EXPERIMENTAL AND NUMERICAL MODELLING OF SPUDCAN PENETRATION IN STIFF CLAY OVERLYING SOFT CLAY SINDHU TJAHYONO NATIONAL UNIVERSITY OF SINGAPORE 2011 EXPERIMENTAL AND NUMERICAL MODELLING OF SPUDCAN PENETRATION IN STIFF CLAY OVERLYING SOFT CLAY SINDHU TJAHYONO (B.Eng., NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Acknowledgements First and foremost, I thank my advisers Prof Leung Chun Fai and Prof Chow Yean Khow for their constant support and critical guidance throughout the course of my PhD study. Without them, this thesis would not materialise. I also acknowledge the financial support in the form of Research Scholarship provided by NUS for my PhD study. I also thank Dr Tho Kee Kiat for his valuable feedbacks especially on the numerical part of this thesis. And not forgetting the centrifuge and geotechnical lab staff Mr Wong Chew Yuen, Mr Tan Lye Heng, Dr Shen Rui Fu, Mr John Choy, Mdm Jamilah and Mr Lam Foo for their assistance in the experimental work presented in this thesis. Thanks also to fellow members of the offshore geotechnical research group including Prof Palmer, Dr Purwana Okky, Dr Zhou Xiaoxian, Dr Xie Yi, Dr Teh Kar Lu, Dr Gan Cheng Ti, Eddy Hu, Cisy and Zongrui, for their constant encouragements and enriching communications. And also to the rest of fellow geotechnical researchers: Dr Chin Keng Ghee, Dr Pang Chin Hong, Dr Cheng Yonggang, Dr Karthikeyan, Dr Chen Xi, Dr Phoon Hung Leong, Dr Zhang Xi Ying, Dr Yi Jiangtao, Dr Ong Chee Wee, Dr Yang Haibo, Dr Karma, Dr Banerjee Subhadeep, Dr Xiao Huawen, Dr Chaudhary Krishna, Dr Tan Andy, Xue Jing, Chong Hun, Czhia Yheaw, Liang Wei, Hartono, Ay Lee, and others. Their friendships have made my research life that much enjoyable. And lastly, to my family for always being there for me. This thesis is dedicated to them. i Table of Contents Acknowledgements i Summary .vii List of Tables ix List of Figures xi List of Symbols .xvii Chapter Introduction . 1.1 Background . 1.2 Objectives of present study 1.3 Outline of thesis Chapter Literature Review 2.1 Introduction 2.2 Two-layer clay in practice 2.3 Bearing capacity solutions 11 2.4 Existing experimental studies on footing penetration in two-layer clay 12 2.5 Existing numerical studies on footing penetration in two-layer clay . 15 2.6 Existing design solutions for spudcan load-penetration response in two-layer clay 18 2.6.1 SNAME (2002)’s guideline 18 2.6.2 Hossain & Randolph (2009a)’s method 20 2.7 Spudcan penetration in sand overlying clay . 25 2.8 Experimental and numerical considerations . 28 2.8.1 Centrifuge modelling 28 2.8.2 Preparation of two-layer clay specimens 28 2.8.3 Soil deformation measurement using particle image velocimetry (PIV) 31 2.8.4 ALE versus Eulerian finite element methods . 32 2.9 Concluding remarks 34 Chapter Experimental Setups and Procedures 45 3.1 Introduction 45 3.2 Centrifuge model setups . 46 iii 3.3 Specimen preparation . 48 3.4 Test procedures . 50 3.5 Properties of clay specimens 51 3.5.1 Basic properties 51 3.5.2 CIU test results for cement-treated clay samples 51 3.5.3 CIU test results for kaolin clay samples . 54 3.5.4 In-flight T-bar penetration test results 55 3.5.5 Additional test results . 58 3.5.6 Summary of specimen properties . 60 Chapter Measured Response during Spudcan Penetration in Two-Layer Clay . 71 4.1 Introduction 71 4.2 Measured spudcan load-penetration response in two-layer clay 71 4.2.1 Test repeatability 71 4.2.2 ‘Series 1’ tests . 72 4.2.3 ‘Series 2’ tests . 74 4.3 Comparison of peak resistance with existing solutions 75 4.3.1 Comparison for ‘Series 1’ tests . 75 4.3.2 Comparison for ‘Series 2’ tests . 78 4.4 Observed soil failure mechanisms during spudcan penetration in two-layer clay . 80 4.4.1 Evolution of failure mechanisms during penetration 80 4.4.2 Effects of thicknesses of upper soil layer . 83 4.4.3 Effects of strength versus depth profiles of lower soil layer 88 4.4.4 Limiting cavity depth above penetrating spudcan 89 4.4.5 Comparison with sand overlying soft clay . 90 4.5 Further discussions . 92 4.6 Summary 96 Chapter Numerical Analysis of Spudcan Penetration in Two-Layer Clay . 123 5.1 Introduction 123 5.2 Brief description of Eulerian finite element method 124 5.3 Numerical model 125 5.4 Preliminary analyses . 126 5.4.1 Tresca versus von Mises models 127 5.4.2 Effects of penetration rate . 128 5.4.3 Effects of mesh fineness . 128 5.4.4 Effects of model boundary 129 iv 5.4.5 Effects of spudcan base inclination angle . 130 5.4.6 Effects of spudcan surface roughness . 130 5.4.7 Effects of E/cu . 131 5.5 Verification of model . 131 5.5.1 Single-layer weightless clay model 131 5.5.2 Two-layer clay model: Hossain & Randolph (2010a; 2010a)’s experiments . 132 5.5.3 Two-layer clay model: present experiments . 133 5.6 Parametric studies . 136 5.6.1 Effects of H/B . 137 5.6.2 Effects of cu2/cu1 137 5.6.3 Effects of γ1′/γ2′ . 138 5.6.4 Effects of cu2/(γ2′B) . 139 5.6.5 Effects of strain-softening behaviour of crust material . 139 5.7 Summary . 143 Chapter Design Method for Estimating Spudcan Load-Penetration Response in Two-Layer Clay 171 6.1 Introduction 171 6.2 Design method 172 6.2.1 Limiting cavity depth 172 6.2.2 Spudcan load-penetration response 175 6.3 Comparisons with existing data 179 6.3.1 Hossain & Randolph (2010a; 2010b)’s experimental data . 179 6.3.2 Present experimental data . 180 6.3.3 Field data reported by Kostelnik et al. (2007) 182 6.4 Comparison for a test case with a low H/B value . 183 6.5 Summary . 183 Chapter Conclusions . 197 7.1 Summary of findings 197 7.2 Areas for further study 201 References 203 Appendix A 209 v Chapter Design Method for Estimating Spudcan Load-Penetration Response in Two-Layer Clay 0.0 40 Spudcan resistance (kPa) 80 120 160 200 240 0.0 250 Test F3b (H/B = 0.31) 0.2 0.2 0.4 0.4 D/B D/B Test F2 (H/B = 0.16) 0.6 Experimental data 0.8 Spudcan resistance (kPa) 50 100 150 200 0.6 0.8 Present FE results Present design method Hossain & Randolph (2009a)'s design method 1.0 1.0 SNAME (2002)'s design method 1.2 0.0 1.2 Spudcan resistance (kPa) 40 80 120 160 200 240 280 0.0 Test F4 (H/B = 0.50) 0.2 0.2 0.4 0.4 0.6 D/B D/B 0.6 0.8 0.8 1.0 1.0 1.2 194 1.2 1.4 40 Spudcan resistance (kPa) 80 120 160 200 240 280 320 Chapter Design Method for Estimating Spudcan Load-Penetration Response in Two-Layer Clay 0.0 40 Spudcan resistance (kPa) 80 120 160 200 240 280 320 360 Test F6 (H/B = 1.00) 0.2 0.4 D/B 0.6 0.8 1.0 1.2 1.4 1.6 Figure 6.7. Comparison of present design method with present experimental data 195 Chapter Design Method for Estimating Spudcan Load-Penetration Response in Two-Layer Clay 0.0 50 Spudcan resistance (kPa) 100 150 200 250 300 350 400 Measured punch-through resistance 0.2 0.4 D/B 0.6 0.8 Present design method Hossain & Randolph (2009a)'s design method 1.0 SNAME (2002)'s design method 1.2 1.4 Figure 6.8. Comparison of present design method with field data reported by Kostelnik et al. (2007) -100 0.0 0.1 -50 Spudcan resistance (kPa) 50 100 150 200 250 B = 10 m H/B = 0.1 cu1 = 80 kPa cu2 = 20 kPa γ1′ = kN/m3 γ2′ = kN/m3 D/B 0.2 0.3 Present design method Hossain & Randolph (2009a)'s design method SNAME (2002)'s design method 0.4 0.5 Figure 6.9. Comparison between design methods for a test case with a low H/B value 196 Chapter 7.1 Conclusions Summary of findings This thesis presents the results of centrifuge tests as well as numerical analysis of spudcan penetration in two-layer clay. The main contributions of the present study beyond the findings of Hossain & Randolph (2010a; 2010b) are summarised as follows. (a) The measured spudcan load-penetration response is shown to change from one that exhibits post-peak reduction in load with penetration to one that exhibits monotonic increase in load with penetration when H/B decreases below a certain critical value. For ‘Series 1’ tests in this study, the critical value is approximately 0.3. This implies that 1) the potential for punch-through decreases as the upperlayer thickness decreases, and 2) there is a certain upper-layer thickness below which punch-through becomes improbable. (b) The observed soil failure mechanisms during spudcan penetration for H/B between 0.14 and 1.04 are similarly characterised by punching failure in the crust layer and the resultant trapping of a crust plug beneath the spudcan, coupled with local or punching shear failure in the soft clay layer. However, the horizontal and upward displacements in the soft clay decreases with larger H/B, which could be attributed to 1) decreasing distortion in the interface between the two clay layers for a given spudcan penetration depth, and 2) greater resistance provided by the thicker crust layers against heaving in the soft clay. The ratio of the final thickness of the crust plug to the original thickness of the crust layer (H) also decreases with larger H/B 197 Chapter Conclusions (from 0.85 for H/B = 0.14 to 0.7 for H/B = 1.04). This decrease could be a result of the larger spudcan resistance generated by larger H/B, thereby causing greater vertical compression in the crust plug. (c) The strength profile of the lower soft clay layer is shown to affect the soil failure mechanism during spudcan penetration in two-layer clay. The extent of soil deformation is shallower but wider in the soft clay layer with increasing strength with depth than in the soft clay layer with uniform strength. The increasing strength profile tends to provide greater resistance against downward soil flow, which in turn makes it easier for the soil to flow laterally. (d) Numerical analysis of spudcan penetration in two-layer clay using Eulerian finite element method is shown to give reasonably good predictions of spudcan loadpenetration responses in comparison with the experimental data, with errors generally less than 15%. The simulated soil deformation patterns during spudcan penetration are also shown to be in good agreement with the experimental observations. (e) The effects of strain-softening behaviour of the crust material on the spudcan loadpenetration response in two-layer clay are analysed using a simple model consisting of three parameters: cu1, p (peak strength), α (ratio of ‘softened’ strength to peak strength), and β (equivalent plastic strain where ‘softened’ strength is first mobilised). The analysis shows that, for β ≤ 5% (and α as low as 0.5), the strainsoftening crust may be approximated by an equivalent non-softening crust with strength equal to α cu1, p , with generally less than 10% errors in the load-penetration response. On the other hand, for β > 5%, the strain-softening crust could not be 198 Chapter Conclusions easily approximated by an equivalent non-softening crust. This is because 1) an equivalent non-softening crust with strength of α cu1, p would significantly underestimate the load-penetration response and the potential for punch-through; whereas 2) an equivalent non-softening crust with strength of cu1, p would significantly overestimate the load-penetration response and the potential for punch-through. The effects of the strain-softening behaviour on the soil failure patterns during spudcan penetration are also analysed. Failure in the strainsoftening crust tends to propagate in a narrower shear zone that extends deeper into the crust layer than in the non-softening crust. (f) A design method for estimating spudcan load-penetration response in two-layer clay is developed using the concepts of the standard bearing capacity theory. The proposed design method is based on the superposition of the spudcan resistance in weightless two-layer clay and the assumed soil surcharge during the spudcan penetration taking into account the effects of soil backflow. In comparison with Hossain & Randolph (2009a)’s design method which is primarily based on curvefitting procedures, the present method has a stronger physical basis and so is considered to be more robust. The proposed design method is shown to give reasonably good agreement with the experimental and the field data, with errors generally less than 15%. The proposed design method also generally gives better predictions than Hossain & Randolph (2009a)’s and SNAME (2002)’s design methods. In addition to the above main contributions, useful findings obtained in the present study are summarised as follows. 199 Chapter Conclusions (1) Comparisons between the measured peak spudcan resistance in ‘Series 1’ tests and Brown & Meyerhof (1969)’s bearing capacity solution show good agreement between them if the strain-softening crust is characterised by the peak strength rather than the post-rupture strength. This is not surprising as Brown & Meyerhof (1969)’s solution is based on an experimental study where the strain-softening clay is characterised by the peak strength. On the other hand, the numerical bearing capacity solutions of Edwards & Potts (2004), Merifield & Nguyen (2006) and Hossain & Randolph (2009a) give better agreement if the crust is characterised by the post-rupture strength rather than the peak strength. This is because these numerical solutions assume perfectly-plastic material behaviour, and the postrupture strength is estimated to be closer to the mobilised average strength of the crust layer during spudcan penetration than the peak strength. (2) Several modifications to Brown & Meyerhof (1969)’s solution are proposed to extend the use of the solution to two-layer clay with linearly-increasing cu2 with depth. These modifications are based on either a) the substitution of the theoretical bearing capacity solution for single-layer clay of linearly-increasing strength for the second term in Brown & Meyerhof (1969)’s solution (i.e. 6.05cu2), or b) the use of the average strength over the depth of B/2 or B from the uppermost level within the lower layer as an equivalent ‘uniform’ strength of the lower layer cu2. The proposed solutions show reasonably good agreement with the measured peak spudcan resistance in ‘Series 2’ tests with errors less than 15%, thereby suggesting validity of these solutions. (3) A simple conceptual model based on the observed failure mechanisms is proposed to provide a physical explanation for the change from a ‘post-peak reduction’ 200 Chapter Conclusions profile to a ‘monotonic increase’ profile in the spudcan load-penetration response with decreasing crust layer thickness. In this conceptual model, considering the crust plug as a free body, the spudcan resistance could be given by the sum of the side resistance and the base resistance on the plug. With increasing spudcan penetration depth, the side resistance tends to decrease whereas the base resistance tends to increase. It could be shown that the interaction between these two opposing components tends to produce a ‘post-peak reduction’ profile for a thick crust layer and a ‘monotonic increase’ profile for a thin crust layer. (4) H/B, cu2/cu1, and cu2/(γ2′B) are shown to have significant effects on the spudcan load-penetration response and the potential for punch-through. For 0.1 ≤ H/B ≤ 1, a decrease in H/B reduces the potential for punch-through. An increase in cu2/cu1 likewise reduces the potential for punch-through, possibly causing a change from a ‘post-peak reduction’ to a ‘monotonic increase’ load-penetration profile. A decrease in cu2/(γ2′B) gives larger spudcan resistance as well as smaller depth of initiation of soil backflow (Dbf). During the initial penetration up to Dbf, smaller cu2/(γ2′B) gives lower potential for punch-through. Beyond Dbf, however, cu2/(γ2′B) does no longer have significant effects on the potential for punch-through. 7.2 Areas for further study The present study is limited to the simplified case of two-layer clay. In practice however, the soil profile is likely to be multi-layered. For example, in the Sunda Shelf, the profile often consists of a layer of soft clay overlying a crust layer followed by another layer of soft clay (Castleberry & Prebaharan, 1985). The effects of the overlying soft clay layer on the spudcan load-penetration response in such multilayered clay have not been investigated. The soil failure mechanisms during spudcan 201 Chapter Conclusions penetration in such multi-layered clay are expected to be different from the mechanisms in two-layer clay as presented in this study. The potential for punchthrough may be accordingly different. Challenges in this area of study include the preparation of multi-layered clay in the laboratory. Another area for further study is the effects of partial consolidation or dissipation of excess pore water pressure during spudcan penetration. While spudcan penetration in clay is normally an undrained process, some unexpected delay during spudcan preloading could cause significant dissipation of excess pore water pressure during the penetration. This may cause ‘set up’ or local stiffening of clay beneath the spudcan, which may result in punch-through (Rapoport & Young, 1988; Young et al., 1984). The local stiffening effects may be more significant when there are variations in soil permeability with depth. In the present study, the Eulerian finite element method (with an explicit integration scheme) is limited to total-stress analysis (i.e. without pore water pressure). Further work is required to incorporate pore water pressure in Eulerian finite element analysis of spudcan penetration. 202 References Almeida, M. S. S. & Parry, R. H. G. (1984). Penetrometer apparatus for use in the centrifuge during flight. Proc. Symposium on The Application of Centrifuge Modelling to Geotechnical Design, Manchester, 47-66. Almeida, M. S. S., Davies, M. C. R. & Parry, R. H. G. (1985). Centrifuge tests of embankments on strengthened and unstrengthened clay foundations. Geotechnique 35, No. 4, 425-441. Aust, T. (1997). Accident to the mobile offshore drilling unit Maersk Victory on November 16 1996. MESA Report Book RB 97/24, Mines and Energy Resources South Australia. Benson, D. J. (1992). Computational methods in Lagrangian and Eulerian hydrocodes. Computer Methods in Applied Mechanics and Engineering 99, No. 2-3, 235-394. Bowles, J. E. (1996). Foundation Analysis and Design. New York: McGraw-Hill. Brennan, R., Diana, H., Stonor, R. W. P., Hoyle, M. J. R., Cheng, C. P., Martin, D. & Roper, R. (2006). Installing jackups in punch-through-sensitive clays. Proc. 2006 Offshore Technology Conference, Houston, OTC 18268. Brown, J. D. & Meyerhof, G. G. (1969). Experimental study of bearing capacity in layered clays. Proc. 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico, 45-51. BS1377-8 (1990). Methods of test for soils for civil engineering purposes, Part 8: Shear strength tests (effective stress). London: British Standard Institution. Burland, J. B. (1990). On the compressibility and shear strength of natural clays. Geotechnique 40, No. 3, 329-378. Castleberry, J. P. & Prebaharan, N. (1985). Clay crusts of the Sunda shelf – a hazard to jackup operations. Proc. 8th Southeast Asian Geotechnical Conference, Kuala Lumpur, 40-48. Chan, N. H. C. (2009). Personal communication. Dier, A., Carroll, B. & Abolfathi, S. (2004). Guidelines for jack-up rigs with particular reference to foundation engineering. MSL Engineering Ltd. Research Report 289, Surrey. Edwards, D. H. & Potts, D. M. (2004). The bearing capacity of a circular footing under 'punch-through' failure. Proc. 9th International Conference on Numerical Modelling in Geomechanics, Ottawa, 493-498. Finnie, I. M. S. & Randolph, M. F. (1994). Punch-through and liquefaction induced failure of shallow foundations on calcareous sediments. Proc. International Conference of Behaviour of Offshore Structures BOSS '94, Boston, 217-230. Gemenhardt, J. P. & Focht, J. A. (1970). Theoretical and observed performance of mobile rig 203 References footings on clay. Proc. 2nd Annual Offshore Technology Conference, Houston, OTC 1201. Goh, T. L. (2003). Stabilization of an excavation by an embedded improved soil layer. PhD thesis, National University of Singapore. Hossain, M. S. & Randolph, M. F. (2009a). New mechanism-based design approach for spudcan foundations on stiff-over-soft clay. Proc. 2009 Offshore Technology Conference, Houston, OTC 19907. Hossain, M. S. & Randolph, M. F. (2009b). New mechanism-based design approach for spudcan foundations on single layer clay. Journal of Geotechnical and Geoenvironmental Engineering 135, No. 9, 1264-1274. Hossain, M. S. & Randolph, M. F. (2010a). Deep-penetrating spudcan foundations on layered clays: centrifuge tests. Geotechnique 60, No. 3, 157-170. Hossain, M. S. & Randolph, M. F. (2010b). Deep-penetrating spudcan foundations on layered clays: numerical analysis. Geotechnique 60, No. 3, 171-184. Hossain, M. S., Hu, Y., Randolph, M. F. & White, D. J. (2005). Limiting cavity depth for spudcan foundations penetrating clay. Geotechnique 55, No. 9, 679-690. Houlsby, G. T. & Wroth, C. P. (1984). Calculation of stresses on shallow penetrometers and footings. Proc. IUTAM Symposium on Seabed Mechanics, Newcastle, 107-112. Houlsby, G. T. & Martin, C. M. (2003). Undrained bearing capacity factors for conical footings on clay. Geotechnique 53, No. 5, 513-520. House, A. R., Oliveira, J. R. M. S. & Randolph, M. F. (2001). Evaluating the coefficient of consolidation using penetration tests. IJPMG-International Journal of Physical Modelling in Geotechnics 1, No. 3, 17-26. Hu, Y. & Randolph, M. F. (1998a). A practical numerical approach for large deformation problems in soil. International Journal for Numerical and Analytical Methods in Geomechanics 22, No. 5, 327-350. Hu, Y. & Randolph, M. F. (1998b). H-adaptive FE analysis of elasto-plastic nonhomogeneous soil with large deformation. Computers and Geotechnics 23, No. 1-2, 61-83. JGS (2000). Practice for making and curing stabilized soil specimens without compaction, JGS T 0821-2000. Tokyo: The Japanese Geotechnical Society. Kostelnik, A., Guerra, M., Alford, J., Vazquez, J. & Zhong, J. (2007). Jackup mobilization in hazardous soils. SPE Drilling & Completion 22, No. 1, 4-15. Lee, F. H., Tan, T. S., Leung, C. F., Yong, K. Y., Karunaratne, G. P. & Lee, S. L. (1991). Development of geotechnical centrifuge facility at the National University of Singapore. Proc. International Conference Centrifuge 1991, Boulder, 11-17. Leung, C. F. (2005). Challenges in offshore geotechnics in Southeast Asia. Proc. 16th International Conference on Soil Mechanics and Geotechnical Engineering, Osaka, 303- 204 References 311. Lu, Q., Randolph, M. F., Hu, Y. & Bugarski, I. C. (2004). A numerical study of cone penetration in clay. Geotechnique 54, No. 4, 257-267. Martin, C. M. & Randolph, M. F. (2001). Applications of the lower and upper bound theorems of plasticity to collapse of circular foundations. Proc. 10th International Conference on Computer Methods and Advances in Geomechanics, Tucson, 1417-1428. McClelland, B., Young, A. G. & Remmes, B. D. (1981). Avoiding jack-up rig foundation failures. Proc. Symposium on Geotechnical Aspects of Coastal and Offshore Structures, Bangkok, 137-157. Merifield, R. S. & Nguyen, V. Q. (2006). Two- and three-dimensional bearing-capacity solutions for footings on two-layered clays. Geomechanics and Geoengineering: An International Journal 1, No. 2, 151-162. Merifield, R. S., Sloan, S. W. & Yu, H. S. (1999). Rigorous plasticity solutions for the bearing capacity of two-layered clays. Geotechnique 49, No. 4, 471-490. Meyerhof, G. G. (1972). Stability of slurry trench cuts in saturated clay. Proc. Specialty Conference on Performance of Earth and Earth Supported Structures, Lafayette, 14511466. Nakase, A., Kimura, T., Saitoh, K., Takemura, J. & Hagiwara, T. (1987). Behaviour of soft clay with a surface crust. Proc. 8th Asian Regional Conference on Soil Mechanics and Foundation Engineering, Kyoto, 401-404. Osborne, J. (2005). Are we good or are we lucky? Managing the mudline risk. OGP/CORE Workshop: The Jack-Up Drilling Option – Ingredients for Success, Singapore. Osbourne, J. J. & Paisley, J. M. (2002). SE Asia jack-up punch-throughs: The way forward? Proc. International Conference on Offshore Site Investigation and Geotechnics, London, 301-306. Paisley, J. M. & Chan, N. H. C. (2006). S E Asia Jack-Up Punch Throughs: Technical Guidance Note on Site Assessment. Proc. Jack-Up Asia Conference and Exhibition, Singapore. Purwana, O. A. (2006). Centrifuge Model Study on Spudcan Extraction in Soft Clay. PhD thesis, National University of Singapore. Randolph, M. F. & Houlsby, G. T. (1984). The limiting pressure on a circular pile loaded laterally in cohesive soil. Geotechnique 34, No. 4, 613-623. Randolph, M. F., Wang, D., Zhou, H., Hossain, M. S. & Hu, Y. (2008). Large deformation finite element analysis for offshore applications. Proc. the 12th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG), Goa, 3307-3318. 205 References Rapoport, V. & Young, A. G. (1988). Foundation performance of jack-up drilling units, analysis of case histories. In Mobile Offshore Structures (eds L. F. Boswell, C. A. D'Mello & A. J. Edwards), 461-476. London: Elsevier. Reardon, M. J. (1986). Review of the geotechnical aspects of jack-up unit operations. Ground Engineering 19, No. 7, 21-26. Roscoe, K. H. & Burland, J. B. (1968). On the generalised stress-strain behaviour of 'wet' clay. In Engineering plasticity (eds J. Heyman & F. A. Leckie), 535-609. Cambridge: Cambridge University Press. Salgado, R., Lyamin, A. V., Sloan, S. W. & Yu, H. S. (2004). Two- and three-dimensional bearing capacity of foundations in clay. Geotechnique 54, No. 5, 297-306. Satyanarayana, B. & Garg, R. K. (1980). Bearing capacity of footings on layered C-φ soils. Journal of the Geotechnical Engineering Division 106, No. GT7, 819-824. Schofield, A. N. (1988). An introduction to centrifuge modelling. In Centrifuges in Soil Mechanics (eds W. H. Craig, R. G. James & A. N. Schofield), 1-9. Rotterdam: Balkema. Simulia (2009). Abaqus 6.9-EF User's Manuals. Providence: Dassault Systemes Simulia. Skempton, A. W. & Henkel, D. J. (1953). The post-glacial clays of the Thames Estuary at Tilbury and Shellhaven. Proc. 3rd International Conference on Soil Mechanics and Foundation Engineering, Zurich, 302-308. SNAME (2002). Guidelines for Site Specific Assessment of Mobile Jack-Up Units, T&R 5-5A. New Jersey: The Society of Naval Architects and Marine Engineers. Stewart, D. P. & Randolph, M. F. (1991). A new site investigation tool for the centrifuge. Proc. International Conference Centrifuge 1991, Boulder, 531-538. Teh, K. L., Leung, C. F., Chow, Y. K. & Cassidy, M. J. (2010). Centrifuge model study of spudcan penetration in sand overlying clay. Geotechnique 60, No. 11, 825-842. Teh, K. L., Leung, C. F., Chow, Y. K., Cassidy, M. J. & Foo, K. S. (2007). Miniature penetrometers responses in sand overlying clay. Proc. 16th Southeast Asian Geotechnical Conference, Kuala Lumpur, 405-410. Teh, K. L., Cassidy, M. J., Leung, C. F., Chow, Y. K., Randolph, M. F. & Quah, C. K. (2008). Revealing the bearing capacity mechanisms of a penetrating spudcan through sand overlying clay. Geotechnique 58, No. 10, 793-804. Terzaghi, K. (1943). Theoretical Soil Mechanics. New York: Wiley. Tho, K. K., Leung, C. F., Chow, Y. K. & Swaddiwudhipong, S. (2009). Application of Eulerian finite element technique for analysis of spudcan and pipeline penetration into the seabed. Proc. 12th International Conference on The Jack-Up Platform – Design, Construction and Operation, London. Vesic, A. S. (1975). Bearing capacity of shallow foundations. In Foundation Engineering 206 References Handbook (eds H. F. Winterkorn & H. Y. Fang), 121-147. New York: Van Nostrand Reinhold. Vreugdenhil, R., Davis, R. & Berrill, J. (1994). Interpretation of cone penetration results in multilayered soils. Iinternational Journal for Numerical and Analytical Methods in Geomechanics 18, No. 9, 585-599. Wang, C. X. & Carter, J. P. (2002). Deep penetration of strip and circular footings into layered clays. The International Journal of Geomechanics 2, No. 2, 205-232. Webb, D. L. (1969). Residual strength in conventional triaxial tests. Proc. 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico, 433-441. White, D. J. & Take, W. A. (2002). GeoPIV: Particle Image Velocimetry (PIV) Software for Use in Geotechnical Testing, Cambridge University Engineering Department, Cambridge, Technical Report D-SOILS/TR322. White, D. J., Take, W. A. & Bolton, M. D. (2003). Soil deformation measurement using particle image velocimetry (PIV) and photogrammetry. Geotechnique 53, No. 7, 619-631. Young, A. G., Remmes, B. D. & Meyer, B. J. (1984). Foundation performance of offshore jack-up drilling rigs. Journal of Geotechnical Engineering 110, No. 7, 841-859. 207 Appendix A. Procedure for smoothing spudcan load-penetration response from numerical analysis A spudcan load-penetration response obtained from a numerical analysis in its raw form is shown in Figure A.1. The raw response is smoothened by simply averaging the raw spudcan resistance values over an interval of D/B equal to 0.02. Hence the smoothened spudcan resistance value at D/B = y is the average of the raw spudcan resistance values from D/B = y−0.01 to D/B = y+0.01. The resultant smoothened loadpenetration curve is also plotted in Figure A.1. 0.0 0.2 100 Spudcan resistance (kPa) 200 300 400 500 600 700 B = 15 m H = 7.5 m cu1 = 100 kPa cu2 = 50 kPa 0.4 0.6 Raw D/B Smoothened 0.8 1.0 D cu1 H 1.2 cu2 B/2 1.4 1.6 Figure A.1. Raw and smoothened spudcan load-penetration response from numerical analysis 209 [...]... pressure during preloading is typically between 300 and 500 kPa The resulting penetration depth varies from several meters for stiff granular soil to as much as several tens of meters for soft clay The average penetration rate is typically about 1 m/hour, and hence the penetration in clayey soil is likely to be undrained Soil profile containing a thin layer of stiff clay or crust overlying soft clay, hereafter... affecting spudcan load -penetration response in two-layer clay; (d) To investigate the effects of the strain-softening material behaviour of the crust layer on spudcan load -penetration response in two-layer clay; (e) To propose a design method for estimating spudcan load -penetration response in two-layer clay based on the experimental and the numerical results obtained In the present study, experiments of. .. area (bearing area) of spudcan B Diameter of spudcan cu Undrained shear strength of soil cu1 Undrained strength of upper-layer clay (crust) cu1,p Peak (initial) strength of upper-layer clay (crust) cu2 Undrained strength of lower-layer clay cu2* Strength of lower-layer clay at depth of ‘dummy’ spudcan cu2,0 Strength at the uppermost level of lower-layer clay cu,avg Average of strengths of upper- and lower-layer... overview of the experimental and the numerical techniques relevant to the present study is given 2.2 Two-layer clay in practice Castleberry & Prebaharan (1985) reported the occurrence of clay crust within thick layers of soft clay in 69 out of 452 soil borings conducted in the Southeast Asian Sunda Shelf Clay crust overlying soft clay was noted to be the most common cause of punch-through in the Southeast... much as 30% in UU tests The strain-softening behaviour may indicate the presence of strong cementation or structure in the crust, which is also reflected in its high overconsolidation ratios On the other hand, the underlying soft clay is not known to exhibit such strain-softening behaviour, possibly owing to the lack of cementation in the soft clay 10 Chapter 2 Literature Review 2.3 Bearing capacity... depths of initiation of soil backflow for Tests H1, H2 and H5 (vectors scaled 10×) 119 Figure 4.22 Comparison of soil deformation patterns between (a) crust overlying soft clay (Test H6) and (b) sand overlying soft clay (after Teh et al., 2008) 120 xiii Figure 4.23 Conceptual model for explaining change from ‘post-peak reduction’ to ‘monotonic increase’ profiles in load -penetration response in two-layer... the effects of strain-softening material behaviour of the crust on the load -penetration response Centrifuge tests of spudcan penetration in two-layer clay are conducted to obtain measurements of load -penetration response as well as observations of soil failure mechanisms during the penetration To enable comprehensive parametric studies, numerical analysis of spudcan penetration in two-layer clay is conducted... knowledge, the only existing experimental study on deep footing penetration in two-layer clay is that of Hossain & Randolph (2010a) In their study, 12 Chapter 2 Literature Review centrifuge model tests of spudcan penetration in two-layer clay were conducted The study considered two strength versus depth profiles in the lower clay layer: uniform and linearly-increasing profiles The strength in the upper crust...Summary A spudcan is a steel conical footing connected to each leg of a jack-up rig It is installed into the seabed by penetration under the self-weight of the rig and the added water ballast In soil profile containing a layer of stiff clay or crust overlying soft clay, herein referred to as two-layer clay, the soil resistance-versus -penetration curve may show a peak resistance... side of spudcan at depth equal to D xvii PI Plasticity Index Q Spudcan bearing load q Spudcan resistance qf Bearing capacity of circular footing on two-layer clay (q/cu1)p Peak spudcan resistance normalised by strength of upper-layer clay sc Bearing capacity shape factor for single-layer uniform soil V Volume of spudcan v Velocity or rate of penetration Vb Volume of embedded spudcan below level of maximum . EXPERIMENTAL AND NUMERICAL MODELLING OF SPUDCAN PENETRATION IN STIFF CLAY OVERLYING SOFT CLAY SINDHU TJAHYONO NATIONAL UNIVERSITY OF SINGAPORE. NATIONAL UNIVERSITY OF SINGAPORE 2011 EXPERIMENTAL AND NUMERICAL MODELLING OF SPUDCAN PENETRATION IN STIFF CLAY OVERLYING SOFT CLAY SINDHU TJAHYONO (B.Eng., NUS) . installed into the seabed by penetration under the self-weight of the rig and the added water ballast. In soil profile containing a layer of stiff clay or crust overlying soft clay, herein referred