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the axial performance of piles in sand remains an area of great uncertainty in geotechnical engineering. Over the years, database studies have shown that the existing method for offshore piles is unreliable. There is therefore a clear need for an impreoved predictive method
INVESTIGATION OF THE END BEARING PERFORMANCE OF DISPLACEMENT PILES IN SAND by Xiangtao XU BEng, MSc A dissertation submitted for the degree of Doctor of Philosophy THE UNIVERSITY OF WESTERN AUSTRALIA A Leading University June 2007 The School of Civil and Resource Engineering DEDICATION To Jianle XU, Jiping LIU & Ting ZHANG For Their Love and Support i D D E E C C L L A A R R A A T T I I O O N N I hereby declare that, except where specific reference is made to the work of others, the contents of this dissertation are original and have not been submitted in whole or in part for consideration for any other degree of qualification at this, or any other, university. …………………… Xiangtao XU June 2007 ii A A B B S S T T R R A A C C T T The axial performance of piles in sand remains an area of great uncertainty in geotechnical engineering. Over the years, database studies have shown that the existing method for offshore piles (e.g. API 2000) is unreliable. There is therefore a clear need for an improved predictive method, which incorporates the state-of- the-art understanding of the underlying controlling mechanisms. This Thesis is dedicated to address the factors influencing the end bearing performance of displacement piles in siliceous sand with a view to proposing and justifying an improved design formulation. Firstly, a database of displacement pile load tests in sand with CPT data was compiled in collaboration with James Schneider (Schneider 2007). It features the widest database with also the latest available pile load test data (e.g. Euripides, Ras Tanajib, Drammen etc) in electronic form. Evaluation of the three new CPT- based methods (Fugro-05, ICP-05 & NGI-05) against this database has revealed a broadly similar predictive performance despite their end bearing formulations being remarkably different. This anomaly promoted the author to extend the database to include additional tests with base capacity measurements to form new base capacity databases for driven and jacked piles, which resulted in the UWA- 05 method for end bearing of displacement piles in sand. This method accounts for the pile effective area ratio, differentiates between driven and jacked piles, and employs a rational q c averaging technique. The UWA-05, together with other three methods has been included in the commentary of the 22 nd edition of API RP2A (API 2006) for design of offshore driven piles. Field tests were performed in Shenton Park, Perth to supplement the database study and, in particular, to examine the effect of the incremental filling ratio (IFR). 10 open-ended and 2 closed-ended piles were tested in compression followed by tension. The test results provide strong support for the UWA-05 method for base capacity evaluation employing the CPT q c values and the effective area ratio. A series of jacked pile tests was carried out on the UWA beam centrifuge, to further explore the factors affecting pile base response. In total, four uniform and four layered centrifuge samples were prepared and tested at various stress levels and relative densities using three separate pile diameters. The resistance ratio (q b0.1 /q c,avg ) is found to be independent of the absolute pile diameter, effective stress and soil relative density. The tests in layered soil enabled quantification of the reduction in penetration resistance when a pile/cone approaches a weak layer and revealed the significant influence on base stiffness of underlying soft clay layers. The stiffness decay curves (G/G IN vs. w/D, where G IN is initial operational shear stiffness) measured in static load tests were found to vary with ratios of G IN /q c , while there was a unique relationship between G/G IN and q b /q c . A detailed parametric study was carried out (using the FE code PLAXIS) by idealising pile penetration using a spherical cavity expansion analogue in layered soil. The numerical predictions compare well with the centrifuge results and their generalization enabled guidelines to be established for end bearing in layered soil. iii A A C C K K N N O O W W L L E E D D G G E E M M E E N N T T S S I am very grateful for the interest and help of my supervisor, Prof. Barry Lehane, who has always been encouraging and generous with his time and has constantly been on hand to provide invaluable guidance and constructive advice when needed. He has also consistently provided feedback on my writing, which greatly improved my academic writing skills. Special thanks are also extended to Prof. Lehane’s wife, Silke, for her hospitality. The contributions from a number of people are acknowledged. Firstly, James Schneider, who introduced me the very efficient tool (Macro) in Microsoft Excel during our collaboration to develop the UWA database, and also provided stimulating discussions and help in many other aspects; James Ayers and Greg Morrig who played a role in setting up and conducting the field test in Shenton Park; Dr. Christophe Gaudin, centrifuge manager, who always found me a testing slot in the busy schedule of the UWA centrifuges. I am also grateful for the invaluable discussions I had with Prof. Tatsunori Matsumoto, Prof. Mark Randolph, Dr. David White, Dr. Kenneth Gavin and Dr. Fiona Chow. Special thanks go to Dr. Susan Gourvenec for giving me the opportunity to play a role in ISFOG and Engineering Camp, which I enjoyed a lot. I am also indebted to Prof. Hanlong Liu, director of Geotechnical Institute of Hohai University for introducing me to the research topic on PCC pile. Moreover, I am very grateful for help from my friend, Jitse Pruiksma in GeoDelft, for his quick response to any of my questions regarding PLAXIS and MATLAB. Technical support from the PLAXIS group in the Netherlands is also highly appreciated. I would like to thank people at the electronic and mechanical workshops for their respective parts in the design and construction of the apparatus, Gary Davies, Frank Tan, David Jones, John Breen, Turan Brown, Shane de Catania and Philip Hortin. Also assistance from Bart Thompson and Don Herley with the centrifuge testing is much appreciated. I also would like to thank all the administrative staff in the School of Civil and Resource Engineering and COFS for their friendship and kind help. In particular, Dr. Wenge Liu, my hometown fellow, who has always offered kind IT help whenever it was needed. I would like to acknowledge all my colleagues, past and present, and academic visitors to the department, some of whom became great friends. I am especially grateful to Dr. Qin Lu and Dr. Jianguo Zhang, who always welcomed me in their happy family and made me feel at home. On a serious note, the financial support (IPRS, UPAIS and ADHOC) I received throughout my time at the University is gratefully acknowledged. Finally, my grandparents, parents and sisters, thank you for your love and support throughout the years. Last but not least, I would like to say ‘thank you’ to my husband, Ting Zhang, who has always been encouraging and supportive with love and great passion for life. iv C C O O N N T T E E N N T T S S Declaration i Abstract ii Acknowledgements iii Contents iv Notation viii Abbreviation xi CHAPTER 1 INTRODUCTION 1-1 1.1 Background 1-1 1.2 Research objectives 1-3 1.3 Organisation of THESIS 1-4 CHAPTER 2 LITERATURE REVIEW 2-1 2.1 Introduction 2-1 2.2 Pile End Bearing Resistance in Sands 2-2 2.2.1 Bearing capacity theory 2-2 2.2.2 Cavity expansion method 2-4 2.2.3 Correlations with CPT data 2-6 2.3 Factors Affecting Pile End Bearing Resistance 2-10 2.3.1 Method of installation 2-10 2.3.2 Surface scale effect 2-12 2.3.3 End condition (open vs. closed) 2-14 2.3.4 Residual stress 2-17 2.3.5 Partial mobilisation 2-19 2.3.6 Scale effect in layered soil 2-20 2.3.7 Assessment of pile base settlement 2-21 2.4 Previous Research for q b in Layered Soil 2-24 2.4.1 Meyerhof approaches (1976-83) 2-25 2.4.2 Vreugdenhil et al. (1994) 2-25 2.4.3 van den Berg & Huetink (1996) 2-27 2.4.4 Ahmadi & Robertson (2005) 2-28 2.5 Summary 2-29 v CHAPTER 3 CPT-BASED METHODS FOR END BEARING OF PILES IN SILICEOUS SANDS 3-1 3.1 Introduction 3-1 3.2 CPT-Based Design Methods for Driven Piles 3-2 3.3 Overview of the UWA Driven Pile Database 3-7 3.4 Evaluation of Fugro-05, ICP-05 & NGI-05 Methods 3-12 3.4.1 Using full UWA database 3-12 3.4.2 Using UWA base capacity database 3-16 3.5 q c Averaging Technique 3-20 3.6 The UWA-05 Method for Driven Piles 3-25 3.7 Jacked Pile Database 3-30 CHAPTER 4 FIELD TESTS AT SHENTON PARK 4-1 4.1 Introduction 4-1 4.2 Soil Conditions 4-1 4.2.1 Shenton Park test site 4-1 4.2.2 CPT results 4-2 4.2.3 Seasonal effect 4-5 4.3 Test Set-up and Procedures 4-6 4.3.1 Test programme 4-6 4.3.2 Pile installation 4-8 4.3.3 Static load tests 4-9 4.4 Analysis of Test Results 4-11 4.4.1 Driving records 4-11 4.4.2 Static load tests results 4-16 4.4.3 Performance of Fugro-05, ICP-05, NGI-05 & UWA-05 4-25 4.5 Analysis of Base Stiffness 4-30 CHAPTER 5 CENTRIFUGE TEST APPARATUS AND PROCEDURE 5-1 5.1 Introduction 5-1 5.2 Centrifuge Modelling 5-1 5.3 Experimental Apparatus 5-3 5.3.1 Geotechnical centrifuge 5-3 5.3.2 Actuator stiffness test 5-5 5.3.3 Model piles and load cells 5-7 5.3.4 Pile cap and guiding plate 5-10 5.4 Testing Programme 5-12 vi 5.5 Material Properties 5-15 5.5.1 Superfine silica sand 5-15 5.5.2 Kaolin clay 5-15 5.6 Sample Preparation 5-16 5.6.1 Sand hopper 5-16 5.6.2 Clay mixer and consolidation press 5-17 5.6.3 Samples A to G 5-19 5.6.4 Sample H 5-19 5.7 Test Details 5-20 5.7.1 Test layout 5-20 5.7.2 Test procedures 5-22 CHAPTER 6 CENTRIFUGE TEST RESULTS 6-1 6.1 Introduction 6-1 6.2 Samples A to G 6-1 6.2.1 Pile Installation 6-1 6.2.1.1 Base resistance 6-1 6.2.1.2 Correlation with D r 6-5 6.2.1.3 Modelling of models 6-8 6.2.2 Static Load Tests 6-8 6.2.2.1 Resistance mobilisation curves 6-8 6.2.2.2 Residual base resistance q b,residual 6-13 6.2.2.3 Factors affecting q b0.1 /q c 6-16 6.3 Sample H 6-19 6.3.1 Pile installation 6-19 6.3.2 Static load tests 6-22 6.3.3 Post sample excavation 6-24 CHAPTER 7 ANALYSIS OF qb IN LAYERED SOIL 7-1 7.1 Introduction 7-1 7.2 Spherical Cavity Expansion in PLAXIS 7-2 7.2.1 Use of Plaxis 7-2 7.2.2 Soil models 7-2 7.2.3 Mesh set-up 7-4 7.2.4 Analysis procedure 7-7 7.3 Numerical Model Verification 7-8 7.3.1 Closed-form solutions to limit pressure p limit 7-8 7.3.2 Comparisons with numerical results 7-10 vii 7.4 Analysis in Two-Layer Soil Profile 7-12 7.4.1 Methodology 7-12 7.4.2 Analysis program 7-14 7.4.3 Pressure expansion curves 7-16 7.4.4 Interpretation of results 7-18 7.4.5 Curve fitting 7-20 7.4.6 Zones of influence 7-23 7.5 Analysis in Three-Layer Soil Profile 7-26 7.5.1 Introduction 7-26 7.5.2 Weak/Strong/Weak 7-27 7.5.3 Strong/Weak/Strong 7-29 CHAPTER 8 ANALYSIS AND DISCUSSION 8-1 8.1 Introduction 8-1 8.2 Assessment of Design Methods for q b0.1 8-1 8.2.1 Driven piles 8-1 8.2.2 Jacked piles 8-5 8.2.3 Implication for design 8-7 8.3 Evaluation of Pile Base Stiffness, G 8-8 8.3.1 Samples A to G 8-8 8.3.2 Sample H 8-18 8.4 Base Resistance Response in Layered Soil 8-19 8.4.1 Analysis procedure in a two-layer soil profile 8-19 8.4.2 Variation of q c /q c,S with H/D 8-20 8.4.3 Variation of G IN with H/D 8-24 8.4.4 Comments on q c averaging technique 8-27 CHAPTER 9 CONCLUSIONS 9-1 9.1 Introduction 9-1 9.2 Findings of Research 9-1 9.3 Recommendations for Future Work 9-5 APPENDIX A………………………………………………………………. A-1 APPENDIX B………………………………………………………………. B-1 APPENDIX C………………………………………………………………. C-1 APPENDIX D………………………………………………………………. D-1 REFERENCE………………………………………………………………. R-1 viii N N O O T T A A T T I I O O N N S S Roman a Current radius of the spherical cavity a 0 Initial radius of the spherical cavity D Pile outer diameter D i Pile inner diameter D 10 Particle size where 10% of the particles are smaller D 50 Particle size where 50% of the particles are smaller D 60 Particle size where 60% of the particles are smaller E Young’s modulus of the soil E p Young’s modulus of an equivalent solid pile E 50 ref Triaxial loading stiffness at p ref E oed ref Oedometer loading stiffness at p ref E ur ref Triaxial unloading stiffness at p ref e initial Initial void ratio e max Maximum void ratio e min Minimum void ratio G s Particle specific gravity of the soil G Shear modulus of the soil G 0 Small strain in-situ stiffness G IN Initial operational shear stiffness h Distance above pile tip level H Distance to the interface of the strong/weak soil layers h cone Height of the sand cone formed beneath pile tip I r Rigidity index = G/p' 0 k Pile base secant gradient =Δq b /Δ(w/D) K 0 NC Earth pressure coefficient for normally consolidated sand [...]... displacement piles in sand The research involved careful examination of the existing databases of static load tests in sand, field and centrifuge testing that targeted uncertainties emerging from the database review, and supporting numerical analyses 1.2 RESEARCH OBJECTIVES The aim of this research is to improve the accuracy and efficiency of the design of axially loaded displacement piles in siliceous sand, with... end- bearing, which accounts for the effective area ratio of open-ended piles, differentiates between driven and jacked piles, and employs a rational qc averaging technique (4) Validate the new proposals using a new set of field tests to be performed at Shenton Park (5) Develop an improved understanding of the physical mechanisms which govern the end- bearing of jacked piles and the end- bearing performance in. .. approaches are often also employed for the analysis of cone penetration, while the last one often avoids the need for assessment of soil properties by using the in- situ test parameter directly in the formulations The various factors potentially influencing qb are then discussed 2-1 Chapter 2 Literature Review Finally, given its relevance to one component of this Thesis, the state -of- the- art for end bearing resistance... was concluded that the installation method affects the axial stiffness of the piles (within ±20%) much less than their bearing capacity The initial stiffness (defined as the initial tangent of a hyperbola fitted to the first three points on the experimental load-settlement curve) depends primarily on the small strain shear modulus of the soil However, it should be noted that this is in particularly true... establishing and testing a design formulation for the base resistance (qb) of closed- and open- 1-3 Chapter 1 Introduction ended piles Factors affecting end bearing, including the installation method, the pile end condition (open or closed), and soil layering will be quantified to assist design Specifically, the key objectives of the research are to: (1) Compile a database of displacement pile load tests in. .. ICP-05 has a tendency to under-predict end- bearing capacity of open-ended piles in loose sands or for large diameter thin walled piles The formulation for endbearing of open-ended piles is seen as a significant limitation that may result in large differences in end- bearing capacity (40 percent) for small changes in pile diameter (1.1m to 1.2m for the uniform dense sands considered) This behaviour may lead... installation, the pile tends to rebound with removal of the pile head force This will result in a reversal in the direction of the skin friction in the upper part of the pile (acting downwards now), while the lower part may still remain in compression At zero pile head load, the base resistance which is in equilibrium with the downwards skin friction is referred to as the residual stress or locked -in base... between various qc averaging techniques As will be discussed below, qb depends not only on the pile dimension and the nature of the sand, but also on the installation method of displacement piles and other factors 2.3 FACTORS AFFECTING PILE END BEARING RESISTANCE 2.3.1 Method of installation According to Mandolini et al (2005), the onshore pile market world is equally subdivided between displacement (driven,... through a series of centrifuge investigations (6) Provide guidance on the influence of soil layering by numerical analyses (using PLAXIS) for a two-layer soil stratigraphy 1.3 ORGANISATION OF THESIS This Thesis comprises nine Chapters Chapter 2 presents a review of the conventional design approaches for base resistance of axially loaded piles in sand In addition, the influence of soil layering on base resistance... would inevitably influence the pile performance in both sand and clay Apart from the main theme of this Thesis, the performance of pipe pile in clay was also investigated through field and centrifuge testing programme Such results are summarised in Appendix D by two published papers Figure 2.12 Schematic streamlines of soil flow and profiles of radial stress; r=radial displacement of soil element at pile . SAND by Xiangtao XU BEng, MSc A dissertation submitted for the degree of Doctor of Philosophy THE UNIVERSITY OF WESTERN AUSTRALIA A Leading University June 2007 The School. degree of qualification at this, or any other, university. …………………… Xiangtao XU June 2007 ii A A B B S S T T R R A A C C T T The axial performance of piles in sand remains. University June 2007 The School of Civil and Resource Engineering DEDICATION To Jianle XU, Jiping LIU & Ting ZHANG For Their Love and Support i D D E E C C L L A A R R A A T T I I O O N N