain

a steady value after a penetration depth of 10 cm. A comparison of the penetration

pore pressure profiles for the specimens tested indicate that reference piezocone

show lower penetration pore pressure than the miniature piezocones. Since steady

values of penetration pore pressure for 3|/jF203 and 3 j/jF 1N4 were not observed, the

dissipation curves for them were not plotted on Figure 5.3b. ui type piezocone

(3|/jF2Nl) reached to steady value of pore pressure slower than that of ui type

(3 i/iF 102). which indicates a different mechanism of pore pressure generation

around the cone. The dissipation test results for each test are shown in Figure 5.3b.

The dissipation curve of reference cone (3j/jFRlO) has an initial increase after

stopping penetration. This can be attributed to incomplete pore pressure build-up

before arresting penetration. The dissipation curve of ui type piezocone (3i/jF102)

is lower than that of uj type ( 3 t/jF2N 1). This trend comes from effect of filter

location, whereby the dissipation of ui type piezocone is faster than that of U 2 type.

5.2.4 Specimen No. 4

Specimen 4 was prepared for replication of specimen 2 reconsolidated

under Ko condition. However, the vertical stress in the slurry consolidation stage

was 193.2 kpa, and the specimen was reconsolidated in the calibration chamber to

an anisotropic stress of 262.2 kpa. A horizontal effective stress of 104.8 kpa was

recorded at the end of the Ko consolidation. This corresponds to a Ko value of 0.40.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 114

The procedure of specimen consolidation was performed based on One-Single

Increment method, same as specimen 2. The piezocones were also penetrated under

boundary condition BC3 (Ko condition; zero lateral strain). The penetration

profiles are shown in Figure 5.4a. Total of 5 piezocone penetration tests were

performed. Two of them were UI type miniature piezocones (43/AFlN l, 43/AFT03),

three of them were U2 type miniature piezocones (43/AF2N0, 43/ArF2N2, 43/AF204).

The reference cone was not penetrated in this specimen. The penetration of

43/AF2N0 was performed on the center location of the specimen, the others were

done on 4 different test locations (Figure 3.3, Table 4.4). All penetrations were

performed in a single stroke. They show increasing trend at end of penetration. This

is probably due to the boundary effect at the bottom of specimen. Specimen 4 was

consolidated in a higher vertical pressure than that of specimen 2. In registering the

penetration pore pressures, 43 /aF1N1 failed to attain steady values. The dissipation

results are shown in Figure 5.4b. The normalized excess pore pressure of U| type

piezocone ( 43/AF103) is lower than those of u3 type due to different filter location

as mentioned previously for the other specimens. The initial drop of excess pore

pressure of U[ type (43/AF103) is more than those of U2 type (43/AF2N0, 43/ArF2N2,

43/aF204).

5.2.5 Specimen No. 5

This specimen was prepared for obtaining a heavily overconsolidated

anisotropic soil sample under zero lateral strain stress condition (Ko). Therefore, the

procedure of preparation of specimen took the same steps as specimens 2 and 4

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 115

until the stage of unloading to ascertain an overconsoildated specimen. The applied

vertical stress in the slurry consolidation stage was 193.2 kpa, and the specimen was

reconsolidated in the calibration chamber to an anisotropic stress of 262.2 kpa as

specimen 2 and 4. A horizontal effective stress of 104.8 kpa was recorded at the

end of the Ko consolidation. This corresponds to a Ko value of 0.42, the same value

as specimen 2. After initially obtaining a normally consolidated soil sample, the

vertical effective stress was reduced to 24.2 kpa which translates to OCR 10.9. The

corresponding horizontal effective stress was recorded as 40.7 KPa, which meant

Ko value was 1.7. The penetration profiles are shown in Figure 5.5a. The cone

resistances recorded are asymptotic to a steady value. Since the initial penetration

profile of 53/aF103 was not registered into the data-acquisition system because of a

technical difficulty, it was resumed to penetrate at depth 30 cm after waiting 6 hours

for complete dissipation of penetration pore pressures. The reference cone shows

almost same penetration pore pressures as the miniature cones. This is different

from the previously tested normally consolidated specimens. The dissipation curves

are shown in Figure 5.5b. As expected, the non typical dissipation curves (or non

standard dissipation curves) were observed in U 2 type cones (53/AF2Nl, 53/AF204),

and a clear larger initial drop of excess pore pressure of ui than that of u? type. This

is attributed to combined effects of initial drop due to normal stress reduction and

stress redistribution around cone after arresting penetration.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 116

5.3 Specimen Homogeneity

Twenty water content samples were taken from each specimen at the end of

the cone penetration tests. The average values of water contents in specimen 1,

2,3,4 and 5 were 17.36%, 19.43%, 17.22%. 17.54% and 16.80%, respectively. The

corresponding standard deviations were 0.67%, 1.18%, 0.86%, 0.43% and 0.63%,

respectively. These water content results indicate uniformity of the soil specimens.

The average values and the proximity with each other of the cone resistances for the

tests performed in each specimen also indicate the uniformity of the specimens. A

summary of the cone penetration locations (Figure 3.3) is given in Table 4.4. The

average values and the standard deviations of the cone resistance, excess pore

pressure results are summarized in Table 5.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6.

ANALYSIS OF TESTS RESULTS

The interpretation of piezocone test results is often complex as it is

influenced by many variables. A number of factors such as stress history, rigidity

index (stiffness), sensitivity, soil anisotropy, soil fabric (macrofabric), and strain

rate influence the results. The design of the penetrometer, especially the thickness,

location and pore size of the filter element along with its susceptibility to clogging

and smearing has a significant effect on the magnitude of the pore pressures

generated and their subsequent dissipation. In this chapter, the piezocone test data

obtained from the chamber tests are analyzed using some of the available

interpretation techniques. The limitations of some of the existing methods were

observed, and factors to be taken into account for a more accurate interpretation of

the data have been identified.

6.1 Initial Excess Pore Pressure Distribution

The "initial" excess pore pressure distribution due to piezocone penetration in

clays, is an important factor affecting the interpretation of the coefficient of

consolidation from the piezocone test. There are essentially two effects that

influence the cone resistance and excess pore pressure during penetration: (1)

viscous and dynamic effects (2) drainage effects (Kurup and Tumay, 1995). The

penetration rate changes abruptly from 2 cm/sec to 0 cm/sec when the cone is

abruptly stopped for a dissipation test, resulting in a steep decrease in cone

resistance accompanied by a sudden drop in the excess pore pressure around the

117

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cone path. However, this sudden drop can not be captured by using a low frequency

data acquisition system. If finer data can be captured continuously (high frequency

data acquisition) during penetration and instaneous halting of the probe to perform a

dissipation test, it will help to investigate better the mechanism of dissipation of

excess pore pressure. In order to capture this "immediate" initial drop of excess pore

pressure and the simultaneous changes in tip resistance, in this research, tests have

been conducted on homogeneous soil specimens and data acquired at very close

time intervals (0.01 seconds) using a digital oscilloscope.

6.1.1 Recording Excess Pore Pressure and Tip Resistance During Inital Phase of Dissipation Using Oscilloscope

The benefit of using oscilloscope for piezocone tests is the ability of

capturing fine data in extremely small time intervals. Oscilloscope (Nicolet model

310) can be set to capture 4000 data points on each sweep time (Figure 6.1). For

example, if it is required to measure the tip resistance or excess pore pressure at

close intervals during 40 seconds (as a sweep time) which consists of 5 seconds

before and 35 seconds after stopping penetration, the oscilloscope (Nicolet model

310) may be set for capturing 100 data points each second, as depicted in the

following equation:

DATA POINTS PER SECOND = 4000 / SWEEP TIME

It means one data point can be registered per 0.01 second for the example above.

Oscilloscope (Nicolet model 310) consists of a display screen storage control, time

per points setting part, trigger controls, and channel control. This unit captures the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119

Figure 6.1 Digital Oscilloscope (Nicolet 310)

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signal information from the cone penetrometer, converts it to digital form, and

stores in magnetic disk. The data is then transferred to the main frame memory for

display purposes when it is needed. All tests were triggered 5 seconds before

ceasing penetration.

6.1.2 Test Results

In this research five large-size cohesive specimens were prepared for

testing in the calibration chamber system. Three specimens were prepared by Ko

consolidation stress condition, and two others isotropicaliy consolidated.

Specimen 2. 3. and 4 were normally consolidated and specimen 5

overconsolidated (OCR 10.9). Eleven miniature piezocone and one 10 cm2

piezocone penetration were performed (Three ui type, eight u: type, and one

reference cone). Table 6.1 gives a summary of oscilloscope tests. Since the

utilization of oscilloscope was adopted after specimen 2. no test results for specimen

1 are shown in Table 6.1. All tests were conducted at the standard penetration rate

of 2 cm/s. The figures (6.2 - 6.13) show the change of normalized excess pore

pressure (Au / Aup) and normalized tip resistance (Aq-n / Aqxp) after penetration

arresting. The normalization of tip resistance is based on following equation: AqTt /

Aqtp = (qn - uo) / ( 9 t p - u o ). qxp is the final corrected tip resistance at the time of

penetration arrest, q-n is the corrected tip resistance at any time t. A sudden drop in

the tip resistance was invariably observed as soon as the penetration ceased for all

tests. However, the immediate change in excess pore pressures were dependent

upon location of filter element and stress condition.

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Table 6.1 Summary of Oscilloscope Tests

Immediate initial Vertical Test Drop (%) Stress Effective Ko Specimen Test Depth Excess Pore Condition Consolidation value No. (cm) Tip pressure Stress (KPa) UI U2 23/aF204 64.0 14 5 -> Ko (NC) 207 0.42 2j/a rF203 58.0 18 2 3,/|F 102 55.4 Isotropic 18 10 J*3 262.2 1.00 3mF203 (NC) 20 | ..... 0 43/aF2N0 51.7 13 0 43/ArF2N2 55.5 8 1 .... 0 *♦1 Ko (NC) 262.2 0.40 43/aF103 59.8 26 14 20 | ..... 0 r .. — — 43/a F204 60.1 53/a FRIO 63.9 14 •I 0 51.2 19 0 JC 53/a F2N1 Ko (OC) 24.2 1.70 53MF103 70.6 (OCR=10.9) 11 11 53/aF204 55.0 15 0

The ut type filter location barely produced an immediate drop in excess pore

pressure. The reason probably is the dominance of shear induced excess pore

pressure around location of U 2 type filter. The immediate drop for U| type filter

location could be clearly identified. This immediate initial drop is primarily due to

the normal stress reduction that occurs when penetration ceases. It also indicates

that the penetration pore pressures around ui type cone is dominated mainly by

normal stresses induced by tip resistance.

6.2 Comparison Experimental and Predicted of Initial Excess Pore Pressure

Distribution.

As indicated in chapter 2. the excess pore pressure generated by the

piezocone can be represented as a combined function of octahedral stress change

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure Au / A Up = (ut - uo) / (Up * Uo) 0.2 0 0 0.5 0.4 0.3 0.6 0.7 0.9 0.8 40 3530 Tip Tip resistance “ Excess Pore pressure 25 20 Time (sec) 204 (seeTable 6.1) F a / 15 of 23 Dissipation stage 5 c a> a> a. Figure 6.2 Immediate initial drop of excesspore pressure and tip resistance 0 10 1.0 12 1.1 08 0.5 0.7 0.3 0.2 0.1 0.6 0.0 CL H O' cr < QJ 15 O (/) /-V (/) u a f .8 13 13 cr * £

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure Au / A Up = (uT - uo) / (up - Uo) 0.5 0.4 0.7 0.6 0.2 09 0.8 0.3 0.0 Tip ResistanceTip Excess Pore pressure 25 30 35 4015 20 Time (sec) of 23/ArF203 (seeof Table 6.1) 23/ArF203 Dissipation stage 10 5 Penetration stage Figure 6.3 Immediate initial dropof excess pore pressure and tip resistance 0 1.1 1.0 0 9 0.4 0.3 0.2 0.1 0.0 0.7

0.8 $ ^ 0.6 o r S ^ 7 05 705 ^ .8 ^ H O '

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure Au / A Up = (uT - uo) / (up - uo) 0.2 0.1 1.3 1.3 1.2 1.1 1.0 0.4 0 3 0.0 0.7 0.7 0 6 0.5 0.8 0.9 Excess pore pressure Tip resistanceTip 3 ./ .F 1 0 2 Time (sec) of 3,/jFl02 (seeTable 6.1) £ Dissipation Stage Figure 6.4 Immediate initial drop of excess pore pressure and tipresistance _ 0 5 10 15 20 25 30 35 40 0.3 0.2 £ 0.4 or <

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure Au / A Up = (ut - uo) / (up - uo) 0.0 0.3 0.2 0.4 1.0 0.5 0.7 0.8 0.6 0.9 40 3530 — resistance Tip Excess pore pressure 25 20 Tim e (sec) 15 of 3|/jF203 (see Table 6.1) 10 Dissipation Stage 5 Figure 6.5 Immediate initial drop of excess pore pressure and tip resistance 0 0.3 0.7 0.2 0.0 0.9 cr * 3 a> 1 o. p 0.6 a 9 £ o,8 H S ' '55 2 H 1 'L 0 5

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure

Au / A Up = ( ut - u o ) / (Up - u o ) 0.2 0.0 0.3 0.6 0.5 0.4 0.9 0.7 0.8 Tip resistanceTip — — Excess Pore pressure Tim e (sec) of 43/AF2N0 (seeofTable 6.1) 43/AF2N0 jjj Dissipation Stage «) «) a • a b a Figure 6.6 Immediate initial drop of excess pore pressure and tip resistance 0 5 10 15 20 25 30 35 40 0.9 0.3 0.2 0.0 l C Q. H H • • 0.8 cr 3 <

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure Au / A Up = (uj - uo) / (up - uo) 0.0 0.2 0.3 0.5 04 0.6 0.8 0.7 0.9 40 35 30 Excess Pore pressure Tip resistanceTip 25 • • . 20 Time Time (sec) 155 of (seeTable 6.1) 4a/ArF2N2 10 Dissipation Stage c a 2 a> w c Figure 6.7 Immediate initial drop of excess pore pressure and tip resistance 0 — 0.2 0.0 0.4 0.6 0.5 0.8 0 3 . 0.7 l C o. • • 0.9 I H H cr § 3 O’ |

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure Au / A Up = (uT - uo) / (up - uo) 0.2 0.0 0.4 0.3 0.7 0.6 0.5 0.8 0.9 40 30 35 resistance Tip Excess Pore pressure I Tim e (sec) 103 (see Table 6.1) F a / of 43 10 20 25515 Dissipation Stage

®3 t a Figure 6.8 Immediate initial dropof excess pore pressure and tip resistance 0.7 0.6 0.2 0.0 or ? 0.8 O’ X 0.9

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure

Au / A Up = ( ut - uo) / (up - uo) 0.2 0 .4 0.0 0 .5 0 .7 0.8 0.6 0 .9 — Tip Tip resistance - Excess pore pressure— 0.3 3 0 3 5 2 5 20 Time (sec) 204 (seeTable 6.1) 13 F a / 1 5 4 0 of 43 10 5 Figure 6.9 Immediate initial dropof excess pore pressure and tip resistance Dissipation stage r- 0 0 .9 0 .7 0.8 0.6 - - 0.2 0.0

0 .3 0 .4 I a- S <

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure

Au / A Up = (ut - Uo) / (u p - uo) 0.2 0.0 0.3 0.4 0.5 0.6 0.8 0.7 0.9 40 35 Excess pore pressure Tip resistance 30 25 20 T imT e (sec) 15 of (seeTable 6.1) 53/AFR10 Dissipation stage 10 Figure 6.10 Immediate initial drop of excess pore pressure and tip resistance Penetration stage 0 5 1.0 1.1 0.4 0.9 08 0.7 0.6 0.5 0.3 0.2 0.1 0.0

\ S cr < w 6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131

Normalized Excess Pore Pressure Au / A Up = (uT - uo) / (up - Uo) o co in CM o o o o

9> 0) m co

to

UJ CO 2N1 (see 2N1 (see Table 6.1) F a / m of of 53

in Figure Figure 6.11 Immediate initial drop of excess pore pressure and tip resistance

o CM o 05 to inco CO CM o o o o (On - dib) / (On - ub) = d-tb v / 1JLbV 30UB1STS3X d(X pSZipjUIJOFsJ

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure Au / A Up = (uT - uo) / (up - uo) 0.0 0.2 0.5 0.4 0.3 0.7 0.6 0.9 0.8 40 35 Excess Pore pressure Tip resistanceTip 30 25 20 Time (sec) 15 of 53/aF103 (see Table 6.1) 10 Dissipation Stage 5 Figure 6.12 Immediate initial drop of excess pore pressure and tip resistance 0

0.5 0.2 0.0 0.9 10 • • 0.7 £ O H H § 25 cr cr O" X 0.8 O O 0.3 ^ 0.4 t/i u li N t •o •o '7' 0.6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Normalized Excess Pore Pressure Au / A Up = (uT - uo) / (up - uo) 0.2 0.1 0.0 0.5 0.4 0.9 0.8 0.7 0.6 40 Excess Pore pressure ® ^ Tip Tip resistance 30 35 25 20 Time (sec) 204 (see Table 6.1) F a / 15 of 53 10 Dissipation Stage 5 Figure 6.13 Immediate initial drop of excess pore pressure and tip resistance 0

0.9 0.8 0.7 0-5 0.2 0.0

l C-. C i H H s CT £ 0.3 ^ < 7 ft L 0 6 H o' *8

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 134

and shear induced pore pressure. The model proposed by Torstensson did not

include the effects of shear induced pore pressure in piezocone penetration. Vesic

(1972) developed the following expression for consideration of the induced

shear pore pressure of the spherical cavity expansion in the analysis of penetration

excess pore pressure by introducing the Henkel pore pressure parameter ctf.

Au =s„ 0.943a t- + 4 In (6.1)

where su, rp, r are as defined earlier. a t- is related to the Skemptons pore pressure

parameter at failure. At-, by a t- = 0.707 (3 Ac - 1).

It was found that the pore pressures predicted by the above equations were

still different from the excess pore pressures measured by the piezocone. A

correction procedure was hence adopted (proposed by Gupta and Davidson, 1986)

for by adjusting equation 6.1 to relate to the measured excess pore pressures. The

following expression was used to obtain the corrected initial excess pore pressure

distribution:

Au. 0.943a, +4 In Au = (6.2) 0.943a , +4 In v V ro J

where Aurc is the corrected spatial excess pore pressure distribution, and Aub is the

actual measured excess pore pressure at the base of the piezocone (i.e. U 2

configuration). The method proposed by Gupta and Davidson (1986) for

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determining the initial excess pore pressure distribution by successive spherical

cavity expansions was used to simulate the piezocone penetration mechanism. The

continuous pore pressure dissipation along the penetration path that takes place

during the piezocone penetration test was also taken into account. This initial pore

pressure distribution was allowed to dissipate during the consolidation phase.

6.2.1 Dissipation Phase

The corrected pore pressure distribution was used in a dissipation analysis

based on the Terzaghi-Rendulic uncoupled consolidation theory. This theory

involves the assumption that the total stress remains constant during the

consolidation process. For an axisymmetric linear uncoupled consolidation

problem, the governing differential equation is

0 2Au c. 3Au <32Au dAu cr — + — + C 7- = --- (6. r d r - r d r r d Z 2 8 t

where r is the radial distance from the axis of the cone. The Crank-Nicolson

(Gupta. 1983) scheme to the governing differential equation is given as

(6.4)

At

where 5 is the central difference operator for the variables. The alternating

direction- implicit scheme using the Douglas-Rachford (Douglas, 1962; Peaceman

and Rachford, 1955) difference method first evaluates the r-term at n + 1/2 and

obtains a first approximation for Au*n+I at time n+1 from

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(6.5)

and then move forward in time in the z-term

(6.6)

The solution of these equations (locally second-order correct in space and time and

unconditionally stable) is obtained by the Thomas algorithm (Ames, 1977) and is

incorporated in program PIEZ (Gupta, 1983). A series of trial computer runs

indicated that taking the drainage boundaries (zero excess pore pressure) at a radial

distance of 20 r0 and the upper drainage boundary at 25 r0 above the cone base and

the lower drainage boundary at 30 r0 below the cone base simulated infinite

boundaries for the maximum dissipation time considered (10000 sec.). However,

the analysis was performed for the actual dimensions of the calibration chamber

with the final location of the cone tip at the dissipation levels (depth) corresponding

to the experiment. The finite difference mesh used for the analysis is shown in

Figure 6.14.

As mentioned earlier, for an accurate consolidation analysis, a knowledge of

the exact spatial pore pressure distribution is essential. Pore pressures at different

locations on the piezocone penetrometer can be monitored with sufficient accuracy.

However, the in situ determination of the spatial pore pressure distribution is very

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137

1 = 21 i = 3 4

III! 1 l t L I I I I I I I

I 1 1

! i T i l r . i I 2 r .

I 1

II ! 1 1 ! 1 1 I 1 1 -V-

i i 1 1 ■ ■

j 1 I i = 1 0 7 1- i

2 r . 4 -V-

i = 1 2 5

Fgure6.14 Finite difference mesh.

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difficult and can be affected by the interaction between the soil and the measuring

instrument. The uncertainties in the alignment of the measuring device can give

inaccurate radial coordinates of the points at which the pore pressures are

monitored.

In the calibration chamber tests conducted, the spatial pore pressure

distribution was measured by pore pressure access ducts extending through the

aluminum base plate into the specimen. They were installed at rwo different

elevations and at various radial distances from the axis of penetration (Figure 6.15,

Table 6.2). Since the method proposed by Gupta and Davidson (1986) has a

limitation in simulating the tip geometry of piezocone (i.e. ui type piezocone

penetration), predictions of dissipation curves from specimen 1 which had only ui

type piezocone penetrations were not shown in Table 6.2. Comparisons of the

predicted dissipation with the dissipation curves monitored at the cone base for the

reference cone and the miniature piezocones for the four specimens (No. 2, 3, 4. and

5) are shown in Figures 6.16a through 6.161. Comparisons with the spatial pore

pressure dissipation curves for the four specimens are shown in Figures 6.17

through 6.28. The spatial pore pressure dissipation curves (at different radial

distances from the axis of penetration) obtained in the present study showed an

initial increase in the pore pressure values followed by a decrease (dissipation) at

greater time. The pore pressure ducts located closer to the piezocone reached a peak

earlier compared to those monitored away from the piezocone. In fact, some of the

pore pressures monitored by ducts located far away from the piezocone were

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I

radius - 5.64 mm

r.= 5.64m m r=RADIAL DISTANCE

PORE PRESSURE DUCTS IN LEVEL 1 : Ui. U2 U3 U<

PORE PRESSURE DUCTS IN LEVEL 2' Us, U*. Uj. UB

Level 1 = h, = o = u1( u2, u3l u4= 602 mm Level 2 = h2 = o = u5( u6l u7( u8= 402 mm

30 view PLAN view

Figure 6.15 Location of pore pressure access ducts in the chamber specimen

I Table 6.2 Location of pore pressure access ducts in the calibration chamber specimen.

Level of Specimen 2 (r/ro) Specimen 3 (r/ro) Specimen 4 (r/ro) Specimen 5 (r/ro) Ducts 23/ aF204 23MrF203 3|/jF2Nl 43/ aF2N0 43/ArF2N2 43,AF204 53/AF2Nl 53/aF204 UI 16.7 32.8 23.9 4.4 31.7 16.7 23.9 16.7 Level U2 8.9 25.4 31.7 4.4 34.6 8.86 31.7 8.86 1 U3 16.0 35.3 23.0 8.9 38.1 16.0 23.0 16.0 U4 9.4 19.5 37.2 8.9 32.6 9.4 37.2 9.40 U5 22.2 33.9 24.8 8.9 26.6 22.2 24.8 22.2 Level U6 22.2 16.3 44.3 17.7 21.1 22.2 44.3 22.2 2 U7 29.8 15.0 52.8 26.6 20.2 29.8 52.8 29.8 U8 29.8 51.6 15.2 26.6 54.4 29.8 15.2 29.8 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.8 1:0 0.6 0.4 0.2 0.0 Fgure 6.16a Measured and predicted dissipation profiles at the cone base cone the at profiles dissipation and predicted Measured 6.16a Fgure 1 10 Time (sec) Time for 2 for 100 :,/ a F204 Predicted Experimental 1000 poue ih emiso o h cprgt we. ute erdcin rhbtdwtot emission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.8 0.6 0.4 0.2 0.0 Figure 6.16b Measured and predicted dissipation profiles at the cone base cone the at profiles dissipation predicted and Measured 6.16b Figure 1 10 Time (sec) for23,ArF203. 100 Experimental Predicted 1000 poue wt pr sin h cprgt we. ute erdcin rhbtdwtot emission. perm without prohibited reproduction Further owner. copyright the f o ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.8 1.0 0.4 0.6 0.2 0.0 iue61c Measuredand Figure predictedatthe dissipationprofiles 6.16c basecone 1 10 Time (sec) Time for3[/jF2Nl. 100 Predicted Experimental 1000 Experimental & Predicted ^ 0.8 <3 u 3 V i V i a a. 0.6

o Z 0 2

0.0 1 10 100 1000 10000 Time (sec)

Figure 6.16d Measured and predicted dissipation profiles at the cone base for 43/AF2N0.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue ih emiso o h cprgt r ute erdcin rhbtdwtotpr ission perm without prohibited reproduction Further er n w o copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.4 0.0 0.2 iue61e Measured andpredicted dissipationFigure 6.16e at base thecone profiles 1 10 Time (sec) Time for43/ArF2N2 100 Predicted Experimental 1000 145 146

1.00

Experimental i 0.80 Predicted 3 < LaV 3C/i C/i Uhm a . 0.60

0.00 1 10 100 1000 Time (sec)

Figure o. I6f Measured and predicted dissipation profiles at the cone base for 43/aF204

Reproduced with permission of me copyright owner Further reproduction prohibited without permission. poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.2 0.8 0.0 1.2 0.4 0.6 1.0 iue61g Measuredand predicted dissipationFigure6.16g at thecone baseprofiles 1 10 Time (sec) 100 for53/AF2Nl 1000 Predicted Experimental 10000 147 poue ih emiso o h cprgtonr Frhrrpouto poiie ihu emission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.7 0.9 0.3 0.5 0.6 0.8 1.2 0.0 0.1 0.2 0.4 1.0 iue61h MeasuredFigure and predicted 6.16h base dissipationat cone the profiles 1 10 100 Time (sec) Time for 53/AF204 for 1000 Predicted Experimental 10000 149

Experimental Predicted

a . 0.8 3 <

3 <

3 GA CA 0.6 eu 0 a.' CA !A 8 0.4 x u ■o 1 ea 1,/iFRlO o “ 0.2 Z

0.0 1 10 100 1000 10000 Time (sec)

Figure 6 .16i Measured and predicted dissipation profiles at the cone surface for ii/i FR10

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.0 0.2 0.4 0.6 0.8 iue6lj Measured and predictedat surfacedissipationprofiles cone the Figure6.l6j 1 10 100 Time (sec) Time for 2 s ,' a FR10 1000 Predicted Experimental 10000 150 1.2

Experimental Predicted 3 1.0 <1 3 <

0.0 1 ■ 1 Mill H i 10 100 1000 10000 Time (sec)

Figure 6.16k Measured and predicted dissipation profiles at the cone surface for 3i„FR10

Reproduced with permission o„he copyrign, „wner. Further reproductjo„ ^ poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / Aup) 0.80 1.00 0.60 0.40 0.20 0.00 Figure 6.161 Measured and predicted dissipation profiles at the cone surface cone the at profiles dissipation predicted and Measured 6.161Figure 1 10 100 for 53for Time (sec) Time / a FRIO 1000 Predicted Experimental 10000 poue ih emiso ofte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the f o ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ov') 0.50 0.40 0.30 0.20 0.10 0.00 Figure 6.17a Spatial pore pressure dissipation at level 1 in specimen 2 specimen 1 in level at dissipation pressure pore Spatial 6.17a Figure for the dissipation test of 23 test of dissipation the for 1 10 1 r/ro16.7u1= u4 r/rou4 =9.4 r/ro16.0u3 = r/rou2 =8.9 Experimental Predicted m Time (sec) Time F 204 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 1 (See level in 204 100 1000 1000 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ct 0.10 0.09 0.07 0.08 0.05 0.06 0.00 0.01 0.02 0.03 0.04 Figure 6.17b Spatial pore pressure dissipation at level 2 in specimen 2 specimen in 2 level at dissipation pressure pore Spatial 6.17b Figure for the dissipation test of 23of test dissipation the for 1 10 Experimental Predicted r/ro =22.2 r/ro=22.9 r/ro =29.8 r/ro =22.2 / a F Time (sec) Time 204 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 1 (See level in 204 100 1000 ULIlf lU 1000 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v' 0.05 0.04 0.03 0.02 0.01 0.00 Figure 6.18a Spatial pore pressure dissipation at level 1 in specimen 2 specimen 1 in level at dissipation pressure pore Spatial 6.18a Figure for the dissipation test of 23/ArF203 in level 1 (See Fig 6.15, Table 6.2). Table 6.15, 23/ArF203Fig 1 of (See test level in dissipation the for 1

10

u3=35.3 r/ro u1=32.8r/ro u2 =25.4 r/ro u4 r/ro = 19.5 u4= r/ro Predicted Experimental Time (sec) Time 100

i m ± Mridili 1000

10000 155 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Fore Pressure (Au / o v' 0.05 0.04 0.03 0.02 0.01 0.00 Figure 6 .18b Spatial .18b poreFigure 6 pressure in 2 dissipationspecimen 2 at level for the dissipation test of test23/ArF203 of Tablefor 6.2).dissipation 6.15. the Fig 1 (See in level 1 0100 10 Predicted Experimental u5 r/rou5 =33.9 u6 r/rou6 16.3= u7 r/rou7 15.0= u8 r/rou8 =51.6 Time (sec) Time

“ “ i 1000 10000

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Normalized Excess Pore Pressure (Au / ctv') 0.04 0.05 0.01 0.02 0.03 0.00 £ 0.00 for the dissipation test of 3i/-,F2Nl in ievel I (See Fig 6.15. Table 6.2). Table 6.15. Fig (See I ievel 3i/-,F2Nl in of test 3 dissipation specimen in Ifor the ievel at dissipation pressure pore Spatial 6.19a Figure 1 u4 rA’ou4 =37.2 rA’ou1 =23.9 u3 rArou3 =23.0 rA’ou2 =31.7 Predicted Experimental Time (sec) Time 100 1000 1000 poue ih emiso o h cprgt we. ute erdcin rhbtdwtot emission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ov') 0.05 0.04 0.03 0.02 0.01 0.00 Figure 6.19b Spatial pore pressure dissipation at ievel 2 in specimen 3 specimen in 2 ievel at dissipation pressure pore Spatial 6.19b Figure for the dissipation testof3i/,F2Nl in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 1 (See level in testof3i/,F2Nl dissipation for the & 1 u6r/ro =44.3 =44.3 u6r/ro u5 r/ro=24.8 u7r/ro =52.8 u8 r/ro15.2= Experimental Predicted Time (sec) Time 1000 10000 158 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu emission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v') 0.50 0.40 0.30 0.20 0.10 0.00 for the dissipation test of 43/AF2N0 in level 1 (See Fig 6.15, Table 6.2). Table 6.15, 43/AF2N0Fig 1 (See of level in test dissipation the for Figure 6.20a Spatial pore pressure dissipation at level 1 in specimen 4 specimen 1 in level at dissipation pressure pore Spatial 6.20a Figure 1 10 u2 r/rou2 = 4.4 r/rou1=4.4 u4 r/rou4 =8.9 r/rou3 =8.9 Experimental Predicted Time (sec) Time 100 1000 1000 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v‘) 0.50 0.40 0.30 0.20 0.10 0.00 Figure 6.20b Spatial pore Figure6.20b pressurein specimen2 4 dissipation atlevel for the dissipation test of 43/AF2N0 in level 1 (See Fig 6.15, Table43/AF2N0 6.15, Fig 6.2). test 1 (See of level forin dissipation the 1 10 u8r/rO = 26.6 r/rO u7: = 26.6 -r/ro u6 = 17.7 - - u5 r/ro= 8.9 - Experimental Predicted Time (sec) Time I - 1000100 10000

160 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v') 0.050 0.040 0.030 0.020 000 0 .0 0 0.010 Figure 6.21a Spatial pore pressure dissipation at level 1 in specimen 4 specimen in 1 level at dissipation pressure pore Spatial 6.21a Figure for the dissipation test of 43/ArF2N2 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 43/ArF2N2 1 (See level in testof dissipation the for 1 10 u4 r/ro = 32.6 = r/ro u4 38.1 = r/ro u3 34.6 = r/ro u2 u1 r/ro = 31.7 = r/ro u1 Experimental Predicted Tim e (sec) e Tim 100

1000 10000 161 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a 0.05 0.04 0.02 0.03 0.00 0.01 Figure 6.21b Spatial pore pressure dissipation at level 2 in specimen 4 specimen in 2 level at dissipation pressure pore Spatial 6.21b Figure for the dissipation test of 43/ArF2N2 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 43/ArF2N2 1 (See level of in test dissipation for the £ 1

10 u5 r/rou5 =26.6 u6 r/rou6 =21.1 u8 r/rou8 =54.4 r/rou7 = 20.2 Predicted Experimental Time (sec) Time 100 1000 / n / 10000 162 poue wt pr sin h cprgt we. ute erdcin rhbtdwtot emission. perm without prohibited reproduction Further owner. copyright the f o ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ctv') 0.50 0.40 0.30 0.20 0.10 0.00 £ £ 0.00 for the dissipation test of 43/AF204 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 1 (See level 43/AF204in of test dissipation 4 the specimen for 1 in level at dissipation pressure pore Spatial 6.22a Figure 1 10 1 r/ro16.7 =u1 u3 r/ro16.0 u3 = u4 r/rou4 =9.4 r/rou2 =8.9 Experimental Predicted lul u il Time (sec) Time 100 L 5 5 — 1000 - \ 10000 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v') 0.020 0.015 0.010 0.005 0.000 Figure 6.22b Spatial pore pressure dissipation at ievel 2 in specimen 4 specimen in 2 ievel at dissipation pressure pore Spatial 6.22b Figure o h ispto ts f 43 of test dissipation the for 1 10 u6 r/rOu6 22.2 = r/rou5 22.2 = u7 r/rou7 29.8 = u8 r/rou8 29.8 = Experimental Predicted / a F Time (sec) Time 204 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 1 (See level in 204 100 1000 *— 10000 t i 164 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v') 0.50 0.40 0.30 0.20 0.10 0.00 Figure 6.23a Spatial pore pressure dissipation at level 1 in specimen 5 1specimen in level at dissipation porepressure Spatial 6.23a Figure for the dissipation test of 53/AF2Nl in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 1 (See 53/AF2Nllevel in test of dissipation the for 1 10 Predicted Experimental Time (sec) Time r/ro = 23.0 r/ro= 31.7 r/ro = 23.9 r/ro= r/ro = 37.2 r/ro = U 1 100 4 - gfd Pmfc f m P ftd g -I -4

1000 10000 165 poue wt pr sin h cprgt we. ute erdcin rhbtdwtot emission. perm without prohibited reproduction Further owner. copyright the f o ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ctv') 0.50 0.30 0.40 0.20 0.10 0.00 fp fp 0.00 Figure 6.23b Spatial pore pressure dissipation at level 2 in specimen 5 specimen in 2 level at dissipation pressure pore Spatial 6.23b Figure o h ispto ts f 53 of test dissipation the for 1 .= i-i ^ i+ r & r ^ T - i rg rrjij' rrjij' rg i - T ^ r & r i+ ^ i-i .= 0 100 10 Experimental Predicted r/ro=52.8 r/ro=24.8 r/ro = 15.2r/ro= r/ro=44.3 / a F 2N1 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 1 (See level in 2N1 Time (sec) Time 1000 10000 166 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ctv') 0.9 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Figure 6.24a Spatial pore pressure dissipation at level 1 in specimen 5 specimen 1 in level at dissipation pressure pore Spatial 6.24a Figure o h ispto eto 5 of test dissipation the for 1 10 u1 r/ro = 16.7 r/ro=u1 u4 r/rOu4 =9.4 =r/ro8.86u2 u3 r/ro =16.0 r/rou3 Experimental Predicted ^/ a F204 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. 1Fig (See level in F204 Time (sec) Time 100 1000 n 10000 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ctv') 0.90 1.00 0.80 u5 r/ro = 22.2 =22.2 r/ro u5 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 Figure 6.24b Spatial pore pressure dissipation at level 2 in specimen 5 specimen in level2 at dissipation pressure pore Spatial 6.24b Figure o h ispto eto 53 of test dissipation for the 1 10 u6 r/ro = 22.2 =22.2 r/ro u6 Experimental / Predicted u8 r/ro = 29.8 = r/ro u8 u7 r/ro = 29.8 29.8 = r/ro u7 a F 2 0 4 in level 1 (See Fig 6.15. Table 6.2). Table 16.15. Fig (See level in 4 0 2 Time (sec) Time 0 1000 100 1000 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ctv') for the dissipation test of l|/iFR10 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. Fig 1 (See level in l|/iFR10 of test 1 dissipation for the specimen 1 in level at dissipation pressure pore Spatial 6.25a Figure 2 3 0 1 1 - u3 r/ro=3.0 u3 - r/ro u2 =2.0 z r/ro= u12.0 c * u4 r/ro =3.0 u4 * 10 Predicted Experimental Time (sec) Time 100 1000 10000 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v' 2.0 0.5 1.5 0.0 1.0 Figure 6.25b Spatial pore pressure dissipation at level 2 in specimen 1 specimen in 2 level at dissipation porepressure Spatial 6.25b Figure for the dissipation test of 1 i/iFR10 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. 1 1Fig (See level i/iFR10 in of test dissipation the for 1 10 Experimental Predicted u7 r/rou7 =8.0 u6r/ro =6.0 u8 r/rou88.0 = u5r/ro3.0 = Time (sec) Time 100 1000 10000 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / ctv') 0.3 0.2 0* .0 0 for the dissipation test of of test dissipation the for Figure 6.26a Spatial pore pressure dissipation at level 1 in specimen 2 1specimen in level at dissipation pressure pore Spatial 6.26a Figure 1 10 u3 r/ro=3.0 u3 r/ro= 2.0 u2 1 r/ro =2.0 u1 u4 r/ro=3.0 u4 Experimental Predicted 23 /AFR Time (sec) Time 10 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. 1Fig (See level in 100 1000 1000 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / riv') 0.10 0.08 0.09 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 for the dissipation test of of test dissipation the for Figure 6.26b Spatial pore pressure dissipation at level 2 in specimen 2 specimen in 2 level at dissipation pressure pore Spatial 6.26b Figure » i 5 rtrrtif =5t =»=i — Lt — A u 10 ll 23 — a a. a — u6r/ro = u7r/ro = r/rou5 = u8 r/ro= Experimental Predicted /aFR Time (sec) Time 1 0 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. 1Fig (See level in 0 100 1000 'I ' i ...... 1000 poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v') 2.0 0.5 0.0 Figure 6.27a Spatial pore pressure dissipation at level 1 in specimen 3 1specimen in level at dissipation pressure pore Spatial 6.27a Figure o h ispto eto 3 of test dissipation the for .0 00 100 10.0 10000.00 1000.00 100.00 10.00 1.00 —ii — — a a- — u s- :s u2 r/ro2.0 = u1r/ro2.0 = u3 r/ro =3.0 u3 r/ro =3.0 u4 r/ro=3.0 Predicted Experimental i / j FRIO -Qra. ^ Time (sec) Time a — — —aa- —a — —a - —aa in level l(See Fig 6.15. Table 6.2). Table 6.15. Fig l(See level in Q _ _ poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v') 2.00 1.50 0.50 1.00 0.00 for the dissipation test of of test dissipation specimen for in the 2 level at dissipation pressure pore Spatial 6.27b Figure 1 10 u8 r/ro8.0 = u7 r/ro8.0 = u6 r/ro6.0 = u5r/ro 3.0 = Experimental 3i,jFR10 3i,jFR10 Predicted Time (sec) Time —0o* in level level in 100

I (See Fig 6. Fig (See 1000 15. Table 6.2). Table 10000 3

174 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / a v) 20.00 15.00 10.00 5.00 0.00 for the dissipation test of 53of test dissipation 5 the for 1specimen in level at dissipation pressure pore Spatial 6.28a Figure 1 10 u2r/ro =2.0 u3r/ro =3.0 u1 r/ro=2.0 u4r/ro =3.0 l — —li Predicted Experimental / a FRIO Time (sec) Time in level 1 (See Fig 6.15. Table 6.2). Table 6.15. 1Fig (See level in 100 A 1000 10000 175 poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure (Au / o v') 2.0 1.5 1.0 0.5 0.0 Figure 6.28b Spatial pore pressure dissipation at level 2 in specimen 5 specimen in 2 level at dissipation pressure pore Spatial 6.28b Figure for the dissipation test of 53/AFR10 in level 1 (See Fig 6.15. Table 6.2). Table 6.15. 53/AFR10 1Fig of (See level test in dissipation the for 1 z □z □□ 01000 10 u5 r/ro = 3.0 r/ro= u5 u6 r/ro = 6.0 =r/ro u6 u8 r/ro = 8.0 =r/ro u8 8.0 r/ro= u7 Experimental Predicted Time (sec) Time 10000 176 177

increasing even at 10000 seconds. These observations clearly indicate that

dissipation occurs primarily in the radial direction. The figures show the time

variation of the spatial pore pressure dissipation results at level 1 (depth of 602 mm)

and at level 2 (depth 402 mm). These reading were obtained with the penetration of

piezocone being stopped at level I (depth of 602 mm). The pore pressures predicted

by the theory underestimates most of the measured values except reference

cone. This could probably mean that the pore pressures below the tip extends to a

distance greater than that predicted by spherical cavity expansion and in fact, the

pore pressure distribution around the cone tip may not even be spherical in shape.

6.2.2 Limitations

It can be seen that the dissipation curves predicted at the cone base match

very well (Figures 6.16a through 6.161) with those obtained during the dissipation

tests conducted in the four specimens. However, the predicted spatial pore pressure

dissipation curves (around the piezocone) do not show an accurate match with those

of the experiment. The comparisons between the predicted and the measured spatial

pore pressure dissipation curves, however, exhibit a fairly good trend agreement

considering the limitations and the many simplifying assumptions in the modified

cavity expansion approach. The tip geometry which has a significant influence on

the pore pressure gradient around the tip cannot be modeled by a simple method

based on a spherical cavity expansion theory. In heavily overconsolidated stiff

clays, very high pore pressure gradients develop around the cone tip. The predicted

curve of 53/aF204 (Figure 6.16h) could not simulate the initial increasing part of the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 178

dissipation curve. The method proposed by Gupta and Davidson (1986) cannot be

applied for such soils because it is incapable of predicting such pore pressure

gradients which produce a non-standard dissipation curve. It does, however, predict

a pore pressure gradient along the length of the penetrometer due to the dissipation

effect during penetration. In addition to the above limitations, the model does not

take into account the following important factors: geometric nonlinearity due to

finite strain rates in the soil during penetration, stress and fabric anisotropy,

remolded zone of soil around the penetrometer, coupling between the total stresses

and pore pressures, inertia and creep effects.

6.3 Application of the Interpretation Models to Chamber Penetration Data

The dissipation results obtained from the chamber studies were used to

evaluate some of the interpretation models described earlier in chapter 2. The cavity

expansion interpretation models proposed by Torstensson (1975, 1977) are

compared with fourteen PCPT dissipation results of five specimens in Figures 6.29a

through 6.29e. The excess pore pressure dissipation, 0.5 mm above the cone base

(for the filter element in the ui configuration) may not have a truly cylindrical

symmetry nor a spherical one. Hence, all comparisons are made with both the

cylindrical cavity expansion and the spherical cavity expansion solutions. It can be

observed from the figures that the spherical cavity expansion solution predicts a

much faster dissipation than those observed in the experiments. This is not

surprising since the radius of the plastic zone (and thereby the spatial extent of the

excess pore pressure distribution) predicted by the spherical cavity expansion theory

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 179

3a 0.8 < 3 <

0.6 |— 50 % consolidation line bmu o a* C/5 C/5 8 0.4 □ cylindrical. E/j,-500 x ■ cylindrical. E/*,-l00 M O spherical. E/s,«500 T3 • spherical. E /s.-100 a "3 € n , o 0.2 Analytical Z

0.0 0.001 0.010 0.100 1.000 10.000 100.000 1000.00 Time Factor. (T=crt / ro2)

Figures 6.29a Comparison of dissipation results in specimen I with the Torstensson's model

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 180

\v < 3 <

d 3 ■si „ _ 0.6 50 % consolidation line a.o C/t □ cylindrical. E/V“500 5U /5 ■ cylindrical. E/s,” 100 g 0.4 O spherical. E/s.=500 u • spherical. E/s,” 100 -a .3 Experimental "3 § o 0.2

0.0 0.001 0.010 0.100 1.000 10.000 100.000 1000.00 Time Factor, (T=crt/ ro2)

Figures 6.29b Comparison of dissipation results in specimen 2 with the Torstensson's model

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 181

3 <

0.6 a . oI— 50 % consoidedon line o 2L V) □ cylindrical. E/s«“ 500

0.0 0.001 0.010 0.100 1.000 10.000 100.000 1000.00 Time Factor, (T=c,t / ro2)

Figures 6.29c Comparison of dissipation results in specimen 3 with the Torstensson's model

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 182

A4«.F103 1.0 — O «v.F2N 0 • 4v.If2N l & c 4„,no« 3 y < 3 0.8 — < a 3cn VI a a . 0.6 — a o - 50 % consolidation line

VI VI □ cylindrical. E/s,” 500 yu x ■ cylindrical. E/s,” 100 U 0.4 — O spherical. E/s,«500 • spherical.' E/s,»IOO a Experimental

Analytical o Z 0.2 —

0.0 1 limit I 1 m UJ 0.001 0.010 0.100 1.000 10.000 100.000 1000.00 Time Factor. (T=crt / ro2)

Figures 6.29d Comparison of dissipation results in specimen 4 with the Torstensson's model

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Normalized Excess Pore Pressure, (Au / Aup) . — 1.0 . — 0.8 0.6 0.4 . — 0.2 0.0 Figures 6.29e Comparison of dissipation results in specimen 5 specimen in results dissipation of Comparison 6.29e Figures 0.001 0%oskainln « 50 %consokdationline -

_LL □ cylindrical.E/S.-500 □ ■ cylindrical. E/v*100 E/v*100 cylindrical. ■ O spherical. E/s,«500 E/s,«500 spherical. O peia. E/S.-IOO spherical. • 0.010 ihteTrtnsns model Torstensson's thewith Analytical "

1 1 1 Expcrimenui mill 0.100 Time Factor, (T=crt / ro2)(T=crt / Factor, Time

l

i mini, 1.000

il ml i nun i i mil n i T Imill 10.000 100.000 V ?5v*fl03

Sv«F2Nl 1000.00 184

is smaller than that predicted by the cylindrical cavity expansion theory. However,

it is possible that during the advance of the piezocone, a deformation pattern is

produced in the vicinity of the tip approximately similar to that during a spherical

cavity expansion. The cylindrical symmetry of displacement contours along the

shaft may be a result of the successive summation of the spherical cavity expansions

(Gupta and Davidson. 1986). In such a case, the cylindrical symmetry of the excess

pore pressure distribution above the cone base (along the shaft) should be taken into

account (the two-dimensional aspect) during the dissipation phase. This would give

a much slower dissipation rate at the cone tip than that predicted by a one­

dimensional spherical dissipation process. The limitations and disadvantages of the

Torstensson's model are:

• Difficulty in defining the equivalent radius, ro for the spherical cavity.

• Difficulty in selecting an appropriate (single) value for the rigidity index. Ir.

• Does not take into account the two-dimensional aspect of the cone

penetration and dissipation process.

• Neglects shear induced excess pore pressures.

• It is based on a simple elastic-perfectly plastic soil model. It does not take

into account geometric nonlinearities, creep effects, remolding, stress history

of the soil, and coupling between total stresses and pore pressures.

However, the main advantage of the model is that it is relatively simple to use and

can give an approximate estimate of cr. In general, the PCPT results obtained from

the present chamber tests were in well agreement with cavity expansion solution,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 185

except specimen 5. As mentioned above, Torstensson’s model can not take into

account the effects of stress history and apparently is the least representative.

The theoretical solutions of Levadoux and Baligh (1986) and Houlsby and

Teh (1988) using the strain path method are compared with the experimental dissipa­

tion results obtained from the chamber PCPTs in Figures 6.30a through 6.30e. The

method proposed by Levadoux and Baligh does not indicate significant difference

between the dissipation rates at the tip of the cone and the cone base probably due to

insufficient modeling of the tip geometry. The method proposed by Houlsby and

Teh overestimated this difference. At 20% degree of dissipation, the modified time

factor. T*, at the cone base is 38 times that at the cone tip. The difference between

the time factors diminish at higher degrees of dissipation. The method uses a

modified time factor. T \ defined as T* = (cr t)/(ro2 Ir ) in order to include the influence

of the rigidity index Ir and the radius of the influenced zone. This approach was

found to give unified dissipation curves for different values ofthe rigidity index, Ir.

However, for any particular degree of dissipation, the difference between the time

factors, T, for various filter locations cannot be a constant. The chamber PCPT

results indicate that the difference between the time factors for different filter

locations at any particular dissipation level was influenced by the stress history of

the soil.

A comparison of the coefficient of consolidation cr at 50% degree of

dissipation using the interpretation models are given in Table 6.3. In Table 6.3, the

coefficients of consolidation cr were estimated with Aup and Au,. The Auj was

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. poue wt pr sin t oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright e fth o ission perm with eproduced R

Normalized Excess Pore Pressure, (Au / Aup) 0.8 1.0 0.6 . — 0.4 0.0 . — 0.2 iue .0 Cmaio fdsiainrslsi pcmn 1 specimen in results dissipation of Comparison 6.30a Figures 0.01 50- % consolidation \ fine ■ Houlsby & Teh (1988), U,type (1988), Teh Houlsby & ■ Levadoux G Houlsby • Ui(1986),type Baligh & Levadoux O — ______------with the strain path method path strain the with 0.10 St Analytical Experimental

St Teh (1988), Uitype (1988), Teh 1 I II I Baligh (1986), Ui(1986),type Baligh Time Factor, (T=ert / ro2) / (T=ert Factor, Time 1.00

10.00 0.01000.00 100.00 y y 71,^103 I O ▼ 1t*fA luvlOl 104 ia FRIO 186 187

.0 O Iv.FR lO

a. 3 0.8 < 3 <

3 cn cn 0.6. H a. H -50% consolidation lii o a. csoa oU X 0.4 u "T3 »*■8 Experimental 'a o 0.2 z □ Levadoux & Baligh (1986). Uj type ■ Houlsby & Teh (1988), (J, type O Levadoux & Baligh (1986). Ui type • Houlsby & Teh (1988), Ui type

0.0 1111 0.01 0.10 1.00 10.00 100.00 1000.00 Time Factor, (T=crt / ro )

Figures 6.30b Comparison of dissipation results in specimen 2 with the strain path method

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 188

0.8 3 < 3 <3 y 3 0.6 cn cn Si a. 50% ccnsdidation line

1 0.4 Xu Ui ■S N Analytical eo 0.2 O □ Levadoux & Baligh (1986), U* type Z ■ Houlsby & Teh < 1988). U2 type O Levadoux & Baligh (1986). Ui type • Houlsby & Teh (1988). U, type

0.0 0.01 0.10 1.00 10.00 1 00.00 1000.00 Time Factor, (T=crt/ ro2)

Figures 6.30c Comparison of dissipation results in specimen 3 with the strain path method

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 < 1 0.8 2 V)3 5/3 2 a 06 o a. 50 % consolidation line VI c/3 u

" Analytical

□ Levadoux 4 Baligh (1986). Uj type ■ Houlsby 4 Teh (1988), U2 type O Levadoux 4 Baligh (1986). Ui type • Houlsbv 4 Teh (1988). Ui type il 0.01 0.10 1.00 10.00 100.00 1000.00 Time Factor, (T=crt / ro2)

Figures 6.30d Comparison of dissipation results in specimen 4 with the strain path method

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission 190

3 < 3 0.8 <3

a ; 0 .6

o % consolidation line CL C /1

.§ 13 Analytical

O □ Levadoux & Baligh (1986). Uj type Z ■ Houlsby & Teh (1988). Ui type O Levadoux & Baligh (1986). U i type • Houlsby & Teh (1988). U i type s' T Ss mil hmiiL 0.01 0.10 1.00 10.00 100.00 1000.00 Time Factor, (T=crt / ro2)

Figures 6.30e Comparison of dissipation results in specimen 5 with the strain path method

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 1.9 4.2 2.2 4.2 Reference (c, x (c, cm2/sec) 10'J Auj 1.0 1.9 1.3 7.7 With Aup 2.6 2.0 1.0 1.0 1.0 1.0 11.312.7 9.3 5.3 2.5 2.5 10.8 6.5 10.711.0 8.1 10.2 5.3 2.0 2.0 2.0 2.0 5.0 Aiij With Houlsby & Teh (1988) AUj 10.5 7.8 13.7 6.8 3.2 2.3 20.6 With With Aup 10.5 11.7 12.7 8.8 44.6 33.6 2.9 29.0 With Estimated (c, x 10'1 cm2/sec) x(c, Estimated 10'1 Atij % dissipation level using andAup 3.1 3.7 9.7 9.7 33.8 7.8 3.7 33.1 2.7 3.73.3 16.0 3.3 3.5 3.9 3.3 2.2 50 With at at Atip 3.4 3.4 4.1 4.1 3.1 31.7 31.7 1.2 1.2 3.4 2.7 3.9 13.5 16.9 3.4 3.5 3.5 33 3.1 3.1 Torslensson( 1975,1977) Torslensson( Levadoux & (1986) Baligh With Table 6.3 C omparisonTable 6.3 betweenC the estimated and reference valuescr F R I 0 F102 F204 FI03 F2()4 lest a a a a / m , /AF2N0 / , j j j i j li/, FRIOli/, 33.8 1 in FIOI in 1 1 i/i F103 i/i 1 1 i/i FI04 i/i 1 3.7 2 21 3 i/i F102 i/i 3 3|/,F2NI 3.1 2 4 4 4 2j/ArF203 4.0 4j,ArF2N2 1 9 4 3 No, * Specimen* 5 was excluded since all interpretation models are valid only for OCR < 5. Specimen

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 192

defined in chapter 2 as a initial excess pore pressure immediately after arresting

penetration. The reference values of cr in Table 6.3 were obtained from oedometer

tests conducted on undisturbed samples obtained from chamber specimens.

The comparison between the estimated and reference values of cr

summarized in Table 6.3 reveal that utilizing Au-, instead of Aup improves

interpretative methods proposed by Levadoux and Baligh (1986) and Houlsby and

Teh (1988) in better representing the experimental results.

6.4 Undrained Shear Strength

The behavior of the soil around an advancing cone is very complex in

nature. The soil elements in front of the tip are subjected to a changing state of

stress (involving rotation of the principal stresses) as they slide along the cone face

up the shaft. Because of the continuous failure and the varying nature and state of

stress, the mode of failure is very much different from any of the laboratory tests

used to determine the undrained shear strength. The strain rates experienced by the

soil elements in the vicinity of the cone is also very high compared to that in a

conventional triaxial test (Tumay, et al„ 1985; Acar and Tumay, 1986). Since su is

not a unique soil parameter, the type of test used to determine su should be stated.

In this research, the reference su has been determined from triaxial compression tests

(Consolidated Isotropic Undrained ,CIU, tests for specimens 1 and 3; Consolidated

Anisotropic (Ko) Undrained, CKoU, test for specimens 2, 4 and 5).

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 193

6.4.1 Interpretation Methods

Many analytical models have been proposed to determine the undrained

shear strength from PCPT data. For the present interpretation, the following models

will be considered:

• Bearing Capacity Model (Terzaghi. 1943; Meyerhof, 1951, 1961)

• Cavity Expansion Models (Vesic, 1972, 1977)

• Steady Penetration Approach (Baligh, 1975)

Strain Path Method (Baligh, 1985. Houlsby and Teh, 1988)

Empirical and Semi-empirical Methods (Lunne, et al„ 1985;

Massarsch and Broms, 1981)

6.4.1.1 Bearing Capacity Models

The cone resistance, qc, during undrained piezocone penetration into cohesive

soils can be expressed by the following bearing capacity equation

qc Nc su ■+■ Ovo (6.7)

where qc = cone resistance, Nc = cone factor, su = undrained shear strength, and ctv0 =

total vertical stress. The above equation may be alternatively expressed as:

(6 .8)

The ultimate bearing capacity theories assume the soil as a rigid perfectly plastic,

incompressible and weightless material. The plane strain slip-line problem for a

continuous strip footing is solved on the basis of the fundamental solution

developed by Prandtl (1921). Empirical depth factors and shape factors are used to

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 194

modify the plane strain bearing capacity problem for application to axisymmetric

deep penetration problems. Based on different assumed failure pattern (geometry of

the plastified zone), the following values of Nc have been suggested:

Nc = 7.4 Terzaghi (1943)

Nc = 9.3 (smooth base) Meyerhoff (1951, 1961)

Nc = 9.7 (rough base)

Nc = 9.6 Durgunoglu and Mitchell (1974)

The objections raised against the bearing capacity theories to analyze deep

penetration problems are:

(1) The boundary conditions are not appropriate for deep cone penetra­

tion problems. In shallow penetration problems, the soil moves

outwards and upwards to the surface, whereas in deep penetration

problems, the displaced soil (inner plastic zone) is accommodated by

the elastic deformations of the soil in the outer zone.

(2) Involves empirical correction factors for depth and shape.

(3) Cannot model the continuous process of the cone penetration

mechanism.

6.4.1.2 Cavity Expansion Models

During the PCPT, some surface heave occurs at shallow depths of penetra­

tion. At larger penetration depths, little surface heave is noticed and it has been

argued that the soil moves predominantly outward in a radial direction. This has led

to the modeling of PCPT as a cylindrical cavity expansion process from zero radius

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 195

to the radius of the cone penetrometer. The limit pressure pu required to expand the

cylindrical cavity is considered as the radial stress on the shaft of the penetrometer.

The general form of soil movement at the penetrometer tip has been visualized as

that due to the expansion of a spherical cavity from zero radius to an equivalent

penetrometer radius, ro. The ultimate cavity pressure required to expand the

spherical cavity is often considered an estimate of the cone resistance (at the tip).

Theories for cylindrical an spherical cavity expansion have been developed by Hill

(1950), Soderberg (1962), Ladanyi (1963), and Vesic (1972). These models are

based on elastic-ideally plastic material. The solution by Hill (1950) does not take

into account effects of volume change in the plastic zone. Based on experimentally

determined stress-strain/volume change relationships from triaxial test, volume

change effects in the plastic zone was included by Ladanyi (1963). Vesic (1972)

developed solutions for spherical and cylindrical cavity expansion in an isotropic

soil media ( total in-situ stress; a0 = crvo) governed by a Mohr-Coulomb failure

criteria. The effects of volume change in the plastic zone were taken into account.

For undrained cavity expansion in cohesive soils, the following limit pressures were

obtained:

Pu = ^ su (l + In I r) (spherical cavity) (6.9a)

Pu = su(l + lnlr) (cylindrical cavity) (6.9b)

where Ir = G/su = rigidity index and G = shear modulus.

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Vesic (1977) assumed the point resistance to be governed by the total

octahedral normal stress (cr0 =

expansion solution for the limit pressure in a cohesive soil.

P.. = ± (l + lnl,)+2.57 (6. 10) J

The above expressions for the limit pressure. Pu, has a form similar to that of

the bearing capacity equation. The cavity expansion theories are one-dimensional

theories and do not take into account the two-dimensional nature of the penetration

process. It involves assumptions for the rigidity index lr and the equivalent

spherical cavity radius ro (during predictions of excess pore pressure distribution).

Cavity expansion studies using work hardening elastoplastic soil models have been

used by Randolph, et al. (1979), Baneijee and Fathallah (1979), and Chopra, et al.

(1992) to analyze PCPT results.

6.4.1.3 Strain Path Method

The steady penetration method (Baligh. 1985: Tumay. et al., 1985) has been

used to analyze PCPT results (Baligh, 1985; Houlsby and Teh, 1988; Teh and

Houlsby. 1991). The method hypothesizes that due to strict kinematic constraints in

deep penetration problems, soil deformations, and strains are independent of the

shearing resistance of the soil and the problem is essentially strain controlled. The

cone penetration problem is analyzed by considering the flow of an incompressible,

inviscid fluid (soil) around a static penetrometer. The strain history for each soil

element is determined from the computed flow pattern. The deviatoric stresses are

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then determined (using appropriate initial stresses) by integration of the appropriate

constitutive laws along the streamlines. The mean normal stress is then determined

using one of the equations of equilibrium (radial or axial) and integrating from an

outer boundary starting from some point sufficiently away from the cone. The

stresses may not satisfy the equilibrium equations reflecting the error in the assumed

flow field. Houlsby and Teh (1988) used a large strain finite element method to

correct the inequilibrium by applying incrementally equal and opposite forces with

the cone held stationary.

The expression for Nc including the effects of cone roughness, rigidity index,

Ir, and initial in-situ stresses is given by

(6. 11)

where otf = cone roughness (0 < otf < 1.0), a s = shaft roughness (0 < a s < 1.0), A =

[(avo - crho)/2su] = horizontal index (- 1.0 < A < 1.0),

lateral stress. Figure 6.31 indicates the influence of Ir, cone roughness, A on the

cone factor Nc.

6.4.1.4 Steady Penetration Approach

The steady penetration approach proposed by Baligh (1975) analyzes the

problem of continuous penetration of the piezocone (or pile) as a steady-state

situation. The work done by the external force per unit area to push the cone a unit

distance was equated to the sum of the work done to push the cone tip (wedge)

alone at a constant velocity over a unit distance and the work done to expand a

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f • *o*

Sptwncai a w * r t-m. Z fi - 10*

'00 . ISO » *r Variation of Nkr with lr

■1* ■ 11 10 -0-7J -0-5 -oas'''I^OJS 04 07S A • f V • i *7 ►

Variation of NkT with A

lr = 300

10 ‘ * 100 • <

100fi/a Effect of shaft roughness on cone factor NkT

Figure 6.31 Factors influencing Nc (Houlsby and The. 1988)

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 199

cylindrical cavity behind the cone. The medium surrounding the wedge was

assumed to be massless, isotropic, rigid-perfectly plastic material with a Tresca

yield criterion and the cylindrical cavity expansion assumes an elastoplastic soil

medium, where the initial in-situ stress state was equal to the total horizontal stress.

For a 60° cone, Baligh arrived at the following expression for the analytical bearing

capacity factor, Nc:

Nc = [l+ lnlr] + ll (6.12)

6.4.1.5 Empirical and Semi-Empirical Methods

The undrained shear strength may be estimated using the following empirical

equation suggested by Lunne, et al. (1985)

(613) N ut

where qj = qc - Uo, N^r = empirical cone factor and a vo = vertical stress. NkT values

have been reported to vary between 4 and 30 in actual practice. Several factors such

as plasticity, stress history, stiffness, sensitivity, fabric are known to be the cause for

such wide variations. Semi-empirical relations based on cavity expansion theories

(Vesic, 1972; Randolph and Wroth, 1979; Massarsch and Broms, 1981) using

penetration induced pore pressures may also be used to estimate su. The initial

excess pore pressure distribution within the plastic zone due to spherical or

cylindrical cavity expansion is given by Massarsch, 1976):

Spherical cavity expansion

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(6.14)

Cylindrical cavity expansion

\ Auc = s„f 21n — + In Ir +1.733Af -0.577 (6.15) (V r J

where Au^“ = excess pore pressure due to spherical cavity expansion, AuL = excess

pore pressure due to cylindrical cavity expansion, ro = equivalent penetrometer

radius, r = radial vector to a point within the plastic zone, and At- = Skempton pore

pressure parameter at failure. The equations above for Au are of the form

Au = su NAu (6.16)

where the correlation factor, NAl1. varies between 2 and 20, depending on the soil

type, in-situ stress state (Ko), rigidity index (Ir), overconsolidation ratio, sensitivity,

and the soil micro and macro fabric. Values for NAu as a function of the plasticity

index (Ip) and/or rigidity index (G/su) and Ar for two different filter locations (cone

tip and cone base) are given in the form of interpretation charts (Figure 6.32,

Massarsch and Broms, 1981).

6.4.2 Application of the Interpretation Methods to the Chamber Specimens

Comparison of the empirical cone factor estimated from the cone

penetration data and the reference su with the analytical cone factor Nc are

summarized in Table 6.4. The N^r values for the cone penetration tests in

specimens 1, 2 and 3 were higher than most of the theoretical Nc values (except that

predicted by the steady penetration approach). The Nkr values for Ko-anisotropically

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 500

- 200

• 100 • SO

Pore pressure / parameter at failure, Af *o 04 Au,

10 8 6 4 2 0 Pore pressure ratio, = Au„/su

» ■ ■ ■ « - » --1-

50 oim

Pore pressure parameter at failure

Aut

Pore pressure ratio, 1^= Au^s,,

Figure 6.32 Interpretation charts for Niu (Massarsch and Broms. (1981).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14.0 13.4 10.9 12.8 9.8 Oo = Oo o,o Strain Path and Teh (1988) Method Baligh (1985) Houlsby 17.1 17.8 18.2 18.0 16.6 Steady m Approach Oo = Oo 0,0 Penetration Baligh(1975) N 12.2 10.8 10 10 1 Spherical Oo — — Ooc, Oo 7.0 11.9 6.1 5.6 O0 = OyO Cylindrical (Vesic 1972,1977) Cavity Expansion Models 0 ,0 9.6 7.2 8.2 Analytical Bearing Capacity Factors Nt Spherical Oo= 7 - 1 0 7 - 10 Models O0 =O0 O,o Bearing Capacity Meyerhof(1951-61) — it 8 8 8 8 8 8 7 - 1 0 9.3 8 9 99 7 - 1 0 9.1 6.8 11.7 11 13 13 11 12 12 13 27 27 28 29 28 N (qrO»o)/Su lerzaghi (1943) FRI0 F103 F204 FIN1 F2N2 F204 F I0 2 FRIO F2N1 FIN2 F103 F204 FRIO Table 6.4 Comparison ofanalytical bearing capacity factor, Nc, with a a a a a a a a a a a , /AF2N0 MF2N 1 MF2N m lest ia j j j 1 i/i FIO i/i I1 FI0 211/, 11 II 7 - 1 0 7.5 I li/,F I 0 3 1 ,/i F ,/i 1041 3,/jF 2033 ,/,F lN 4 12 2 2 53/ 53/ 53, 23/ 3,/, FRIO 3,/i F2NI 3,/i F I0 2 2,/ 43/ 4 43/ 53/ 53/ 43 43/ 23MrF 203 1 5 3 2 4 No. Specimen

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without perm ission. 203

consolidated specimens were lower than those for the isotropically consolidated

specimens signifying the importance of the horizontal stress on NkT. Analysis based

on the strain path method along with large strain finite element analysis (Houlsby

and Teh, 1988) have indicated the importance of horizontal stress and rigidity index

on the cone factor and this is confirmed by the present study. Tests on specimen no.

5 (OCR = 10.9) gave higher NkT values than the others indicating the influence of

the soil stress history on NkT-

6.5 Lateral Stress Coefficient

6.5.1 Interpretation Methods

Empirical correlations have often been used, usually based on some theoreti­

cal framework, to correlate the data measured during a PCPT to the engineering soil

parameters. In recent years, efforts have been made (Sully and Campanella, 1991;

Mayne and Kulhawy, 1992) to correlate the pore pressure measured on the face of

the cone and that measured behind the tip to the lateral stress coefficient (Ko). The

method (Sully and Campanella, 1991) is based on the interdependence of Ko and

OCR in non-cemented soils in which the preconsolidation have developed by a

simple mechanical loading-unloading process. The development of the correlation

between Ko and the measured pore pressures were based on the following argu­

ments.

The excess pore pressure Aui measured on the cone tip and Au 2 measured

behind the tip could be expressed as a proportion of the total corrected cone

resistance, qr during a PCPT, i.e.:

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. 204

Aui = fi(q-r) (6.17)

Aui — fzlqT) (6.18)

Hence, Aim - Aut = ui - ui = tKq-r), where ui and ui are the total pore pressures

measured on the cone face and above the cone tip, respectively. Also, due to the

fact that the cone resistance in clays is related to the horizontal effective stress, o'h

(as confirmed by the present chamber studies), meant

ui - u2 = f3(q-r) = f4(cr'h) (6.19)

Hence, it can be argued that normalized pore pressure parameter. PPSV, defined as

PPS V = ^ ~ U: = f5 (K0) (6.20) tfvO

gives a correlation between the measured pore pressures and the lateral stress condi­

tion. The analysis of published data (mostly field test results and one calibration

chamber study) collected from a number of research sites around the world indicates

a definite trend between the PPSV and Ko (especially site specific). A linear

relation between PPSV and Ko was suggested (Sully and Campanella, 1991; Figure

6.33)

Ko = a + b (PPSV) (6.21)

where a and b are constants. The value of "a" is less than the normally consolidated

value of Ko and an approximate value of 0.11 was suggested for b. The method

suggested by Sully and Campanella was verified by Mayne and Kulhawy (1992)

using PCPT data from tests conducted on kaolinitic clay in a fixed wall calibration

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. poue ih emiso ofhe cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright e fth o ission perm with eproduced R

Lateral stress coefficient, iue63 PS-ocreainfrcas(ul n apnla 1991). Campanella. and (Sully clays for correlation PPSV-Ko 6.33 Figure 4 2 - - * % Grangemouth e o e e e aaaaa axmxtKDHaga OO0OO a a a a a *«**• • • ■ r 3 St 232 Lr i i i i I " | 'V | "I I I I •••• •••• Orammen rn Cross Brent Onsoy tog Pit Strong Rio Cowdon Madingley de

r—r 5 * ■ * •

a * Janeiro

—i | "i i -r i r i i r - | i i i " i | i i— r—

PVfo PCPT from PPSV 0 5 20 15 10 i i i”n 1 i > i i 1 n ” i i i i i i PV= u u2)/av* (ur = PPSV ooooo ooooo aoooo aoooo t Alban St. s t s e t Emmerstod * * * * * o. hmber cham Col. 0 0 0 0 0 A A ags MIT Saugus. coad Farm McDonald "Grad

0.11

205 206

chamber. Their results showed good agreement with the proposed method (Figure

6.34).

6.5.2 Evaluation of the Interpretation Methods

The comparison of PPSV vs. Ko for the four chamber specimens with the

method suggested by Sully and Campanella is shown in Figure 6.35. The results

can be evaluated on the basis of the stress history of each specimen. From the

present calibration chamber results, it was observed that (comparing the penetration

pore pressures in specimens 1, 2, 3 and 4) for normally consolidated specimens, the

difference between U| and uj, i.e., the magnitude of ui (and hence, the value of

PPSV). diminishes with an increase in Ko. Since specimen 1 had only ui

penetration, figure 6.35 does not include the results of specimen 1. The reason why

the results for specimen 3 fall away from the line proposed by Sully and Campanella

is that one rarely comes across soils that are normally consolidated having a Ko

value equal to unity (i.e., isotropic, normally consolidated specimen). A Ko value of

unity in the field would usually mean overconsolidated soils for which the PPSV

will be high due to the high pore pressure gradient around the tip for such soils.

In specimen 5, the overconsolidation ratio (OCR = 10.9) was achieved by

anisotropic unloading of the anisotropic, normally consolidated specimen. This

stress history is the same as in the field. In the field, the soils usually have a FQ,

value less than unity during the NC stage, and it increases as OCR increases.

Specimen 2 and 4 were normally consolidated under conditions of zero lateral

Reproduced with permission ofthe copyright owner. Further reproduction prohibited without permission. Fixed-Wall Chamber Tests on Kaoiinitic Clay c (3) jo

Figure 6.34 PPSV-Ko comparison in a fixed wall chamber test (Mayne and Kulhawy, 1992).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Lateral Stress Coefficient, Ko 3.0 2.0 0.0 — Figure 6.35 PPSV-Ko comparison for the four chamber specimens. chamber four the for comparison PPSV-Ko 6.35 Figure -5.0 Specimen - Specimen I Specimen - Specimen Specimen 0.0 ul n apnia 1991 Campaneila, Sully and PPSV = (U |-U 2 ) / CTv / ) 2 |-U (U = 5.0 10.0 15. 208 209

strain (similar to in-situ soils). It can be seen in Figure 6.35 that Ko vs PPSV for

these specimens are very close to the line proposed by Sully and Campanella. Thus,

in general, the method proposed by Sully and Campanella to determine the lateral

stress condition from PCPT data seems to be qualitatively effective. They have also

cautioned that site specific correlations be used (because of the influence of other

factors such as the presence of fissures and soil type (i.e., the plasticity index on the

PPSV value). Due to the influence of other factors, such as plasticity index, Ko value

for anisotropic normally consolidated specimens under different stress paths need to

be studied futher by PCPT’s conducted in cohesive soils in well controlled laboratory

calibration chambers.

6.6 Overconsolidation Ratio

The overconsolidation ratio (OCR) defined as the ratio of the preconsolida­

tion pressure. cr'p. and the existing effective overburden pressure, a 'vo, is an

important factor governing the strength, stress-strain behavior, and the compres­

sibility characteristics of soils. Knowledge of the OCR is hence essential in

selecting relevant soil parameters for a proper design of geotechnical systems. The

conventional method of determining OCR is from laboratory oedometer tests on

undisturbed samples obtained from the field. The determination of a 'p is influenced

by the type and procedure of testing (Crawford, 1964) and also by the unavoidable

sample disturbance. If a continuous profile of OCR with depth is required, the

conventional laboratory method becomes time consuming and expensive, requiring

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 210

an elaborate testing scheme. In recent years, the estimation of OCR from in-situ

tests (piezocone, dilatometer, vane shear) has gained a lot of attention.

6.6.1 Interpretation Methods

The use of PCPT data for estimating OCR have been suggested by a number

of researchers (Schmertmann, 1978; Baligh, et al., 1980; Tumay, et al.. 1982;

Senneset, et al., 1982, 1988; Wroth, 1984; Robertson, et al., 1986; Konrad and Law,

1987; Mayne, 1987; Mayne and Holtz. 1988; Mayne and Bachus, 1988; Sully et al.,

1988; Sandven, et al., 1988; Mayne, 1991, 1992). Some of the suggested interpre­

tation methods are evaluated using the chamber PCPT data obtained in this study.

6.6.1.1 Schmertmann Method

The cone resistance qx has been recognized as a measure of the undrained

shear strength su which itself is a function of the OCR (Ladd, et al., 1977;

Schmertmann, 1978 ). Hence, the cone resistance should reflect the OCR of the soil

deposit. Based on the above argument. Schmertmann (1978) suggested the

following method to estimate OCR;

(1) Using the relationship proposed by Skempton (1957), estimate the

normalized, normally consolidated undrained shear strength, i.e.:

f \ S 1# = 0.11 + 0.0037Ip (6.22) VCTvO J

where Ip is the plasticity index.

(2) From the corrected cone resistance, qx, calculate

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 211

/ \

*-1 t ~ C T vO s.. a s = ^ - = A ^ (6.23) CTvO ^ k T

where crVo is the total overburden pressure and NkT is the cone factor.

(3) Estimate OCR using the relationship

, vl.l3-r0.04 - j OCR = (6.24) V s i )

6.6.1.2 Pore Pressure Parameters

Various pore pressure parameters have also been used in the past to directly

correlate to the OCR:

u. Baligh, et al. (1980) 4c

Au Tumay, et al. (1982)

Au Smits (1982) (qB - uo)

B =—— Senneset and Janbu( 1984), (6.25) qT-<*vo

where u = pore pressure at the cone base. u0 = equilibrium pore pressure, Au = u - Uo

= excess pore pressure, and o V0 = total overburden pressure. It is the shear induced

pore pressure that reflects the stress history of the soil and any pore pressure

parameter used to estimate OCR should relate a change in the pore pressure to

changes in the octahedral and shear stress around a penetrating cone (Wroth, 1984).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Because of the similarity between Bq and the Skempton's pore pressure parameter at

failure (At) (Skempton, 1954), Bq was considered as a promising parameter to

estimate OCR. The following expression was suggested

2.3B OCR = t ^ (6.26) (3.7Bq-l)

As mentioned earlier, it is the shear induced pore pressure that reflects the stress

history of the soil. The Bq method was considered a promising parameter to

evaluate the OCR. However, this method does not allow the shear induced pore

pressures to be separated from those generated by the octahedral stresses. Research

performed by various investigators (Battaglio, et al.. 1986; Campanella. et al., 1986;

Jamiolkowski. et al.. 1985; Lunne, et al.. 1985) have shown that no universal corre­

lation exists between Bq and OCR. Moreover, in soft clays, the accuracy of tip

resistance may be considered unreliable (Tumay and Acar, 1985). The existence of

a large pore pressure gradient around the tip especially in overconsolidated clays has

been pointed out by a number of investigators (Baligh. et al.. 1981; Tumay, et al.,

1982; Campanella, et al., 1986; Jamiolkowski. et al., 1985; Lunne, et al., 1986).

Using this principle. Sully, et al. (1988) suggested the following possible pore

pressure parameters to predict OCR.

(1) Pore pressure ratio (PPR)

PPR = (6.27) u ,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (2) Excess pore pressure ratio (PPR1)

(6.28)

(3) Pore pressure difference (PPD)

(6.29) u

The relationship between PPR. PPRl and OCR is shown in Figure 6.36a. The

following correlation between PPD and OCR (Figure 6.36b) was proposed:

OCR = 0.49 + 1.50 (PPD) (6.30)

Sully, et. al., 1988 also stated that PPR, PPRl were not sufficiently sensitive to

changes in stress history to be used as indicators of OCR, especially in soft soils,

and that PPD appears to give a good indication of the stress history.

6.6.1.3 Cavity Expansion/Modified Cam-Clay Methods

Using the critical-state soil mechanics and the cylindrical cavity expansion

theory, Mayne (1987) and Mayne and Holtz (1988) suggested the following expres­

sion for determining OCR

I( n „ Au A V 79 OCR = 0.317 — (6.31)

V C T v0

where Au is the excess pore pressure measured immediately behind the cone tip.

Using the modified Cam-Clay and the cavity expansion theory, Mayne and

Bachus (1988) suggested the following expressions for estimating OCR.

For cylindrical cavity expansion

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 214

15.0 PPR

PPR1

a: o 10.0 — 0 PPR « U1/U2 o* 1 PPR1 s (U1-Uo)/(U2-Uo) c o — Proposed PPR a S “ Proposed PPR1 8 c Specimen 2 PPR 8 Specimen 2 PPR1 s 5.0 — Specimen 3 PPR o POINT 1 Specimen 3 PPR1 Specimen 4 PPR Specimen 4 PPR1 Specimen 5 PPR POINT 2 d Specimen 5 PPR1

0.0

0.00 5.00 10.00 15.0 Pore pressure ratios PPR and PPR1

Figure 6.36a PPR and PPRl vs. OCR (after Sully, et al.. 1988).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue ih emiso o h cprgt we. ute erdcin rhbtd ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

Overconsolidation ratio, OCR 14.0 12.0 10.0 iue63b P-C orlto ncas(fe ul,e l. 1988). al.. et Sully, (after clays in correlation PPD-OCR 6.36b Figure 8.0 6.0 4.0 2.0 0.0 . 2.0 0.0 OCR = 0.49 + 1,5(PPD)+ 0.49 = OCR 4.0 PPD = (U1-U2)/Uo= PPD PPD A Specimen 5 Specimen A - Specimen 4 Specimen - - Specimen 3 Specimen - > Specimen 2 Specimen > 6.0 Proposed relation Proposed 8.0 215 10.0 216

^CTvO J OCR = 2 (6.32a) r\AM \ ( g n In -I V 2y ^Su )

For spherical cavity expansion

Au

Va vO J OCR = 2 (6.32b) fI 2 3 M lJ In f ° l -1

where M = (6 sin ')/(3 - sin

(Wroth. 1984).

Mayne (1991. 1992) suggested the following expressions for predicting

OCR using the Cavity Expansion/Modified Cam-Clay (CE/MCC) approach:

f \ I 33 1 OCR = 2 qT - ub, (6.33a) 1.95M + 1 V CTvO J

1.33 1 q T - u OCR = 2 ^ + 1 (6.33b) 1.95M \ CTvO

where Ubt = U 2 = pore pressure measured just above the cone base and ut = ui = pore

pressure measured on the tip.

6.6.1.5 Kurup Method

The methods proposed by Mayne (1991. 1992) was developed based on the

spherical cavity expansion theory of Vesic (1975) which has been formulated for the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 217

octahedral normal stress (

(equation 6.33a), a '0 has been taken equal to ct'vo since the in-situ lateral stress is

difficult to determine. The method proposed by Kurup (1993) and Tumay, et. al.,

(1995) utilizes the technique of Ko profiling suggested by Sully and Campanella

(1991) (i.e., equation 6.21) and is combined with equation 6.33a (after substituting

for a 0 = CT0Ct instead o f ct0 = a vo). The resulting expression for OCR is given by

*■* c \ 1.33 J OCR = 2 9 r " u : (6.34) _(l.95M + l)avok l + 2 K 0

or in terms of Ui and U 2

f \ 1J 3 qT - u 2 OCR = 2 (6.35) “ (1.95M + 1)l CTvoO+ 2a)+ 2b(u, -u,)J

The values of 'a1 and 'b' suggested by Sully and Campanella may be used for in-situ

predictions of OCR. Since chamber specimens 1, 2 and 3 were isotropically

consolidated and also because Kc for all the specimens were known, equation 6.34

has been used to verify the validity of the approach (Table 6.5).

6.6.2 Evaluation of the Interpretation Methods

The estimated OCR from the chamber PCPT data, using the earlier

mentioned interpretation methods are given in Table 6.5. The method proposed by

Schmertmann data (1978) overestimated the OCR for all the specimens. The

significant scatter in the predicted and actual values of OCR using the methods

suggested by Konrad and Law (1987) have also been reported by other investigators

(Kabir and Lutenegger, 1988; Robertson, et al., 1988). They have also shown that

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ro oe 1.2 2.8 2.8 Tumay Kurup & (1993, 1995) 1.2 1.4 28.6 17.2 Mayne (1992) Mayne (1987) 0.4 0.6 1.5 (1988) Sully et al. Predicted OCR -9.7 3.4 10.1 Bq Method 5.33.8 1.3 1.1 0.6 0.9 4.1 1 1.8 4.7 1.4 0.4 0.5 (1978) Table 6.5 Interpretation ofOCR from PCPT data Schmertmann 10.9 40.8 OCR 1 1 5 23 1 1 4 1 No. Specimen

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the proposed method was only marginally better than the Bq method and it appeared

to have the same restrictions as the Bq method.

From Table 6.5, it appears that the Bq method gives a negative OCR for

specimen 5. The calculation of OCR resulted from average values of Table 5.1. If

the Bq method of specimen 5 is estimated based on the measured values of excess

pore pressure, OCR could be 31. Therefore, the Bq method may have a limitation in

evaluation of heavily overconsolidated soils. In addition to findings of

this investigation, research performed by various investigators have shown that no

universal correlation exists between Bq and OCR (Battaglio, et al.. 1986;

Campanella, et al., 1986; Jamiolkowski, et al., 1985: Lunne. et al., 1985). The Bq

method uses only one value of measured pore pressure (above the cone base) and

hence it is difficult to evaluate the shear induced pore pressure which is believed to

provide a basis for the estimation of OCR. Mayne and Bachus (1988) have shown

that Bq is more of a site specific parameter. Robertson and Campanella (1983)

postulated that the Bq method will be influenced by variations in soil plasticity and

sensitivity. Moreover, the unreliability of the tip resistance in soft clays (Tumay

and Acar. 1985) can add to the errors in estimating OCR using the Bq method.

Significant scattering has been observed in Bq vs OCR at low overconsolidation

ratios (Kabir and Lutenegger, 1988; Robertson, et al., 1986). Sully, et al. (1988)

have suggested an interesting method to estimate OCR using pore pressure

parameters (pore pressure ratios and pore pressure difference) determined from pore

pressure measurements at the cone face and above the cone tip. The method uses

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the pore pressure gradient existing around the tip and its dependence on OCR.

Figure 6.36a shows the PPR and PPRl vs. OCR for the four specimens and also the

relationships suggested by Sully, et al. (1988). The pore pressure ratios for the four

specimens are also given in Table 6.6. It can be observed that the pore pressure

ratios are not sufficiently sensitive to changes in OCR. This observation is

consistent with the observations of Sully, et al. (1988). The PPD method (Figure

6.36b). however, seems to predict OCR reasonably well qualitatively. An important

assumption of the PPD method requires special mention. The method assumes that

the water table is close to the ground surface. Hence, for interpreting the chamber

PCPT results, an equivalent u0 should be determined (which may not be the same as

the backpressure) knowing the density of soil and water and calculating the depth

(and the hydrostatic pressure, Uo) corresponding to the effective vertical stress on the

chamber specimens. In the field. u<, can be determined from dissipation tests carried

out to completion if the phreatic level is close to the ground surface. If the phreatic

level is not close to the ground surface, it should be determined from the soil

Table 6.6 Pore pressure ratios and pore pressure difference for the chamber specimens.

Specimen Aui AUt Equivalent PPR PPRl PPD No. u0 2 467.0 482.9 69 0.97 0.96 -0.1

3 793.6 778.7 34.5 1.02 1.02 0.06

4 535.0 547.8 138 0.98 0.97 -0.06

5 308.1 266.1 96.6 1.12 1.12 1.94

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density variation if known or by using an approximate density (which could lead to

some errors). For using the PPD method, the pore pressure filter location, height

and thickness have to be standardized since these can significantly affect the

magnitude of the measured pore pressure (Sully, et al., 1987).

The predicted OCR's using the methods suggested by Mayne (1987) and

Mayne (1991, 1992) are shown in Table 6.5. These prediction methods have been

formulated from the theories of cavity expansion and critical-state soil mechanics.

The method proposed by Mayne (1987) using the excess pore pressure measured

above the cone base gave good predictions of the OCR. The method suggested by

Mayne (1992) gave good predictions of OCR for pore pressures measured above

the cone base except specimen 5. Interpretation using pore pressures measured at

the cone tip overestimated the OCR's.

The method proposed by Kurup (1993) and Tumay, et. al., (1995) provide

better prediction of OCR than Mayne (1992) in specimen 5. The reason probably

is due to consideration of influence of lateral stress on stress history.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 7

SUMMARY, CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH

7.1 Summary

In this research, four reference piezocone (10 cm2 projected area) and twenty

one miniature piezocone (1.0 cm2 projected area) were conducted on large

instrumented cohesive soil specimens in a computer controlled calibration chamber.

By using a two-stage slurry consolidation technique, homogenous cohesive soil

specimens of very high quality were prepared. The soil specimens were instru­

mented to monitor the spatial pore pressure distribution along the axial and radial

penetration path during slurry consolidation and subsequent cone penetration and

dissipation tests inside the calibration chamber. The performance of the piezocone

penetrometer test (PCPT) to predict consolidation and flow characteristics are

evaluated. Immediate changes in excess pore pressure and cone tip resistance after

penetration arrest for dissipation were experimentally identified. In order to capture

the immediate changes in excess pore pressure and cone penetration resistance

penetration data was acquired at very close time intervals (0.01 seconds) using a

digital oscilloscope. The method proposed by Gupta and Davidson (1986) for

determining the initial excess pore pressure distribution by successive spherical

cavity expansions was used to simulate the piezocone penetration mechanism. The

extremely time consuming and laborious process involved in preparing large size,

222

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. instrumented cohesive soil specimens, limited the number of tests that were

conducted.

The chamber PCPT data was evaluated using some of the existing state-of-

the-art interpretation models. The undrained shear strength, influence of lateral

stress and overconsolidation ratio on the penetration pore pressures, and the

coefficient of consolidation were investigated. Limitations of the current

interpretation models and the need to incorporate factors not included in the previous

models were identified. Areas requiring further research in testing (laboratory as well

as in-situ) and in the analytical models were recommended to further help resolve the

complexities involved in piezocone penetration testing.

7.2 Accomplishments and Conclusions

7.2.1 Accomplishments

(1) The two stage slurry consolidation technique was successfully used to

prepare large size cohesive soil specimens of known stress histories for

calibration chamber testing. The specimens prepared were reproducible and

homogeneous as was indicated by the settlement and pore pressure

dissipation histories and by the water content results obtained from samples

taken from the chamber specimens. The homogeneity of the specimens was

additionally confirmed by the cone penetration results (qr, Au profiles) in

each specimen.

(2) For specimen 1 and 3 the isotropic consolidation was applied, and specimens

2,4, and 5 were subjected to anisotropic Ko reconsolidation. The procedure of

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anisotropic Ko reconsolidation is not as simple as isotropic consolidation. The

performance of the reconsolidation followed the One increment (or Single

increment) procedure suggested by Campanella and Vaid (1972). Campanella

and Vaid utilized a rigid wall chamber to eliminate lateral strain.

LSU/CALCHAS used in this research eliminates lateral strain (Ko

condition) with its servo-controlled double flexible wall system.

(3) In order to capture the immediate excess pore pressure drop and the

simultaneous changes in cone tip resistance penetration data was acquired at

very close time interval (0.01 seconds) using a digital oscilloscope.

7.2.2 Conclusions

(1) High frequency data acquisition using a digital oscilloscope clearly indicated

sudden drops in the corrected tip resistance, and substantial changes in the tip

excess pore pressure (m type filter), when the penetration was arrested to

conduct dissipation tests. This is primarily due to the normal stress reduction

at the tip, as the penetration rate changes abruptly from 2 cm/s to 0 cm/s.

Dissipation and pore pressure redistribution around the tip could also

contribute to this effect. The ui type filter located just above the cone base,

barely showed any instantaneous excess pore pressure drop. This is because

of the fact that above the cone base there is a normal stress release, and the

excess pore pressures are predominantly induced by shear (compared to

excess pore pressure at the cone tip, which are primarily dominated by

octahedral normal stresses).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 225

(2) Comparison between the estimated and reference values of cr reveal that

utilizing the initial excess pore pressure values immediately after the sudden

drop (Au,) instead of the penetration excess pore pressure (Aup) improve the

interpretative methods proposed by Levadoux and Baligh (1986), and

Houlsby and Teh (1988) in better simulating experimental results.

(3) The method proposed by Gupta and Davidson (1986) simulates the

piezocone penetration process as a successive spherical cavity expansion,

and thereby extends the one-dimensional solution proposed by Torstensson

to a two-dimensional, axisymmetric problem. The method also takes into

account the dissipation that occurs during penetration, and empirically

corrects the predicted excess pore pressure at the cone base to exactly match

with the measured excess pore pressures. Due to this empirical correction,

the method gives very good comparisons between the predicted and actual

dissipation profiles above the cone base. However the predicted spatial pore

pressure distribution during the dissipation phase show only qualitatively

agreement with the experimental results (recorded at the ducts situated along

the penetration path). This is probably due to the limitations and simplifying

assumptions in the method, because of which the predicted initial spatial

excess pore pressures do not exactly match with the actual initial spatial

excess pore pressure distribution.

(4) The empirical cone factor (Nk-r) to estimate the undrained shear strength was

found to be very high for the overconsolidated specimen. This is probably

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because the penetration boundary condition BC3 (zero lateral strain and

constant vertical stress), appears to develop a stiff specimen response,

thereby yielding a higher corrected cone resistance qr. The field boundary

conditions are known to be between BC1 and BC3, with the actual

conditions being closer to BC1 (constant lateral stress and constant vertical

stress). BC1 provides a less stiff response compared to BC3 because of the

lateral yielding permitted by BC1 (to maintain constant lateral stress).

(5) The method proposed by Sully and Capanella underpredicted the lateral

stress coefficient (Ko). for the overconsolidated specimen (by a factor of

two). This is probably because the pore pressure Ui and ut (and hence the

normalized pore pressure parameter, PPSV) were influenced by the

penetration boundary condition BC3. A similar observation was made for the

corrected cone resistance in the overconsolidated specimen. It, however,

appears from this study and previous studies (Mayne and Kulhawy, 1992:

Kurup, 1993; tumay et al., 1995) that the method proposed by Sully and

Campanella (1991) give good Ko predictions for specimens with stress

histories and boundary conditions similar to in situ deposits.

(6) The method proposed by Schmertmann overestimated the OCRs of all five

specimens by a factor of almost four. The Bq method yielded a negative

value for the OCR in specimen 5. Research performed by various

investigators in the past have shown that no universal correlation exists

between Bq and OCR. The method proposed by Sully et al. underestimated

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the OCRs of all five specimens. The equation proposed by Mayne (1987)

that makes use of the excess pore pressure above the cone base (ui filter

location) was found to give very good OCR predictions in all five specimens.

However it should be mentioned that this method will predict a negative

OCR in heavily overconsolidated stiff clays, when negative excess pore

pressure develop at the U 2 filter location. The OCR prediction methods based

on critical-state soil mechanics and cavity expansion theories proposed by

Mayne (1991, 1992). Kurup (1993), Tumay et al. (1995) were found to give

acceptable OCR predictions. Once again the overprediction can be attributed

to the penetration boundary condition BC3 (zero lateral strain and constant

vertical stress) that appears to develop a stiff specimen response, thereby

yielding a higher corrected cone resistance qr, and high predicted OCR

values. From specimen 5 pore pressure data it also appears that the

penetration boundary condition BC3 significantly suppresses the high excess

pore pressure gradients that would normally develop around heavily

overconsolidated soils.

7.3 Recommendations for Future Research

The outcome of this research identified some areas which require further

research and/or refinement:

(1) A comprehensive statistically designed experimental investigation (i.e.

factorial analysis) to further expand the limited data base of chamber

piezocone penetration tests in cohesive soils is recommended.

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(2) The labor intensive two-stage slurry consolidation and chamber

reconsolidation can be streamlined and be less prone to specimen loss/failure

due to transfer problem if the slurry consolidometer can be mounted on the

calibration chamber during slurry consolidation. This may be achieved with a

laboratory ceiling height of 15 ft instead of the 11 ft ceiling height currently

available. This will provide enough headroom for the free manipulation of

the 2 ton overhead crane which is essential in the operation of the calibration

chamber.

(3) PCPT’s need to be conducted at different penetration rates (slower and faster

than 2 cm/sec) to study the inertia (viscous) effects, and also to refine the

effect of immediate pore pressure drop and simultaneous changes in cone

resistance, after penetration at different rates is arrested.

(4) It is essential to prepare soil specimens, instrumented not only to monitor

pore pressures along the vicinity of the penetration path, but also to monitor

stress changes (using total stress cells ,TSC) and soil displacements (using

lead shots or fibers and x-ray techniques). The influence of change in soil

fabric should also be studied using scanning electron microscopy (SEM) on

soil samples taken from around the cone penetrometer path.

(5) Comparisons of soil engineering parameters evaluated from laboratory

pressuremeter tests and dilatometer tests conducted on identical soil

specimens should be made to study the different mechanisms which control

their behavior.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 229

(6) Development of Gupta and Davis’ method should include the dissipation

effect during piezocone penetration to determine the initial excess pore

pressure distribution, Auj, for a non-standard dissipation analysis.

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Beyongseock Lim was bom in Seoul, Korea, on June 14, 1964. He obtained

the degree of Bachelor of Engineering from Civil and Environmental Engineering

Department at Korea University, Seoul, Korea, in 1990. He graduated from the same

university with the degree of Master of Engineering in Geotechnical Engineering

division in Civil and Environmental Engineering Department in 1993. He had served

in Republic of Korea Air Force as a private from 1984 to 1987. He is a member of

the American Society of Civil Engineers and a member of the National Society of

Professional Engineers. In August 1994, he entered the graduate program at

Louisiana State University and is currently a candidate for the degree of Doctor of

Philosophy in Civil and Environmental Engineering.

244

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: Beyongseock Lim

Major Field: Civil Engineering

Title 0f Dissertation: Determination of Consolidation Characteristics in Fine Soils Evaluated by Piezocone Tests

Approved:

lirman

dean Lot leVGraduate School

EXAMINING COMMITTEE:

(Co-Chair)

D ate of Baanination:

October 11, 1999

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