Centrifuge model study on spudcan pile interaction

CENTRIFUGE MODEL STUDY ON SPUDCAN-PILE INTERACTION XIE YI NATIONAL UNIVERSITY OF SINGAPORE 2009 CENTRIFUGE MODEL STUDY ON SPUDCAN-PILE INTERACTION XIE YI (B.Eng., ZJU) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 ACKNOWLEDGEMENTS It has been a great pleasure for me to have the opportunity to pursue my graduate study in the Centre for Soft Ground Engineering and Centre for Offshore Research and Engineering at National University of Singapore (NUS). First I would like to express my sincere gratitude to my supervisors, Professor Leung Chun Fai and Professor Chow Yean Khow for their continuous guidance and support throughout the course of research. Their invaluable comments, patience and encouragement are highly appreciated. I would like to acknowledge the financial support of NUS research scholarship and the sponsorship from the spudcan-pile Joint Industry Project (JIP) initiated by NUS, ExxonMobil, Keppel, Shell, TOTAL and ABS. I am grateful for the kind suggestions given by Tho Kee Kiat who worked together with me in this JIP and helped to carefully review this thesis. Special thanks are also given to Patrick Wong of ExxonMobil, Okky Purwana of Keppel and Zhang Xiying of ABS who have given me valuable advice as well as helped me to link the academic research findings to practical applications. I am deeply grateful to Prof. Phoon Kok Kwang and Dr. Chew Soon Hoe as my research committee members for their helpful inputs and I have learnt a lot from attending their lectures, too. In addition, thanks should definitely be given to the technical staff of the NUS Geotechnical Laboratory: Wong Chew Yuen, Tan Lye Heng, Jamilah, Shaja, Loo Leong Huat and Choy Moon Nien. Without their help, the centrifuge model tests could not have been accomplished. The members of our offshore geotechnical group should never be forgotten for their great help and friendship: Okky Purwana, Teh Kar Lu, Zhou Xiaoxian, Gan Cheng Ti, Sindhu, Eddie and Xue Jing. Special thanks to Okky Purwana (again) and Teh Kar Lu for their selfless and numerous advice throughout my experiments and analysis of results. Great appreciation is also given to the members of our research group on piles: Shen Ruifu and Ong Chee Wee. I deeply enjoyed the discussions with them. The latter i always shares his feelings with me and his optimistic attitude encourages me a lot. I wish to thank Dr. Dave White of University of Western Australia (UWA) for his permission to use the PIV software and valuable input on the PIV analysis. I am also grateful to the industry expert Colin Nelson of Transocean for his inputs when I was just touching this spudcan-pile area and Dr. Wang Jianguo of UWA for his kind advice on my research. In addition, thanks are also due to Professor Mark Cassidy and Dr. Susan Gourvenec of UWA, Dr. Johnny Cheuk of University of Hong Kong and Professor Sarah Springman of Swiss Federal Institute of Technology Zurich, for their discussions with me when they came to visit NUS. I would like to thank other previous and current friends at NUS: Zhang Yaodong, Pang Chin Hong, Chen Xi, Phoon Hung Leong, Cheng Yonggang, Yi Jiangtao, Yang Haibo, Subhadeep, Ye Feijian, Yeo Chong Hun, Wang Lei, Xiao Huawen, Chen Jian, Sun Jie, Wu Jun, Krishna, etc. I would never forget those seniors who have left the campus but are still willing to share their experiences with me in any aspects of life. My great appreciation is also given to Professor Tang Xiaowu of Zhejiang University who led me to the research in geotechnical engineering. His invaluable encouragement during those few years will never be forgotten. Lastly, I want to specially thank my parents for their love, support and blessing everyday throughout the course of my studies. ii TABLE OF CONTENTS ACKNOWLEDGEMENTS i TABLE OF CONTENTS iii SUMMARY ix LIST OF TABLES xi LIST OF FIGURES xii LIST OF SYMBOLS xxv Chapter – Introduction 1.1 Mobile Jack-Up Rigs and Spudcan Foundations 1.2 Permanent Jacket Platforms and Pile Foundations 1.3 Interaction between Spudcan and Piles 1.4 Objectives and Scope of Study 1.5 Outline of Thesis Chapter – Literature Review 2.1 Introduction 15 2.2 Soil Flow Mechanism during Installation and Extraction of Footings 15 2.2.1 2.2.2 Installation of footing 16 2.2.1.1 Single soft soil profile 16 2.2.1.2 Soft overlying stiff soil profile 19 Extraction of footing 20 2.3 Effect of Lateral Loading on Piles 22 2.3.1 Active pile 22 2.3.2 Passive pile 24 iii 2.3.3 Limiting soil pressure on active and passive piles 2.4 Effect of Vertical Loading on Piles 25 26 2.4.1 Pile subjected to downward soil movement 26 2.4.2 Pile subjected to upward soil movement 27 2.4.3 Interaction between lateral and vertical loading on piles 29 2.5 Effect of Spudcan on Adjacent Piles 30 2.5.1 Spudcan resting on soil surface 32 2.5.2 Spudcan penetrating deeply in soil 34 2.5.2.1 Experimental studies 34 2.5.2.2 Numerical studies 37 2.5.3 Spudcan operation 38 2.5.4 Spudcan extraction and post-spudcan extraction 39 2.5.5 SNAME (2002) 41 2.6 Effect of Pile on Adjacent Piles 41 2.7 Summary 43 Chapter – Experimental Setup and Procedures 3.1 Introduction 76 3.2 Experimental Modeling Concepts 76 3.2.1 3.2.2 Centrifuge modeling 76 3.2.1.1 Why centrifuge? 76 3.2.1.2 Centrifuge scaling relationships and model error 77 3.2.1.3 NUS Geotechnical Centrifuge 78 Deformation measurement technique 79 3.3 Experimental Set-Up (Full-Spudcan Test) 80 3.3.1 Model container and loading actuators 81 3.3.2 Model spudcan 82 3.3.3 Model piles and pile caps 82 3.3.3.1 In free-headed pile tests 82 3.3.3.2 84 3.3.4 In fixed-headed pile tests Sensors 3.4 Experimental Set-Up (Half-Spudcan Test) 85 87 iv 3.5 Soil Sample 89 3.5.1 Soil preparation 89 3.5.2 Shear strength profile 90 3.6 Data Acquisition and Control Systems 92 3.6.1 Data acquisition 92 3.6.2 Servo-controlled loading system 92 3.7 Experimental Procedure 3.7.1 3.7.2 93 Experimental procedure (full-spudcan test) 94 3.7.1.1 94 Pile installation 3.7.1.2 Spudcan penetration 96 3.7.1.3 Spudcan operation and extraction 97 Experimental procedure (half-pudcan test) 97 Chapter – Soil Flow Mechanism for Spudcan in Soft Clay 4.1 Introduction 113 4.2 Test Program 113 4.3 Soil Responses with Spudcan in Single Clay Layer 115 4.3.1 Load-displacement curve 115 4.3.2 Failure mechanism around spudcan 116 4.3.2.1 Spudcan penetration 116 4.3.2.2 Spudcan operation 119 4.3.2.3 Spudcan extraction 121 4.3.3 Effect of spudcan penetration depth 123 4.3.4 Soil movements at pile location 124 4.3.4.1 Spudcan penetration 124 4.3.4.2 Spudcan operation and extraction 128 4.4 Soil Responses with Spudcan in Clay/Sand Layered Profile 129 4.4.1 Load-displacement curve 129 4.4.2 Failure mechanism around spudcan 130 4.4.2.1 Spudcan penetration 130 4.4.2.2 Spudcan operation 135 4.4.2.3 Spudcan extraction 135 v 4.4.3 4.4.4 Soil movements at pile location 138 4.4.3.1 Spudcan penetration 138 4.4.3.2 Spudcan extraction 142 Effect of pile presence on soil movement 142 4.5 Comparison with Previous Studies 144 4.5.1 Comparison with studies on shallow footing induced soil movement 144 4.5.2 Comparison with studies on pile induced soil movement 4.6 Summary 145 147 Chapter – Effect of Spudcan on Free-Headed Pile 5.1 Introduction 217 5.2 Typical Test Results (Test F1) 217 5.2.1 Lateral pile responses 218 5.2.1.1 Spudcan penetration 218 5.2.1.2 Spudcan operation 222 5.2.1.3 Spudcan extraction 223 Axial pile responses 225 5.2.2.1 Residual load before spudcan penetration 226 5.2.2.2 Spudcan penetration 227 5.2.2.3 Spudcan operation 230 5.2.2.4 Spudcan extraction 231 5.3 Test Series Fa: Effect of Pile Installation Method 235 5.2.2 5.3.1 Lateral pile responses 235 5.3.2 Axial pile responses 236 5.4 Test Series Fb: Effect of Spudcan-Pile Clearance 237 5.4.1 Lateral pile responses 237 5.4.2 Axial pile responses 239 5.5 Test Series Fc: Effect of Operation Period 241 5.5.1 Lateral pile responses 241 5.5.2 Axial pile responses 243 5.6 Test Series Fd: Effect of Preload Ratio 5.6.1 Lateral pile responses 244 245 vi 5.6.2 Axial pile responses 5.7 Summary 246 247 Chapter – Effect of Spudcan on Fixed-Headed Pile 6.1 Introduction 279 6.2 Typical Test One: Spudcan Far above Underlying Sand Layer (Test A12) 279 6.2.1 6.2.2 Lateral pile responses 280 6.2.1.1 Spudcan penetration 280 6.2.1.2 Spudcan operation 283 6.2.1.3 Spudcan extraction 284 Axial pile responses 285 6.2.2.1 Residual load before spudcan penetration 285 6.2.2.2 Spudcan penetration 286 6.2.2.3 Spudcan operation 287 6.2.2.4 Spudcan extraction 288 6.3 Typical Test Two: Spudcan Close to Underlying Sand Layer (Test A5) 6.3.1 6.3.2 289 Lateral pile responses 289 6.3.1.1 Spudcan penetration 289 6.3.1.2 Spudcan operation and extraction 292 Axial pile responses 293 6.4 Test Series A: Effect of Pile Embedded Length in Clay 294 6.4.1 Lateral pile responses 294 6.4.2 Axial pile responses 296 6.5 Test Series B: Effect of Pile Installation Method 297 6.5.1 Differences in pile responses 297 6.5.2 Stress state surrounding pile 298 6.5.2.1 Spudcan penetration 299 6.5.2.2 Spudcan operation and extraction 302 6.6 Test Series C: Effect of Spudcan Size 303 6.7 Test Series D: Effect of Spudcan-Sand Layer Clearance 304 6.8 Test Series E: Effect of Pile Socket Length in Sand 307 6.9 Test Series F: Effect of Soil Squeezing 309 vii 6.10 Test Series G: Effect of Spudcan-Pile Clearance 311 6.11 Test Series H: Effect of Operation Period 314 6.12 Summary 315 Chapter – Conclusions 7.1 Introduction 361 7.2 Summary of Findings 361 7.2.1 Soil failure mechanism 361 7.2.2 Effect of spudcan on free-headed pile 363 7.2.3 Effect of spudcan on fixed-headed pile 365 7.3 Practical Implications 368 7.4 Recommendation for Further Studies 370 REFERENCES 372 viii Chapter Conclusions usually just a small fraction of the pile length and therefore the pile behaves more like a propped cantilever beam. With respect to the axial pile responses, a longer pile would have a smaller increase in the compressive axial force during spudcan penetration. In the case that spudcan penetrating to the same soil depth with different clearances between the base of the spudcan and the underlying stiff sand layer (test series D), a smaller clearance between the base of the spudcan and the sand layer leads to a smaller magnitude of induced bending moment on the pile socketed into the sand layer. The study on the effect of pile socket length in sand in test series E shows that with a longer socket length, the induced maximum bending moment during spudcan penetration decreases. However, beyond a certain critical socket length of approximately 2.4 times pile diameter, the incremental benefits of a longer socket length diminish. In a layered soil profile consisting of clay overlying sand, an increase in lateral soil movement due to soil squeezing as the spudcan approaches the clay-sand interface is thought to increase the induced lateral loads on the adjacent floating pile. However, if the pile is directly socketed into the underlying sand layer as commonly adopted in practice, the restraints provided by the dense sand layer, occurring simultaneously with the soil squeezing effect, acts to reduce the induced moment. The net effect as observed from test series D is that the maximum pile bending moment actually decreases with decreasing clearance between the spudcan and sand layer. As an attempt to isolate the effect of soil squeezing from the restraint on the pile socket segment by the sand layer (test series F), it is observed that induced bending moment on the floating pile is greater in the presence of an underlying stiff sand layer as 367 Chapter Conclusions opposed to the case of single clay profile, despite the increase is only 7%. Therefore, through interpretation of results from these parametric studies, it is established that for socketed pile, the upper soft clay layer with a greater thickness (i.e. longer pile embedded length in overlying soft clay) is more critical, while for non-socketed pile, clay/sand layered soil profile associated with soil squeezing effect is more onerous. However, it should be noted that the study on the non-socketed pile case is only for academic purpose, since the pile should be socketed into the underlying sand layer if the pile toe is close to it. As intuitively expected and similar to that in the free-headed pile, the induced bending moment in the pile increases with decreasing clearance between the pile and the spudcan (test series G). It is found that in single clay layer, at a very close spudcan-pile clearance of quarter spudcan diameter, the pile bending stress is over 85% of the pile yield stress and hence the pile performance is heavily affected. In contrast to the free-headed pile responses during spudcan extraction, a longer operation period induces a smaller bending moment in a fixed-headed pile. The magnitudes of bending moment are further found to be much smaller than those during spudcan penetration, regardless of the duration of operation period. 7.3 Practical Implications This study systematically investigated spudcan-induced pile responses under different scenarios. Study on free-headed pile revealed the basic pile bending mechanism, and suggested patterns and magnitudes of lateral pile deflections. On the other hand, the fixed-headed pile responses provided insights on the degree of disturbance to the lateral pile performance. The results have shown some potential cases where 368 Chapter Conclusions fixed-headed piles may be loaded laterally to quite a large increase as to post some danger to the pile performance and in the most svere condition, the induced bending stress can reach as high as over 85% of the pile yield stress. The latter induced bending stress in fixed-headed pile is usually more critical compared to free-headed pile, as in practice the pile head is restrained by the upper jack-up leg, and thus the actual lateral pile deflection and rotation should be much less than those measured in the present free-headed pile tests. Axial pile responses are less critical compared to the lateral since there is no major increase in compressive axial pile stress induced by spudcan penetration and extraction. It should be noted that the actual pile responses in the field with a partially fixed head are expected to lie in-between the two extremes, i.e. free-headed pile and fixed-headed pile. For floating pile, the difference in pile length could change the pile bending pattern as well as bending moment magnitudes as demonstrated in test series A. This test series on a wide range of pile lengths is expected to provide the potential stress in the pile for the designers to check the pile stability of various lengths. With regard to a pile resting or socketed into the underlying stiff layer, the soil squeezing effect during spudcan penetration is found to be insignificant to the pile responses. However, the socket length in stiff layer, the pile embedded length in soft clay are established to play an important role, as established from test series D to F. Again, the effect of spudcan-pile clearance for both single soft clay and soft overlying stiff soil profile has been evaluated (test series Fb and G), which is deemed to be the most important parameter. Engineers should seriously pay attention, and properly locate the spudcan at a suitable distance from the adjacent pile, without causing any 369 Chapter Conclusions undesirable stress on piles. Soil movements captured in half-spudcan tests successfully reveal the soil failure mechanism. Numerical methods, such as finite element method, finite difference method and boundary element method coupled with beam-column model could be used to evaluate the pile responses from the measured free-field soil movement as a preliminary estimation, as explained in Section 2.3.2. However, the distortion of the soil during spudcan penetration/extraction should be carefully taken into account. 7.4 Recommendation for Further Studies The present research investigated several fundamental behaviors of spudcan-pile interaction. From the insights obtained in this study, the following topics are recommended for further studies: 1. The centrifuge study presented in this dissertation focused on spudcan-pile interaction problems in soft clay and soft clay overlying sand layer. Considerable scope exists for expanding the study to other stratified soil profiles. 2. In practice the spudcan penetration can be completed in a relatively calm environment. However, during spudcan operation, environmental or storm loading is inevitable. Hence, cyclic loading as well as combined lateral and vertical loading on spudcan and pile during spudcan operation could be further modeled. The cyclic loading on spudcan is expected to cause remolding in the adjacent soil and further affect the adjacent pile responses. Lateral and vertical loading on a pile may have a significant effect on the pile performance, if combined with the pile lock-in stresses induced during spudcan penetration. 370 Chapter Conclusions 3. A certain amount of lock-in bending stress is observed after spudcan extraction. This stress should be considered in design combined with potential environmental and storm loadings after a jack-up rig leaves the location. The pile responses under constant horizontal loading at the pile head after spudcan extraction have been studied by Stewart (2005) without considering the lock-in pile bending stresses as well as the duration of soil consolidation after spudcan extraction. Accordingly, experiments could be conducted to apply designed horizontal constant and cyclic loading on the pile head at different periods after spudcan extraction. These loadings can be in different directions relative to the spudcan footprint. 4. It is common that the foundation under each platform leg consists of a pile group. Therefore, parametric studies could be carried out to investigate the interaction between piles in a pile group as the adjacent spudcan penetrates and extracts. 5. Vertical piles were employed in the present study. However, raked piles are also common in practice, and the pile inclination could provide greater resistance against lateral loading. Moreover, this inclination could cause the spudcan-pile clearance to vary as the spudcan penetrates deeply in soft clay. Further study on the raked piles could enable the comparison with the responses of vertical piles and shed light on the effect of pile inclination. 371 REFERENCES Adams, J.I. and Radhakrishna, H.S. (1973). The lateral capacity of deep augured footings. Proc. 8th Int. Conf. Soil Mech. and Found. Eng., Moscow, 2, pp. 1-8. Adrian, R.J. (1991). 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Numerical solution for laterally loaded piles in a two-layer soil profile. J. Geotech. Geoenviron. Engrg., 132(11), pp. 1436-1443. Young, A.G., Remmes, B.D. and Meyer, B.J. (1984). Foundation performance of offshore jack-up drilling rigs. J. Geotech. Engrg. Div., 110(7), pp. 841-859. Yu, H.S. (2000). Cavity expansion methods in geomechanics, Dordrecht: Kluwer Academic Publishers. 382 [...]... potential spudcan- pile interaction problem and provide rational suggestions for practical pile foundation design under such a situation 6 Chapter 1 Introduction Owing to complexity and difficulties in numerical analysis of large soil deformation during spudcan penetration/extraction, the present study concentrated on the physical modeling of the spudcan- pile interaction The tests were carried out on the... Geotechnical Centrifuge to simulate the installation of piles and spudcan, as well as the extraction of spudcan, using appropriate control modes Spudcan operation was simplified as the period with a constant vertical loading on the spudcan Only vertical piles were investigated in the present study As piles are partially fixed at the pile head in practice, both free head (free rotation and free deflection) and... study the effect of soil squeezing on pile responses when a spudcan approached the underlying sand layer Several contributing factors such as spudcan- pile clearance, pile socket length in sand, and spudcan 7 Chapter 1 Introduction operation period were also examined To facilitate the study on pile responses, the free field soil movements and soil flow mechanisms during spudcan penetration/extraction... National University of Singapore (NUS) to investigate the spudcan- pile interaction mechanism The study is a part of a joint industry project sponsored by ExxonMobil, Keppel, Shell, TOTAL and ABS The aim of this study was to enhance the understanding of the interaction mechanism between spudcan and adjacent pile foundations Specifically, the study intended to: a) study the lateral and axial pile responses... other, the pile under each leg can be simplified as a single pile, and the interaction between the piles mainly through the connection provided by 4 Chapter 1 Introduction the tubular jacket above can be simplified as the pile with a certain degree of restraint at the pile head In addition, it is generally accepted that only the nearest spudcan is considered to have an effect on the working condition of... Schematic of model set-up for free-headed pile tests (all dimensions in mm) Figure 3.4 Photograph of model set-up for free-headed pile tests Figure 3.5 Model spudcan for free-headed pile tests (all dimensions in mm) Figure 3.6 Model spudcan for fixed-headed pile tests (all dimensions in mm) Figure 3.7 Instrumented pile showing elevation of strain gauges and pile hook assembly for free-headed pile tests... vertical pile head movement during spudcan extraction (test F1) Figure 5.27 Unit shaft friction along the pile during spudcan extraction (test F1) Figure 5.28 Schematic diagram of spudcan extraction mechanism Figure 5.29 Development of incremental axial force during spudcan extraction (test F1) Figure 5.30 Comparison of bending moment between high-g and 1-g pile installation during spudcan penetration (series... superposition: (a) residual load distribution before spudcan penetration; (b) load transfer after pile installation; (c) downdrag load due to soil reconsolidation (test A12) Figure 6.8 Induced pile axial force during spudcan penetration (test A12) Figure 6.9 Induced pile axial force during spudcan unloading and operation (test A12) Figure 6.10 Induced pile axial force during spudcan extraction (test... fixed head (no rotation and no deflection) conditions were adopted to simulate the two extreme pile head conditions The actual pile responses in the field are expected to lie in-between the two extremes The effects of spudcan penetration and extraction on both lateral and axial responses of a single vertical pile were examined in isolation of all other loading cases such as environmental and storm loads... factor for pressure on a plate k Permeability N Ratio of centrifugal to the gravitational acceleration P Soil loading on pile pfront Front soil pressure on pile prear Rear soil pressure on pile q Uniform pressure on a plate R Centrifuge radius RB Centrifuge radius from the axis of rotation to the sample base xxv Re Effective centrifuge radius s Depth in sand layer t Mobilized shaft friction t1, t2 Time . CENTRIFUGE MODEL STUDY ON SPUDCAN-PILE INTERACTION XIE YI NATIONAL UNIVERSITY OF SINGAPORE 2009 CENTRIFUGE MODEL STUDY. Introduction 76 3.2 Experimental Modeling Concepts 76 3.2.1 Centrifuge modeling 76 3.2.1.1 Why centrifuge? 76 3.2.1.2 Centrifuge scaling relationships and model error 77 3.2.1.3 NUS Geotechnical Centrifuge. interaction problem is hence necessary. In the present study, the penetration and extraction effects of a spudcan on adjacent single piles were studied by means of a series of small-scale model