Centrifuge model study on spudcan extraction in soft clay

CENTRIFUGE MODEL STUDY ON SPUDCAN EXTRACTION IN SOFT CLAY OKKY AHMAD PURWANA NATIONAL UNIVERSITY OF SINGAPORE 2006 CENTRIFUGE MODEL STUDY ON SPUDCAN EXTRACTION IN SOFT CLAY OKKY AHMAD PURWANA (B.Eng., Unpar; M.Eng., ITB) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2006 To my wife, Nianisi Wilanindia Mutia Acknowledgments I wish to express my foremost gratitude to Prof Leung Chun Fai and Prof Chow Yean Khow not only for having given me constant guidance and advice during the study period but also for showing me a direction to my future in Offshore Geotechnics. I hope the outcome of this study is worth all the trust and patience you have put on me over the past four years. I would also like to acknowledge the supports of National University of Singapore research scholarship and Keppel Professorship fund for research equipments. I am grateful for trust and attention given by A/Prof Choo Yoo Sang (CORE director), Dr Foo Kok Seng and Dr Matthew Quah of OTD KeppelFELS in the NUS-Keppel collaboration project. In addition, the laboratory tests could not have been accomplished without the inspiring and fruitful discussions with Mr Wong Chew Yuen as well as technical assistances from Mr Tan Lye Heng, Shen Rui Fu, Shaja Kassim, Mr Loo Leong Huat, Mr John Choy, Mdm Jamilah Mr Foo Hee Ann and all other staffs. Thank you, for making The Centrifuge Laboratory feel like a second home to me. I am also fortunate to have Mr Martin Loh fabricating all the equipments with his full attention. The soil deformation analysis could have not been accomplished without kind helps of Dr D.J. White of Cambridge University for sharing the GeoPIV8 software. All my colleagues in The Spudcan Club: Dr Zhou Xiaoxian, Teh Kar Lu, Xie Yi, Ong Chee Wee, Gan Cheng Ti, Sindhu Tjahyono, Yang Haibo and my senior Dr Zhang Xi Ying, working together with you turned all the hard works and hard times into an enjoyable journey. Keep up the spirit, guys! My sincere appreciation also goes to Prof Masyhur Irsyam, Prof Dradjat Hoedajanto, and Mdm Siska Rustiani for having led me to this path and my best pals Dr Akhmad Herman Yuwono and Sentot Suryangat for all the encouragement. Finally, my both parents. Thank you for having brought me this far and making me tough in living this life. ii Table of Contents Dedication i Acknowledgments ii Table of Contents iii viii Summary List of Tables ix List of Figures x xviii List of Symbols Chapter Introduction 1.1 Spudcan: Foundation of Mobile Jack-up Rigs 1.2 Jack-up Rig Installation Procedures 1.3 Spudcan Extraction Problems 1.4 Objective and Scope of Research 1.5 Outline of Thesis Chapter Literature Review 2.1 Introduction 12 2.2 Overview of Spudcan-related Studies 13 2.3 Studies on Spudcan Extraction in Clay 14 2.4 Breakout Phenomenon and Related Studies 15 2.4.1 Basic definition 16 2.4.2 Major studies on breakout phenomenon 17 2.4.2.1 US Naval Civil Engineering Laboratory (1960s) 17 2.4.2.2 Vesic (1971) 18 2.4.2.3 Byrne & Finn (1978) 20 2.4.2.4 Rapoport &Young (1985) 21 2.4.2.5 Baba et al. (1989); Shin et al. (1994) 22 2.4.2.6 Mehryar et al. (2002) 24 iii 2.5 2.6 2.7 2.4.2.7 Thorne et al. (2004) 25 2.4.2.8 Rattley et al. (2005) 26 2.4.2.9 Other studies 28 Uplift Capacity of Plate Anchors 29 2.5.1 Review by Kulhawy (1985) 31 2.5.2 Studies by Merifield et al. (2001, 2003) 31 Water Jetting System 32 2.6.1 33 Studies by Lin (1987, 1995) Deformation Measurement Technique 34 2.7.1 36 Particle Image Velocimetry 2.7.2 Close range photogrammetry 2.8 39 2.7.2.1 Camera model 39 2.7.2.2 Refraction through viewing window 42 2.7.2.3 Method for locating control points 43 2.7.2.4 Transformation procedures 44 Summary 45 Chapter Experimental Setup and Procedures 3.1 Introduction 64 3.2 Centrifuge Modeling Technique 65 3.2.1 Why centrifuge? 65 3.2.2 Centrifuge scaling laws and errors 66 3.2.3 NUS Geotechnical Centrifuge 69 3.3 Experimental Setup 70 3.3.1 Full spudcan test 70 3.3.1.1 Model container and loading actuator 70 3.3.1.2 Model full-spudcan 72 3.3.2 3.3.3 3.3.1.3 Sensors 73 3.3.1.4 75 Soil specimen Half spudcan test 77 3.3.2.1 Model container and loading container 78 3.3.2.2 Model half-spudcan and soil specimen 78 3.3.2.3 Image capturing system 80 Data acquisition and control systems 80 3.3.3.1 Data acquisition 80 iv 3.3.3.2 3.3.4 3.3.5 Servo-controlled loading system 81 Apparatus for shear strength profiling 82 3.3.4.1 Cone penetrometer 82 3.3.4.2 T-bar penetrometer 83 3.3.4.3 Vane shear 86 Properties of soil specimen 87 3.3.5.1 Degree of consolidation 87 3.3.5.2 Water content and unit weight 88 3.3.5.3 Undrained shear strength 89 3.4 Selection of Displacement Rate in Centrifuge Tests 91 3.5 Experimental Procedures 93 3.5.1 Installation of spudcan 94 3.5.2 Waiting period 95 3.5.3 Extraction of spudcan 96 3.6 Summary 97 Chapter Assessment of Breakout Force & Its Contributing Factors 4.1 Introduction 119 4.2 Typical Test Results 121 4.2.1 Load-displacement response 121 4.2.2 Soil-surface movement 122 4.2.3 Stress state during installation 123 4.2.4 Stress state during waiting period 129 4.2.5 Stress state during extraction 131 4.3 Parametric Studies 134 4.3.1 Effect of waiting period 134 4.3.2 Effect of maintained vertical load 139 4.3.3 Implications of waiting period and maintained vertical load to breakout force of spudcans 140 4.4 Qualitative Assessment on Excess Pore Pressure Response 141 4.5 Summary 145 v Chapter Breakout Failure Mechanism of Spudcans in Soft Clay 5.1 Introduction 166 5.2 Photogrammetry correction 168 5.3 Test Program 169 5.4 Typical Test Results 169 5.4.1 169 Validity of stress measurement 5.4.2 Penetration 171 5.4.3 176 Waiting period 5.4.4 Extraction 5.5 178 5.4.4.1 Comparison with breakout failure mechanism of anchors 184 5.4.4.2 Factors affecting separation 186 Breakout Failure Mechanism for Specific Cases 189 5.5.1 Immediate extraction cases 190 5.5.2 Long term cases with base cavitation 192 5.6 Overview of Observed Breakout Failure Mechanism 194 5.7 Post-test Investigation 196 5.8 Summary 197 Chapter A Proposed Method for Easing Spudcan Extraction 6.1 Introduction 241 6.2 Postulated Concepts of Easing Spudcan Extraction 243 6.3 Basic Experimental Setup and Test Program 244 6.4 Method A: Repenetrating spudcan prior to extraction 244 6.5 Method B: Connecting top and base of spudcan 246 6.6 Method C: Applying external pressure to spudcan base 247 6.6.1 Specific additional setup 248 6.6.2 Test program 250 6.6.3 Typical test results 6.6.4 251 6.6.3.1 Load-displacement response 252 6.6.3.2 Stress state during extraction 252 6.6.3.3 Equilibrium of forces in vertical direction 256 Parametric studies 257 6.6.4.1 Effect of pressure-outlet area 257 6.6.4.2 Effect of applied pressure-level 261 vi 6.6.5 Qualitative assessment on potential hydraulic fracturing 264 6.7 Overview of Proposed Extraction Method 266 6.8 Summary 267 Chapter Further Interpretation of Results 7.1 Introduction 288 7.2 Assessment on Undrained Shear Strength 288 7.2.1 Test program 289 7.2.2 Shear strength test results 290 7.3 7.4 Assessment of Soil Resistance above Spudcan 292 7.3.1 Gain in shear strength after reconsolidation 292 7.3.2 Prediction of soil resistance above spudcan 294 Assessment of Base Resistance 297 7.4.1 Drag down of soil below spudcan and its implication 297 7.4.2 Verification of reverse bearing capacity assumption 300 7.4.3 Potential correlation between shear strength and base suction 302 7.4.3.1 303 Estimation of undrained shear strength below spudcan 7.4.3.2 Prediction of suction factor 7.5 Summary 306 309 Chapter Conclusions 8.1 Introduction 324 8.2 Summary of Findings 325 8.2.1 Breakout force and its components 325 8.2.2 Breakout failure mechanism 326 8.2.3 Method for eliminating base suction 328 8.3 Practical Implications 328 8.4 Recommendation for Further Studies 329 References 331 Appendix A 341 Appendix B 348 vii Summary Operators of mobile jack-up rigs often face difficulties when extracting spudcan foundations of the jack-up rigs with deep leg penetration particularly in soft clay. Besides posing a vulnerability to the jack-up structure, this problem also causes significant economic consequences to the offshore industry. The current guidelines for jack-up rigs operation procedure has yet to address this issue. In the present study, centrifuge modeling technique was adopted to simulate a simplified operation of an individual spudcan in normally consolidated soft clay. With an intensively instrumented model spudcan, the experimental study was performed to quantify the uplift resistance of spudcan and its contributing factors, with special attention paid to the development of suction pressure at the spudcan base. In addition, soil movement patterns surrounding the spudcan throughout the simulation were also revealed from a series of half-spudcan tests. This involved the use of particle image velocimetry coupled with close range photogrammetry technique to accurately quantify the soil displacements. The experimental results showed that the top soil resistance and base suction constitute the net uplift resistance of spudcan. These two components were substantially influenced by the waiting (operation) period of a jack-up rig. From the observed soil movement patterns, it was revealed that some similarities exist between extraction of spudcan and uplift of anchor. It was also established that the individual components could be reasonably predicted using existing anchor theories provided that an accurate estimate of undrained shear strength above and below the spudcan prior to extraction are available. Based on the findings that highlight the importance of base suction, an improved method for easing spudcan extraction in clay was proposed and evaluated. Under laboratory conditions, the proposed method was proven capable of eliminating the spudcan base suction and thus substantially reducing the spudcan breakout force. Key words: jack-up rig, spudcan, extraction, clay, breakout, suction. viii References Ninomiya, K., K. Tagaya and Y. Murase. (1972). A study on suction and scouring of sit-onbottom type offshore structure. Proc. 4th Offshore Technology Conf., Texas, USA,Vol. I, pp. 869-879. Osborne, J. J. (2006). Personal communication. Overy, R.F. and A.R. Dean. (1986). Hydraulic fracture testing of cohesive soil. Proc. Offshore Technology Conf., Texas, USA, OTC 5226. Potts, D.M. (1976). Behavior of lined and unlined tunnels in sand. Ph.D Thesis, Cambridge University. Poulos, H.G. (1988). Marine Geotechnics. Unwin Hyman, London. Prewitt, J.M.S. (1970). Object enhancement and extraction. Picture processing and psychopictropics. Lipkin R.S. and Rosenfeld A. (eds), pp. 75-149. Purwana, O.A., C.F. Leung, Y.K. Chow and K.S. Foo. (2005). Influence of base suction on extraction of jack-up spudcans. Géotechnique, Vol. 55(10), pp. 741-753. Randolph, M.F. and G.T. Houlsby. (1984). The limiting pressure on a circular pile loaded laterally in cohesive soil. Géotechnique, Vol. 34(4), pp. 613-623. Randolph, M.F. and S. Hope. (2003). Effect of cone velocity on cone resistance and excess pore pressures. Proc. Int. Symp. Engineering Practice and Performance of Soft Deposits, Osaka, pp. 147-152. Rapoport, V. and A.G. Young. (1985). Uplift capacity of shallow offshore foundations. Proc. Uplifts behaviour of anchor foundations in soil, ASCE, pp. 73-85. Rattley, M.J. B.M. Lehane, D.J. Richards and C. Gaudin. (2005). An experimental and numerical study of rate effects for plate anchors in clay. Proc. Int. Symp. Frontiers in Offshore Geotechnics, Perth, Australia, pp. 197-203. Reardon, M.J. (1986). Review of the geotechnical aspects of jack-up unit operations. Ground Engineering, Vol. 19(7), pp. 21-26. Ridley, A.M. and J.B. Burland. (1993). A new instrument for the measurement of soil moisture suction. Géotechnique, Vol. 43(2), pp. 321-324. Roderick, G.L. and A. Lubbard. (1975). Effect of object in-situ time on bottom breakout. Proc. 7th Offshore Technology Conf., Texas, USA, Vol. I, pp. 360-380. Roscoe, K.H. and J.B. Burland. (1968). On the generalized stress-strain behavior of wet clay. In Engineering plasticity, Cambridge University Press, pp. 535-609. Roscoe, K.H., J.R.F. Arthur and R.G. James. (1963). The determination of strains in soil by an X-Ray method. Civil Engineering & Public Works Review, Vol. 58, pp. 873-876. Rowe, R.K. and E.H. Davis. (1982). The behaviour of anchor plates in clay. Géotechnique, Vol. 32(1), pp. 9-23. Saada, A.S., L. Liang, J.L. Figueroa and C.T. Cope. (1999). Bifurcation and shear propagation in sands. Géotechnique, Vol. 49(3), pp. 367-385. 337 References Santa Maria, P.E.L. (1988). Behavior of footings for offshore structures under combined loads. D.Phil Thesis, The University of Oxford. Sawicki, A. and J. Mierczynski. (2003). Mechanics of the breakout phenomenon. Computer and Geotechnics, Vol. 30, pp. 231-243. Schofield, A.N. (1980). Cambridge geotechnical centrifuge operations. Géotechnique, Vol. 30(3), pp. 227-268. Shin, E.C., R.N. Das, M.T. Omar, B.M. Das and E.E. Cook. (1994). Mud suction force in the uplift of plate anchors in clay. Proc. 4th Int. Offshore and Polar Engineering Conf., Osaka, Japan, pp. 462-466. Siciliano, R.J., J.M. Hamilton, J.D. Murff and R. Phillips. (1990). Effect of jackup spudcans on piles. Proc. Offshore Technology Conf., Texas, USA, OTC 6467. Skempton, A.W. (1951). The bearing capacity of clays. Proc. Building Research Congress, London, pp. 180-189. Slama, C.C. (1980). Manual of photogrammetry, 4th Ed. Americal Society of Photogrammetry. SNAME. (2002). Guidelines for site specific assessment of mobile jack-up units. Society of Naval Architecs and Marine Engineers. Technical and Research Bulletin 5-5A, New Jersey. Springman, S.M. and A.N. Schofield. (1998). Monotonic lateral load transfer from a jack-up platform lattice leg to a soft clay deposit. Proc. Centrifuge ’98. Balkema, Rotterdam, pp. 563-568. Sutherland, H.B. (1988). Uplift resistance of soils. Géotechnique, Vol. 38(4), pp. 493-516. Stewart, D.P. (1992). Lateral loading of pile bridge abudments due to embankment construction. Ph.D Thesis, The University of Western Australia. Stewart, D.P. (2005). Influence of jackup operation adjacent to a piled structure. Proc. Int. Symp. Frontiers in Offshore Geotechnics, Perth, Australia, pp. 543-550. Stewart, D.P. and I.M.S. Finnie. (2001). Spudcan-footprint interaction during jack-up workovers. Proc. 11th Int. Offshore and Polar Engineering Conf., Norway. Stewart, D.P. and M.F. Randolph. (1991). A new site investigation tool for centrifuge. Proc. Centrifuge ’91. Balkema, Rotterdam, pp. 531-538. Take, W.A. (2003). The influence of seasonal moisture cycles on clay slopes. Ph.D Thesis, Cambridge University. Take, W.A. and M.D. Bolton. (2002). A new device for the measurement of negative pore water pressures in centrifuge models. Proc. Int. Conf. Physical Modelling in Geotechnics ICPMG’02, pp. 101-106. Tan, F.S.C. (1990). Centrifuge and theoretical modeling of conical footings on sand. Ph.D Thesis, Cambridge University. 338 References Tan, T. S. and R. F. Scott. (1985). Centrifuge scaling considerations for fluid-particle systems. Géotechnique, Vol. 35(4), pp. 461–470. Tani, K. and W.H. Craig. (1995). Development of centrifuge cone penetration test to evaluate the undrained shear strength profile of a model clay bed. Journal of Soil and Foundations, Vol. 35(2), pp. 37-47. Taylor, R.N. (1995). Centrifuges in modelling: principles and scale effects. In Geotechnical Centrifuge Technology, Blackie Academic and Professional, London, pp. 19-59. Taylor, R.N., R.J. Grant, S. Robson and J. Kuwano. (1998). An image analysis system for determining plane and 3-D displacements in soil models. Proc. Centrifuge’98. Balkema, Rotterdam, pp. 73-78. Teh, K.L., C.F. Leung and Y.K. Chow. (2005). Spudcan penetration in sand overlying clay. Proc. Int. Symp. Frontiers in Offshore Geotechnics, Perth, Australia, pp. 529-534. Thanadol, K. (2003). Behavior of an embedded improved soil berm in an excavation. Ph.D Thesis, National University of Singapore. Thorne, C.P., C.X. Wang and J.P. Carter. (2004). Uplift capacity of rapidly loaded strip anchors in uniform strength clay. Géotechnique, Vol. 54(8), pp. 507-517. Treacy, G. (2003). Reinstallation of spudcan footings next to existing footprints. Honours Thesis, The University of Western Australia. Vlahos, G., C.M. Martin, M.S. Prior and M.J. Cassidy. (2005). Development of a model jackup unit for the study of soil-structure interaction on clay. Int. Journal of Physical Modeling in Geotechnics, Vol. 2, pp. 31-48. Vesic, A.S. (1971). Breakout resistance of objects embedded in ocean bottom. Journal of Soil Mechanics and Foundations Division, ASCE, Vol. 97(SM9), pp. 1183-1205. Wagget, P.R. (1989). The effects of lubricants on the interaction between soil and perspex. Cambridge University Part II Project Report. Watson, P.G., N. Suemasa and M.F. Randolph. (2000). Evaluating undrained shear strength using the vane shear apparatus. Proc. 10th Int. Offshore and Polar Engineering Conf., Seattle, pp. 485-493. White, D.J. (2002). An investigation into the behavior of pressed-in piles. Ph.D Thesis, Cambridge University. White, D.J. and W.A. Take. (2002). GeoPIV: Particle Image Velocimetry (PIV) software for use in Geotechnical testing. D-Soils-TR322, Cambridge University Engineering Department Technical Report. White, D.J., W.A. Take and M.D. Bolton. (2003). Soil deformation measurement using particle image velocimetry (PIV) and photogrammetry. Géotechnique, Vol. 30(1), pp. 49-65. 339 References Xie, Y., C.F. Leung and Y.K. Chow. (2006). Effects of spudcan penetration on adjacent piles. Proc. 6th Int. Conf. Physical Modeling in Geotechnics ICPMG, Hongkong, pp. 701-706. Young, A.G., B.D. Remmes and B.J. Meyer. (1984). Foundation performance of offshore jackup drilling rigs. J. Geotech Engng Div., ASCE, Vol. 110(7), pp. 841-859. Zhou, X. (2006). Numerical modelling of extraction of spudcans. Ph.D Thesis, National University of Singapore. 340 APPENDIX A CALIBRATION OF PRESSURE SENSORS A.1 Pore Pressure Transducers A major aspect in the experiment is the reliability of the pore pressure transducers to measure changes in pore water pressure, particularly during spudcan extraction in which a rapid change was expected. The Druck© PDCR-81 miniature pore pressure transducer which was originally developed for use in centrifuge modeling applications is widely known to be reliable for measuring positive pore water pressures. Though the spudcan extraction is designed to be initiated at relatively high water pressure in order to eliminate the potential of cavitation, there are still some possibilities that the total pore pressure at the spudcan base drops below atmospheric pressure. 341 Appendix A Some researchers found that PDCR-81 transducer is not suited for measurement of high suction as the caused large outward deflections of the silicon diaphragm tends to compromise the connection integrity between the diaphragm and the supporting glass cylinder (Ridley, 1993; Take & Bolton, 2002). However, in this experiment the total pore pressure was not expected to fall far below the atmospheric pressure which otherwise allowing cavitation to occur. Therefore, apart from the positive range, the pore pressure transducers were also ensured to be able to register negative pressure in the range of to -100 kPa (-1 atm). The calibration was carried out using a Druck© DPI-601 digital pressure indicator by which a prescribed level of air pressure can be applied to the transducer. The calibration curve was obtained by varying the applied pressure in positive and negative ranges. In addition, the transducer was also recalibrated in high-g condition using water for a range of positive pressures. The typical calibration chart is presented in Figure A.1 in which the relation between the applied pressure and the corresponding output reading appears linear in both positive and negative ranges. The calibration factor obtained from both calibration methods was also found very similar. For the negative pressure range, it was found that the lowest pressure measurable by the PDCR-81 transducers used in the present study was typically 90~95 kPa. This suggests that the measured of negative pore pressure up to this level is still valid. It is shown in Chapter that in fact the lowest pore pressure occurred at the spudcan base was about -50 kPa. In Chapter 5, it is also revealed that cavitation occurred below the spudcan base when the measurement of negative pressure hit the maximum at -95 kPa. 342 Appendix A A.2 Total Pressure Transducers Unlike pore pressure transducers, the calibration factor of total stress transducer is not unique but depends on testing conditions. Lee et al. (2002) pointed out that the conditions in which a total stress transducer is calibrated need to be similar with its usage conditions, that is, in terms of soil type, degree of saturation, stress level, as well as loading and boundary conditions. As such, in the present study calibration of the total stress transducers was conducted under same conditions as that of the spudcan simulation and taking into account some considerations highlighted in the study of Lee et al. (2002). The total stress transducers were calibrated under the soil self weight loading in high-g conditions. To impose the same loading and boundary conditions as that in the spudcan tests, the model container of 550-mm diameter and 500-mm high was used with a certain thickness of saturated kaolin clay to replicate a representative loading range during the actual spudcan tests. At the bottom of the container, the total stress transducer was placed on top of a 30-mm thick porous stone surrounded by a sand layer of the same thickness. For the 500 kPa capacity transducer, the de-aired kaolin slurry was placed into the container as much as 240-mm high whereas for that of MPa capacity the container was filled by 420-mm high of kaolin slurry. The latter is the maximum volume of slurry can be put in the model container. A little water was added on the slurry surface to prevent from drying out during the initial stage of the centrifuge spin. In addition to total stress transducers, a pore pressure transducer was also installed in the mid-height of the soil to monitor the degree of consolidation. The total stress transducers were connected to strainmeter for data capturing and therefore 343 Appendix A the output would be in terms of microstrain (μs). The test layout is schematically shown in Figure A.2. After measuring net weight of the kaolin slurry, the setup was spun up. By varying the acceleration level, various stress level acting on the active face of the stress transducer induced by the total overburden stress above which can be generated. During the actual spudcan tests, the degree of consolidation may vary and therefore the transducer was calibrated at two extreme conditions, i.e. slurry state and full consolidation state. The sequence of loading and unloading for the calibration tests is as follows: 1) Increase the g-level in stages: 1g to 20g, 40g, 60g, 80g, and 100g, with about minutes duration at each stage. At this point, the clay is still in slurry state. 2) Reduce the g-level in the reverse sequence 3) Spin-up to 100g for about hours to fully consolidated the sample indicated from the monitoring of pore pressure level. 4) Reduce the g-level in stages: 100g, 80g, 60g, 40g, 20g, with about minutes duration at each stage. 5) Increase the g-level in the reverse sequence 6) Spin down and measure final weight of the clay With the above procedure, one cycle of loading and unloading process acting on the total stress transducer at slurry and full consolidation states was produced. As shown in Figure A.3, the calibration factor in the slurry and full consolidation states for both transducers was slightly reduced in consistent manner, i.e. 0.30 to 0.28 kPa/μs for 500 kPa capacities and 0.54 to 0.52 kPa/μs for MPa capacities. In view of the minor difference in the calibration factor associated with the degree of consolidation of the 344 Appendix A sample, it is hence justified to use the average value for each transducer as the representative factor throughout the spudcan simulation event. 345 Appendix A 700 Applied pressure (kPa) 600 500 y = 84.3 x + 3.0 R2 = 400 300 200 100 Compression Tension -100 -2 -1 10 Output (V) Figure A. Typical calibration curve for pore pressure transducer used in the present study Container 500 kPa MPa Container Kaolin clay slurry Kaolin clay slurry 500 PPT 420 PPT 240 TST1 TST2 sand 30 TST3 TST4 Porous stone 550 550 Figure A. Layout of setup for total stress transducer calibration in high-g 346 Appendix A 500 TSC MPa 450 Applied stress (kPa) 400 350 300 TSC 500 kPa y = 0.54 x + 1.1 250 y = 0.30 x + 2.3 200 150 100 Loading 50 Unloading 0 100 200 300 400 500 600 700 800 900 1000 Measured strain (μs) (a) calibration at slurry state 500 TSC MPa 450 Applied stress (kPa) 400 350 300 TSC 500 kPa y = 0.52 x + 1.7 250 y = 0.28 x + 2.0 200 150 100 Loading 50 Unloading 0 100 200 300 400 500 600 700 800 900 1000 Measured strain (μs) (b) calibration at fully consolidated state Figure A. Typical result of total stress transducer calibration in saturated clay 347 APPENDIX B VALIDATION OF CAMERA CALIBRATION The camera calibration scheme adopted in the present study, as described in Section 2.7.2, has been validated and adapted from the calibration procedure of White (2002). The calibration procedure involved the photography of a two-dimensional or coplanar array of targets from at least two different angles (Heikkila, 1997). The camera calibration routine was hence carried out based on the known actual coordinates of the targets array in object space and the corresponding coordinates in image space extracted from each image using the method for locating circular control points of Heikkila (1997). The accuracy of the calibration procedure was assessed by comparing the true and the mean predicted object-space coordinates of the targets array. As the target object, a calibrated photogrammetric target from Edmund© Industrial Optics was used (calibration certificate SN 0000-0294, Max Levy Autograph, Inc.). 348 Appendix B The 150 mm × 150 mm target consists of three grids of 0.75, 1.5, and 3.0 mm diameter reference dots with size accuracy of 0.013 mm. As shown in Figure B.1, the target object was positioned immediately behind a 50-mm thick perspex, which was also used in the half-spudcan tests. The image of the target was then photographed from three different angles from which the coordinates of the 3-mm diameter dots array at each image could be located using the method of Heikkila (1997). To derive the transformation parameters, a subset of 16 out of 480 dots spread across the image was used, as shown by the square boxes in Figure B.1. Figure B.2 illustrates an example of the reconstruction of the circular dots which were used as the calibration target. The edge of the circular marker (Figure B.2a) was first identified by applying a simple detector, in this case Prewitt (1970) operator, to the grayscale image. This rough pixel edge (Figure B.2b) was then reconstructed using the moment and curvature preserving ellipse detection technique (Heikkila, 1997). In Figure B.2c, the sub-pixel edge was refined and the control point had been reconstructed to an elliptic feature. The center of this point was then determined using the renormalization conic fitting of Kanatani (1994), as shown by the cross marker at the center of the sub-pixel edge in Figure B.2c. Using the transformation parameters derived from the 16 target markers, the observed image coordinates of the 460 dots need to be projected back to two-dimensional coordinates in object space. The inverse distortion model of Heikkila (1997) and the inverse pinhole camera model were used for executing this task. Figure B.3 shows the discrepancy vectors between the true and predicted object-space coordinates of the target dots. The apparent systematic spatial variation in discrepancy vector is believed to be attributed to the transformation system. The systematic error in the camera 349 Appendix B parameters deduced from the calibration routine would be translated into the same nature of error during the transformation from the image-space to object-space coordinates. Figure B.4 corresponds to the normalized histogram of spatial discrepancy between the actual and predicted object-space coordinates combining the vectors components in both x- and y-directions. It is apparent that the mean discrepancy vector is 0.0 μm with a standard deviation of 15.6 μm or corresponds to an error of 1/15000th of the field of view. It is believed that this accuracy is more governed by the accuracy of the photogrammetric target used. As indicated in the certificate, the accuracies of dot size and dot-to-dot spacing are 13 μm and μm, respectively. This suggests that accuracy of the camera calibration routine used in the present study is satisfactory for use in geotechnical modeling. 350 Appendix B Y X Figure B. Reconstruction of dots using Edmund Scientific© photogrammetric target behind 50-mm thick perspex plate (a) captured control marker (b) detected edge pixels (c) reconstructed edge pixels Figure B. Example of control marker edge reconstruction and determination of its center using centroiding technique adopted in the present study 351 Appendix B Figure B. Vector of discrepancy between actual dot positions and those calculated from camera calibration 0.6 Standard deviation 0.0156 mm deduced from 480 dots Normalized frequency 0.5 0.4 0.3 0.2 0.1 -0.1 -0.08 -0.06 -0.04 -0.02 0.02 0.04 0.06 0.08 0.1 Magnitude of discrepancy in x and y directions (mm) Figure B. Normalized histogram of dot positions discrepancy in both x- and y- directions 352 [...]... significant reduction in soil consolidation duration In the present study, a single spudcan was tested on a specimen of normally consolidated soft clay constituted from Malaysian kaolin clay The use of kaolin clay allows relatively rapid consolidation of large specimen from a slurry state The simulation mainly consisted of spudcan installation, operation, and extraction The spudcan was installed in- flight to... spudcan extraction 2.3 Studies on Spudcan Extraction in Clay Craig & Chua (1990b) pioneered a specific experimental research on extraction of jack-up rig spudcan- type foundations In their study, centrifuge model tests were carried out to simulate spudcan installation and the subsequent extraction under undrained loading in uniform soft clays having undrained shear strength in the range of 12~40 kPa The model. .. enhance the understanding of breakout phenomenon associated with spudcan extractions in soft clay as follows: a To assess the components of breakout force and its contributing factors for spudcan extractions in normally consolidated clay b To investigate the breakout failure mechanism of spudcan extraction in soft clay c To provide an estimate of breakout force of spudcan in soft clay d To propose an... evaluation of the components of breakout force Chapter 5 presents and discusses the experimental findings on the breakout failure mechanism of spudcan in soft clay Chapter 6 proposes a new method for easing spudcan extraction in soft clay Chapter 7 provides further interpretations of the findings including the estimation of uplift resistance Chapter 8 summarizes the main findings established in the... REVIEW 2.1 Introduction This chapter presents a survey of literature pertinent to studies on breakout phenomenon in clay with particular reference to those concerned with suction beneath foundations In addition, existing studies on the application of water jetting system in spudcan extraction are discussed This will provide an insight into the potential drawbacks of the current extraction method which... effective method of spudcan extractions and evaluate its performance under laboratory conditions In view of the complexity of simulating the spudcan extraction problem numerically associated with large soil deformation, centrifuge model technique has been adopted in this study This modeling technique allows a proper simulation of the entire process of spudcan operations using small scale models in laboratory... vertical and pore pressures at spudcan top during installation (Test GS5) 150 Total vertical and pore pressures at spudcan base during installation (Test GS5) 150 Total pore pressures in soil below spudcan during installation (Test GS5) 151 Figure 4 8 Schematic diagram of measured stresses on spudcan during installation 152 Figure 4 9 Stresses development on spudcan around installation stage (Test GS5) 152... 1.5 times spudcan diameter under undrained condition At this stage, the loadings incurred by the spudcan during operation were simplified as a constant vertical loading maintained for a period of time termed as “waiting period” Besides measuring the spudcan breakout force, special 7 Chapter 1 Introduction attention was given to stress development above and beneath the spudcan and the surrounding soil... latter model will allow the simulation of structure-soilfluid interaction which perhaps will create the state-of-the-art jack-up study in the near future Despite the intensive research on spudcan and jack-up behaviors particularly under combined loading, very little attention has been given to the investigation into spudcan extraction in clay The author is unaware of any published literature on this... floating structure into a “fixed” one and vice versa Majority of jack-up rigs in use today are equipped with a rack and pinion system for each leg thus allowing continuous jacking of the hull In contrast to the old pin and hole system, this latest system enables hull positioning at any leg position (Bennet & KeppelFELS, 2005) An idealized description of spudcan installation process is illustrated in . CENTRIFUGE MODEL STUDY ON SPUDCAN EXTRACTION IN SOFT CLAY OKKY AHMAD PURWANA NATIONAL UNIVERSITY OF SINGAPORE 2006 CENTRIFUGE MODEL STUDY ON SPUDCAN. diagram of spudcan extraction mechanism in soft clays for relatively long waiting period cases 238 Figure 5. 46 Sample surface conditions after penetration and extraction (half spudcan) 239. operation of an individual spudcan in normally consolidated soft clay. With an intensively instrumented model spudcan, the experimental study was performed to quantify the uplift resistance of spudcan