Centrifuge and numerical modelling of sand compaction pile installation

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Centrifuge and numerical modelling of sand compaction pile installation

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CENTRIFUGE AND NUMERICAL MODELLING OF SAND COMPACTION PILE INSTALLATION YI JIANGTAO (B.Eng.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 ACKNOWLEDGEMENTS I wish to express my deepest and heartfelt gratitude to my supervisors, Professor Lee Fook Hou and Dr. Goh Siang Huat, for their constant guidance and dedicated assistance throughout this research program. It is with their invaluable advice, continuous support, and crucial encouragement that I can tackle various challenges and achieve my research goals. I would like to convey my sincere gratitude to Professor Mark Randolph (UWA) for his guidance and encouragement on the research of cone penetration rate effect (Chapter 5). I also feel grateful to the technicians in the NUS Geotechnical Centrifuge laboratory, Mr. Wong Chew Yuen and Dr. Shen Rui Fu, for their help in operating the centrifuge equipment and improving the experimental set-up. My sincere appreciation is also extended to Mr. Tan Lye Heng, Mr. John Choy, Madam Jamilah, Mr. Foo Hee Ann and Mr. Shaja Khan for the assistance they provided me in the course of the experimental work. The support from the National University of Singapore is gratefully acknowledged, both for granting me the research scholarship and providing me with a stimulating research environment from which I benefited greatly. Special thanks are given to my fellow research scholars in the Center for Soft Ground Engineering for their friendship, kindness and help. i TABLE OF CONTENTS Page ACKNOWLEDGEMENTS .i TABLE OF CONTENTS ii SUMMARY vii LIST OF TABLES ix LIST OF FIGURES .x LIST OF SYMBOLS . xviii Chapter 1: INTRODUCTION 1.1 Background .1 1.2 The “Set-up” effect in soil 1.3 Research scope and objective .6 Chapter 2: LITERATURE REVIEW 11 2.1 Introduction . 11 2.2 Design methodology for the SCP-treated ground 11 2.2.1 Bearing capacity evaluation .12 2.2.2 Settlement analysis 18 2.2.3 Stability analysis 24 2.3 Research investigation on the sand compaction pilling .25 2.3.1 Field studies .25 2.3.2 Reduced-scale 1g-model tests .29 2.3.3 Centrifuge model tests 31 2.3.4 Numerical analysis 40 ii 2.4 Knowledge gaps and outstanding issues .44 Chapter 3: CENTRIFUGE EXPERIMENTAL PROCEDURE 64 3.1 Fundamentals of centrifuge modelling 64 3.2 Centrifuge experimental set-up .65 3.2.1 NUS in-flight SCP installation system .66 3.2.2 Further modification to the in-flight SCP installer 67 3.3 Centrifuge experimental procedure 70 3.3.1 Sample preparation .70 3.3.2 SCP In-flight installation 72 3.3.3 In-flight shear strength profiling .73 3.4 Instrumentation in centrifuge testing 74 3.4.1 Linear-motion potentiometer .74 3.4.2 Pore pressure transducer (PPT) 74 3.4.3 Total stress transducer (TST) .75 3.4.4 T-bar penetrometer .78 Chapter 4: CENTRIFUGE MODEL TESTING: RESULTS AND ANALYSIS .89 4.1 Introduction .89 4.2 Results and discussion of Type I tests .92 4.2.1 Stress and pore pressure variations during SCP installation 92 4.2.2 Comparison with previous centrifuge studies 97 4.2.3 Initial strength state of clay bed .100 4.2.4 Comparison of measured radial stress and pore pressures with analytical solutions .102 iii 4.2.5 Summary of the single pile installation .108 4.3 Results and discussion of Type II Tests .109 4.3.1 Overview of tests .109 4.3.2 Consolidation effect . 112 4.3.3 Pile group effect . 115 4.3.4 Conclusion remarks 116 Chapter 5: NUMERICAL STUDY OF CONE PENETRATION RATE EFFECTS 131 5.1 Introduction .131 5.2 Literature review .132 5.3 Numerical modelling aspects .139 5.3.1 Model geometry .139 5.3.2 Large-sliding soil-cone interface 141 5.3.3 Large deformation formulation .142 5.3.4 Elastic-plastic soil behavior 142 5.4 Analysis results 143 5.4.1 Effect of different penetration rates 144 5.4.2 Fully undrained penetration response 145 5.4.3 Fully drained penetration response .147 5.4.4 Undrained and drained plastic zones 149 5.4.5 Partially drained response 149 5.4.6 Effect of soil stiffness and strength on backbone curves .151 5.4.7 Comparison with Randolph and Hope’s (2004) centrifuge experimental results .152 iv 5.4.8 Effect of volumetric yielding 153 5.4.9 Effect of modulus profile 155 5.5 Application to soil properties evaluation 156 5.6 Concluding remarks .158 Chapter 6: FINITE ELEMENT ANALYSIS OF SAND COMPACTION PILE INSTALLATION 180 6.1 Overview 180 6.2 Finite element model .180 6.2.1 Model geometry and boundary conditions .181 6.2.2 Model discretization 184 6.2.3 Modeling procedure 188 6. Computed soil responses during SCP installation 188 6.3.1 Soil deformation and strain 189 6.3.2 Soil stresses and pore water pressure 191 6.3.3 Comparison of ABAQUS results with centrifuge data using kaolin clay .193 6.3.4 Comparison of ABAQUS results with previous experimental data by Juneja (2002) 196 6.4 Post-installation stress and strength conditions in the soil 197 6.4.1 Pore pressure and stress field following pile installation .198 6.4.2 Strength improvement effect 199 6.5 Strength improvement profile – Parametric studies 201 6.5.1 Changes in numerical modeling aspects .202 6.5.2 Strength improvement profiles 202 v 6.5.3 Effects of friction angle and modulus ratio 203 6.6 Concluding remarks .206 Chapter 7: CONCLUSIONS .231 7.1 Summary of findings .231 7.2 Recommendations for future research 235 REFERENCES .237 vi SUMMARY The installation of sand compaction pile (SCP) has been known to have a considerable impact on the surrounding soils. This research work focuses on evaluating the influence of sand compaction piling, particularly the resulting strength set-up in the adjacent clay. The study comprises both centrifuge experimental and numerical modelling. The centrifuge tests were carried out to measure the changes in radial stresses and pore pressures in soft clays during and after the in-flight installation of sand compaction piles. It was noted that the measured peak increases in stress and pore pressure could be reasonably estimated by cavity expansion theory. Substantial strength improvements in the clay were observed after pile installation. The strength enhancement was considerably affected by consolidation effects, as well as the number of piles. For pile group installation, the dissipation of excess pore pressures between successive pile installations had a significant influence on the strength set-up effect. The numerical analysis work in this study comprises two phases. The first phase was undertaken to validate the proposed numerical approach for modeling deep penetration problems involving consolidation effects. For this phase, the study problem was selected as the penetration of the cone penetrometer under various rates. Coupled consolidation finite element analyses were carried out to simulate the deep cone penetration using ABAQUS/Standard V6.6. A wide range of penetration rates was considered to cover the full spectrum of consolidation or drainage conditions. vii As the penetration rate decreased, the transition from undrained to partially drained, and then to fully drained was clearly observed. The numerical results from the extremely fast and slow penetration, corresponding to the limiting undrained and drained conditions, compare favorably with various analytical and numerical solutions. The computed normalized backbone curve, which illustrates the effect of cone penetrate rate, was found to agree well with published centrifuge results. Using the hyperbolic curve fitting approach, a simplified procedure was proposed to derive the backbone curve for a soil with given strength and stiffness properties. The second phase of the numerical study uses the deep penetration modeling techniques established in the first phase to carry out finite element analysis of sand compaction pile installation. Reasonable agreement was obtained between the numerical results and those obtained from the centrifuge experiments. By carrying out additional parametric studies, the numerical results provide a comprehensive information database which describes changes in the strains, stresses, pore pressures, and strengths during and after pile installation. More importantly, the extent and magnitude of the strength set-up effect may be defined and quantified by the computed strength improvement radial profiles. A logarithmic function was proposed to approximate these strength improvement profiles, which uses two fitting parameters that are correlated with the soil’s properties. This led to the development of a simple and practical means for predicting the long-term strength increase due to the sand compaction pile installation. viii LIST OF TABLES Table 3.1 Centrifuge scaling rules (after Schofield, 1980 & 1988; Taylor, 1995) .82 Table 3.2 Properties of the kaolin clay (after Goh, 2003, Purwana et al. 2005) 82 Table 4.1 Centrifuge models test details 118 Table 4.2 Estimation of dimensionless time T 118 Table 4.3 Summary of Juneja’s (2002) centrifuge experimental information from selected tests . 118 Table 4.4 Cam-clay properties of kaolin clay (Goh, 2003) . 119 Table 5.1 Soil Properties for SR 18 and SR 49 (data from Kim (2005)) .161 Table 5.2 Back-fitted parameters and estimated soil properties for SR 18 and SR 49 161 Table 6.1 Summary of Juneja’s (2002) centrifuge experimental information from selected tests .208 ix rates of penetration, and (ii) simulation of sand compaction pile installation and the accompanying strength changes in the soil. The former was included herein to examine penetration rate effects and, more importantly, to validate the various numerical techniques proposed for modelling deep penetration problems with consolidation effects. By incorporating the coupled-consolidation and updated Lagrangian large-strain formulations, finite element analyses were carried out to study the cone response for a wide range of penetration rates. The numerical results compared favorably with analytical and other numerical solutions, as well as centrifuge experimental results. The use of the normalized back-bone curve allowed the results to be presented in a compact and meaningful way for studying the influence of penetration rate. The observed correlation between the back-bone curves and the soil properties led to the development of a practical method for deducing some key soil properties from penetration test results. The same numerical techniques used for modeling the deep cone penetration were adopted for simulating the sand compaction pile installation. The numerical results provided useful information on the response of various variables such as strains, stresses and pore pressures. The post-installation, post-consolidation strength field could also be deduced based on the computed effective stresses. Reasonable agreement was obtained between the computed and measured stress and pore pressure histories, as well as the strength improvement effect. The numerical results were post-processed to illustrate the magnitude and extent of SCP-induced strength improvement. It was found that the logarithmic function may be used to 232 characterize the variation of strength improvement ratio with radial distance from the pile axis. These findings were used to propose a practical approach for considering the strength set-up effect in sand compaction pile design. In summary, the findings from this research study may be expressed as follows: i) The installation of sand compaction pile was noticed to exert significant influence on the surrounding clay, where considerable changes in the radial stress and pore pressure were registered. Generally, the peak values of stress and pore pressure increases recorded during casing jack-in and withdrawal could be reasonably predicted by cavity expansion theories. The results from the present study using kaolin clay are generally consistent with those reported by Juneja (2002) using Singapore marine clay. ii) Substantial strength enhancements were observed in the soil after pile installation. The magnitude of the strength improvement was noted to be affected by consolidation effects, as well as the number of piles. By carrying out the tests for two different installation sequences, two distinct consolidation scenarios were produced which in turn led to different strength improvement behaviour. It was found that the timely dissipation of excess pore pressures between successive pile installations can enhance the set-up effect. Besides, increasing the number of piles can also result in additional strength improvement in the soil. iii) The finite element analyses was able to reasonably replicate the response of the cone penetrometer for a wide range of penetration rates, associated with 233 the full spectrum of consolidation conditions. The calculated results form the limiting drained and undrained results were noted to agree well with published analytical solutions. Both the drained and undrained net cone resistance increased with the increasing modulus ratio and friction angle. On the other hand, the ratio of the drained to undrained net cone resistance appeared to be quite insensitive to the friction angle, but increased with the modulus ratio. iv) By plotting the computed cone resistances from different penetration rates in the form of the normalized back-bone curve, the influence of cone penetration rate was clearly illustrated. Favorable agreement was observed between the computed back-bone curve and the centrifuge experimental results of Randolph and Hope (2004). In addition, the characteristic back-bone curves were found to be influenced by the local strength and stiffness properties of the soil in the immediate vicinity of the cone tip. Based on the numerical results, a procedure was developed from which important soil properties such as the modulus ratio, the angle of friction and the coefficient of consolidation can be deduced from the cone penetration test results. Its practical application was illustrated through a field example. v) The numerical simulation of sand compaction pile installation provided useful information on how the stresses, strains and pore pressures change during and after the pile installation. The strength fields could also be inferred from the computed effective stresses. The calculated radial stress and pore pressure histories compared well with the present experimental results and Juneja 234 (2002)’s centrifuge measurements. Furthermore, the deduced short-term and long-terms strength profiles were also in favorable agreement with the experimental measurements. vi) The strength improvement radial profile (Isu~r/Rs curve) quantified the magnitude and extent of the SCP-induced strength improvement in the soil. It was found that significant strength improvement was approximately concentrated within the plastic zone defined by the cavity expansion theory. The Isu~r/Rs curve may be approximated by the logarithmic function, whose fitting parameters were correlated to the soil’s stiffness and strength properties. Based on results from the numerical parametric studies, a simplified procedure was proposed to estimate the magnitude of SCP-induced strength enhancement. This provides a means for incorporating the set-up effect in the design of sand compaction piles, 7.2 Recommendations for future research The following issues may be examined in future research work in this area: i) There is a need to investigate the effect of increasing confinement on the strength gain in sand. As discussed in the earlier chapters, the installation process of sand compaction pile involves displacing the soft clay with sand, which would lead to a substantial stress and subsequent post-consolidation strength set-up in clay. The 'improved' clay in turn increases the lateral confinement on the sand pile itself. Such enhancement will result in 235 additional strength improvement in the sand column, thus contributing to the overall strength of the improved composite ground. This effect, which was not considered in the present study, should be examined in a future study so as to obtain a more complete picture of the overall strength improvement characteristics. ii) Another pertinent issue is the stress concentration ratio n, which helps to evaluate the relative strength contribution of sand column and clay in the composite ground. Thus far, the estimation of stress concentration ratio is largely empirical or based on field measurement data. There remains a lack of reliable approach to predict the stress concentration ratio for various replacement ratios. 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(Kitazume, 2005) In Singapore, sand compaction piles were adopted in land reclamation projects, such as those at Marina Bay, Tanjong Rhu and Tuas (e.g Wei & Khoo, 1992; Wei et al., 1995) In addition, sand compaction piling was also used in the constructions of port and harbor facilities in Singapore For instance, sand compaction piles with diameters of 2m and area replacement ratio of 70% were used in the... Background Construction works in soft grounds often encounter problems originating from weak engineering properties of soft soils such as low bearing capacity, excessive settlements and ground movements Various ground improvement methods are thereby developed and implemented to treat soft soil, one of which is the sand compaction pile (SCP) method The method of sand compaction pile improves weak soil stratums... the sand piles itself, the overall shear strength of the composite ground being taken to be some weighted average of that of the sand pile and the in-situ strength of the soft clay This often led to rather conservative design wherein relatively high area replacement ratios up to 80% were used (e.g Kitazume, 2005) Partly because of this, SCP has not found wider usage in other types of construction and. .. in the ground improvement of soft clayey or loose sandy soil, to increase bearing capacity, reduce settlement, enhance stability and even prevent liquefaction (for loose silty or sandy deposits) in seismic areas This chapter presents a literature review on the design principles for sand piles and sand columns and previous studies The first part of the chapter is a review of existing design methodologies... number of factors which, as summarized by Terashi et al (1991b), include: i) the shear strength of sand piles as well as the strength profile of the soft clay, ii) the area replacement ratio “as” which is defined as the ratio of the area occupied by sand piles to the overall area of the improved ground, iii) the geometric conditions such as the ratio of the width of improved area over the width of foundation,... 12 A, Ac and As the cross sectional areas of unit cell, as well as those of the clay and the sand pile within the unit cell, c and s the vertical stress on the clay and sand pile respectively, and as the area replacement ratio, which is defined as As/A Owing to the stiffness disparity, stress concentration is expected to be present in the unit cell, with more stress on the stiff sand column and less... foundation, 11 iv) the ratio of the length of sand piles over the thickness of soft soil layer, and v) the external loading conditions (e.g loading rate, load eccentricity and inclination) A number of design approaches have been proposed in the literature to evaluate the bearing capacity, settlement and stability of the composite ground Due to the complexity of the problem, most of these methods are developed... Extension of the numerical techniques used in the cone penetration 6 problem to simulate the short-term and long-term effects due to SCP installation v) Use of centrifuge model data obtained in this study and those obtained by Juneja (2002) to benchmark numerical analyses of SCP installation process vi) Numerical parametric studies on the effects of various factors on the state of the surrounding soft clay... (after Kitazume, 2005) Vibro Hopper Sand Casing pipe Setting Sand Penetration Forming the Pile and Compact Figure 1.4 Compozer method of the SCP installation (after Aboshi & Suematsu, 1985) 9 Figure 1.5 Non-vibratory SCP installation (after Tsuboi et al., 2003) 10 Chapter 2: LITERATURE REVIEW 2.1 Introduction As a versatile ground improvement method, the sand compaction pile (SCP) method has been widely... applications in both on-land and near-shore projects such as constructions of building foundation, embankment, port and harbor facilities and sea revetment (e.g Aboshi et al., 1979; Moroto & Poorooshasb, 1991; Kitazume, 2005) As Figure 1.1 indicates, the cumulative length of sand compaction 1 pile in Japan increased rapidly in the last several decades and reached up to 350 thousand kilometer in 2001 (Kitazume, . CENTRIFUGE AND NUMERICAL MODELLING OF SAND COMPACTION PILE INSTALLATION YI JIANGTAO (B.Eng.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF. radial stresses and pore pressures in soft clays during and after the in-flight installation of sand compaction piles. It was noted that the measured peak increases in stress and pore pressure. influence of sand compaction piling, particularly the resulting strength set-up in the adjacent clay. The study comprises both centrifuge experimental and numerical modelling. The centrifuge

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