The Cone Penetration Test Has Been Widely Used for Determining In-Situ
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TOWARDS COMPREHENSIVE INTERPRETATION OF THE STATE PARAMETER FROM CONE PENETRATION TESTING IN COHESIONLESS SOILS by MOHSEN GHAFGHAZI B.Sc., Isfahan University of Technology, 2002 M.Sc., Sharif University of Technology, 2004 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Civil Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2011 © Mohsen Ghafghazi, 2011 Abstract The Cone Penetration Test (CPT) is widely used for determining in-situ properties of soil because of its continuous data measurement and repeatability at relatively low cost. The test is even more attractive in cohesionless soils such as sands, silts and most tailings due to difficulties associated with retrieving undisturbed samples in such soils. Behaviour of soils is highly dependent on both density and stress level. The state parameter is widely accepted to represent the soil behaviour encompassing both density and stress effects. Hence, determining the in-situ state parameter from CPT is of great practical interest. The CPT was analysed using a large strain spherical cavity expansion finite element code using a critical state soil model (NorSand) capable of accounting for both elasticity and plastic compressibility. The constitutive model was calibrated through triaxial tests on reconstituted samples. The state parameter was then interpreted from CPT tip resistance, and the results were verified against an extensive database of calibration chamber tests. The efficiency of the method was further investigated by analysing two well documented case histories confirming that consistent results could be obtained from different in-situ testing methods using the proposed framework. Consequently, cumbersome and expensive testing methods can be substituted by a combination of triaxial testing and finite element analysis producing soil specific correlations. One of the difficulties in analysing the cone penetration problem is the less researched effect of high stresses developing around the cone on the behaviour of the soil. A hypothesis was put forward on the particle breakage process at the particle level and its implications for the behaviour of sands at higher stress levels were discussed. A series of triaxial tests were performed, focusing on the effects of particle breakage on the location of the critical state line. The hypothesis was used to explain the observed behaviour. Particle breakage was shown to ii cause additional compression and a parallel downward shift in the critical state line. The magnitude of the shift was linked to the amount of breakage and it was argued that significant breakage starts after the capacity for compression due to sliding and rolling is exhausted. iii Preface The research topics presented in the thesis were chosen jointly by Dr. D.A. Shuttle and me. I was responsible for performing all the numerical analyses, triaxial compression tests on Fraser river sand and data reduction work associated with the research. Initial versions of all thesis chapters were drafted by me, reviewed by Dr. D.A. Shuttle (and also Dr. Roberto Olivera for Chapter 7), and based on comments received I revised the thesis contents prior to submission. Dr. Roberto Olivera also provided guidance on the laboratory testing procedures described in Appendix A. The following is the list of publications presented in this thesis: Ghafghazi M., and Shuttle D.A. 2006. Accurate Determination of the Critical State Friction Angle from Triaxial Tests. Proceedings of the 59th Canadian Geotechnical Conference, Vancouver, 278-284, (Chapter 2) . Ghafghazi M., and Shuttle D.A. 2008. Evaluation of Soil State from SBP and CPT: A Case History. Canadian Geotechnical Journal, (45)6: 824-844, (Chapter 5) . Ghafghazi M., and Shuttle D.A. 2008. Interpretation of Sand State from Cone Penetration Resistance. Géotechnique, 58(8): 623–634, (Chapter 4) . Ghafghazi M., and Shuttle D.A. 2009. Confidence and Accuracy in Determination of the Critical State Friction Angle. Soils and Foundations, 49(3): 391-395, (Chapter 3) . Ghafghazi M., and Shuttle D.A. 2010. Interpretation of the In-situ Density from Seismic CPT in Fraser River Sand. Proceedings of the 2nd International Symposium on Cone Penetration Testing, Huntington Beach, California, (Chapter 6) . Ghafghazi M., Shuttle D.A., and Olivera R.R. 2011. Particle Breakage and the Critical State of Sand. submitted, (Chapter 7) . iv Table of Contents Abstract .........................................................................................................................................ii Preface ..........................................................................................................................................iv Table of Contents..........................................................................................................................v List of Tables............................................................................................................................. viii List of Figures ..............................................................................................................................ix List of Symbols and Abbreviations......................................................................................... xiii Acknowledgements....................................................................................................................xvi Dedication..................................................................................................................................xvii Chapter 1. Introduction and Background ..................................................................................1 1.1. Introduction ..................................................................................................................1 1.2. Verification and Validation .........................................................................................8 1.3. The NorSand Constitutive Law ..................................................................................9 1.4. Literature Review.......................................................................................................14 1.4.1. Cone Penetration Test........................................................................................14 1.4.2. Interpretation of the State Parameter from CPT Tip Resistance..................16 1.4.3. Calibration Chamber Testing ...........................................................................19 1.4.4. Analytical and Numerical CPT Interpretation Methods................................26 1.4.5. Particle Breakage ...............................................................................................42 1.5. Overview of the Proposed Research .........................................................................46 1.6. References ...................................................................................................................63 Chapter 2. Determination of the Critical State Friction Angle from Triaxial Tests ............78 2.1. Introduction ................................................................................................................78 2.2. Definition of the Critical State ..................................................................................79 2.3. Stress-Dilatancy Definitions ......................................................................................81 2.4. Determination of the Critical State Friction Ratio..................................................82 2.4.1. End of Test (ET) Method...................................................................................82 2.4.2. Maximum Contraction (MC) Method..............................................................83 2.4.3. Bishop Method (BM)..........................................................................................83 2.4.4. Stress-Dilatancy (SD) Method...........................................................................84 2.5. Obtaining the Critical State Stress Ratio of Ticino Sand .......................................85 2.5.1. Ticino Data Processing.......................................................................................86 2.5.2. Discussion of Results for Ticino Sand...............................................................87 2.6. Obtaining the Critical State Stress Ratio of Erksak Sand......................................88 2.6.1. Erksak Data Processing .....................................................................................88 2.6.2. Discussion of Results for Erksak Sand.............................................................89 2.7. Summary .....................................................................................................................90 2.8. References ...................................................................................................................96 Chapter 3. Confidence and Accuracy in Determination of Critical State Friction Angle...98 3.1. Introduction ................................................................................................................98 3.2. Triaxial Database .....................................................................................................100 3.3. Methodology Used for c Determination................................................................101 3.4. Determination of c from Limited Number of Triaxial Tests ..............................103 3.5. Validation against