Numerical study of a large diameter shaft in old alluvium

NUMERICAL STUDY OF A LARGE DIAMETER SHAFT IN OLD ALLUVIUM TAN RWE YUN NATIONAL UNIVERSITY OF SINGAPORE 2004 NUMERICAL STUDY OF A LARGE DIAMETER SHAFT IN OLD ALLUVIUM TAN RWE YUN (B Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004 Dedicated to my family and friends ACKNOWLEDGEMENTS The author would like to express her gratitude to her supervisors, Associate Professor Harry Tan Siew Ann and Associate Professor Leung Chun Fai for their guidance and encouragement throughout her course of study The author has learnt much through their mentorship and meaningful discussions, and she deeply appreciated their patience and generosity with time, in spite of their busy schedules The author would like to thank Mr Mansour Makvandi and Mr R Balamurugan, from Econ Corporation Ltd, for their kind assistance in the collection of project information and explanation of technical details of the project The author is also grateful to Dr Wong Kwong Yan, from Soil Mechanics Pte Ltd, and Ms Teo Li Lin, from CEP Services Pte Ltd, for their support in the compilation of results of instrumentation works The author is thankful to Mr Ni Qing, a NUS research student, for sharing some of his experimental results on Old Alluvium with her She is also very appreciative of the support provided by Mr Shen Rui Fu, from the NUS Geotechnical Laboratory The author would like to express her heartfelt thanks to Mr Dennis Waterman and Mr Andrei Chesaru, from PLAXIS BV, for clarifying her doubts regarding the use of the PLAXIS and PLAXFLOW programs The author has also received much encouragement and support from her family and friends, especially Mr Tho Kee Kiat They have been a source of strength in the course of this project and their kind gestures are greatly appreciated i TABLE OF CONTENTS ACKNOWLEDGEMENTS Page i TABLE OF CONTENTS ii SUMMARY v NOMENCLATURE vii LIST OF FIGURES xiii LIST OF TABLES xix CHAPTER INTRODUCTION 1.1 Background 1.2 Current Issues and Problem Definition 1.3 Scope and Objectives CHAPTER LITERATURE REVIEW 2.1 Introduction 2.2 Singapore Old Alluvium Formation 2.3 Design of Vertical Shafts 19 2.4 Summary 39 CHAPTER CASE HISTORY 55 3.1 Introduction 55 3.2 General Site Condition and Instrumentation 55 ii Page 57 3.3 Site Investigation 3.4 Soil Profile 58 3.5 Excavation Support System and Sequence 58 CHAPTER THE HARDENING-SOIL MODEL 68 4.1 Introduction 68 4.2 Formulation of Hardening-Soil Model 68 4.3 Determination of Model Parameters 76 4.4 Determination of Hardening-Soil Model Parameters of Old Alluvium 80 CHAPTER PLAXFLOW 100 5.1 Introduction 100 5.2 Material Models 100 5.3 Material Sets Available in PLAXFLOW 103 5.4 Verification of Axisymmetrical Groundwater Flow 105 CHAPTER FINITE ELEMENT ANALYSIS 113 6.1 Introduction 113 6.2 Finite Element Model 113 6.3 Finite Element Analysis 117 6.4 Results and Observations 126 6.5 Zone of Influence 133 6.6 Convergence Study 134 iii 6.7 Limitations of Finite Element Model 6.8 Summary CHAPTER PARAMETRIC STUDIES Page 135 139 157 7.1 Introduction 157 7.2 Influence of Soil Strength 159 7.3 Effect of Hardening-Soil Stiffness Modulus 160 7.4 Influence of Soil Stiffness 161 7.5 Influence of Over-Consolidation Ratio 162 7.6 Influence of Soil Permeability 163 7.7 Influence of Interface Strength 165 7.8 Influence of Grade of Concrete of Circular Shaft Wall 165 7.9 Influence of Grade of Concrete of Ring Wall 166 7.10 Summary 167 CHAPTER CONCLUSION 182 8.1 Concluding Remarks 182 8.2 Recommendations for Further Research 185 REFERENCES 186 APPENDIX A 197 APPENDIX B 200 iv SUMMARY In this research, consolidation finite element analyses are performed to simulate the time-dependent behaviour of a circular shaft excavation in Singapore Old Alluvium This 70 m deep excavation is conducted for Influent Pumping Shaft at the Changi Water Reclamation Plant PLAXIS, a finite element package, is used to simulate the excavation process PLAXFLOW is used in conjunction with PLAXIS to perform axisymmetrical groundwater flow computations The outer diameter of the shaft is 42.6 m The excavation support system consists of a circular diaphragm wall Internal ring walls are cast against the diaphragm wall after each excavation stage The Hardening-Soil model is employed to simulate the constitutive behaviour of Old Alluvium A method proposed by Schanz and Bonnier (1997) to determine the values of parameters for the Hardening-Soil model is critically assessed Their proposed equations are independently derived and oedometer element tests are simulated using PLAXIS to verify the validity of the method Schanz and Bonnier’s method is found to be suitable for estimating Hardening-Soil model parameters for cohesionless soils with a power for stress-dependency of stiffness that ranges from 0.5 to 0.7 Laboratory oedometer and triaxial tests conducted on Old Alluvium soil samples are simulated using the Hardening-Soil model to obtain representative soil parameters The use of equal value for the reference secant stiffness modulus and the reference tangential oedometer stiffness modulus is found to be appropriate for Old Alluvium v The duration of each excavation and construction stage are carefully considered in the axisymmetrical finite element model The convergence of the mesh used in the analyses is verified through a convergence study Significant temperature variations during and after casting of the ring walls are observed A method to account for these thermal effects in the finite element model is proposed Hoop strains of the shaft wall usually reflect the excavation sequence and the numerical hoop strains agree well with instrumentation results It is evident from the finite element analyses that neglecting the thermal effects would lead to an unconservative design for circular shafts with cast in-situ ring walls Extensive parametric studies are performed to study the behaviour of such circular shafts in Old Alluvium The influences of soil strength, soil stiffness, overconsolidation ratio, soil permeability, wall interface strength and stiffness of walls on the maximum hoop force, bending moment, shear and deflection of the shaft wall are investigated Keywords: consolidation, finite element analysis, circular shaft, Old Alluvium, Hardening-Soil model, temperature effects vi NOMENCLATURE A A linear regression coefficient B A linear regression coefficient c’ Effective cohesion ci Cohesion of interface cincrement Increment of effective cohesion in Hardening-Soil model csoil Cohesion of soil cu Undrained cohesion E Young’s modulus of elasticity of shaft lining E’ Effective modulus of elasticity E50 Stiffness modulus of soil under primary drained triaxial loading E50ref Reference stiffness modulus of soil under primary drained triaxial loading Eoed Stiffness modulus of soil under primary oedometer loading Eoedref Reference stiffness modulus of soil under primary oedometer loading EPMT Pressuremeter modulus from the first cycle of test Er Pressuremeter unloading-reloading modulus of the second cycle of test Eu Undrained stiffness modulus of soil Eur Unloading stiffness modulus of soil Eurref Reference unloading stiffness modulus of soil EA Axial stiffness EI Bending stiffness (Eoedref)input Reference stiffness modulus of soil under primary oedometer loading inputted in Hardening-Soil model (Eoedref)predicted Reference stiffness modulus of soil under primary oedometer loading predicted by (Schanz and Bonnier, 1997) vii ⎛ p ref Eoedref = ⎜⎜ B ⎝ (1 − m) e = ⎞ ⎟⎟ ⎠ p ref A eB 199 Appendix B 200 201 202 203 204 205 206 207 208 209 210 211 212 213 [...]... Saturated degree of saturation Sres Residual saturation T Temperature TA Transmissivity of aquifer t Thickness of shaft lining V Maximum shear at final excavated depth in parametric study Vo Maximum shear at final excavated depth using basic parameters W(u) Well function w Water content z Depth zch Changeover depth zo Depth of shaft α An auxiliary model parameter in Hardening-Soil model αc Coefficient of. .. Coefficient of thermal expansion of concrete αr Radio of radial earth pressure to Berezantzev’s active earth pressure β An auxiliary model parameter in Hardening-Soil model x δ Maximum wall deflection at final excavated depth in parametric study δo Maximum wall deflection at final excavated depth using basic parameters εvp Plastic volumetric strain εvpc Plastic volumetric cap strain εv p Rate of Plastic volumetric... extend back to late Pliocene Aleva et al (1973) suggested that the Old Alluvium in Singapore could be deposited during the time of Upper Tertiary to Pleistocene age as the characteristics, stratigraphy and environments of deposits of Old Alluvium in Singapore appeared to correlate well with the Alluvial Complex in Singkep and Bangka of Indonesia The maximum recorded depth of the Old Alluvium in Singapore... presented in the later part of this chapter 2.2 Singapore Old Alluvium Formation The Republic of Singapore consists of a main island and many outlying islands totalling some 620 square kilometres in area The geology of Singapore is shown in Figure 2.1, as collated by PWD (1976) Nine geological formations have been identified to describe the stratigraphy of Singapore They are the Sajahat Formation, Gombak... Norite, Palaeozoic Volcanics, Bukit Timah Granite, Jurong Formation, Old Alluvium, Huat Choe Formation, Kallang Formation and Tekong Formation In particular, the Old Alluvium Formation will be of interest in this research The Old Alluvium Formation is an extension of a deposit found in southern Johore of Malaysia and it exists as an extensive sheet in the offshore zone to the east of Singapore PWD... (a) and (b) Stresses acting on a small element of soil at a distance r from centreline of a shaft; (c) and (d) Assumptions on which the computation of earth pressure are based (Terzaghi, 1943) 44 Figure 2.9 (a) Distribution of radial pressure on lining of shaft in sand and distribution of radial stresses on cylindrical section with radius r; (b) Approximate distribution of radial, circumferential and... quartzite and cryptocrystalline silica and they have an average pebble size of approximately 20 mm Fresh alkali feldspar pebbles may be found occasionally The sand grains are mostly subangular and they have similar composition as the pebbles No unequivocal signs of tilting of the beds have been found in Old Alluvium Faulting is rarely found and it is mainly restricted to small-scale displacements of. .. vertical shaft ix Rf Ratio of ultimate deviatoric stress to asymptotic shear stress in Hardening-Soil model Rinter Interface strength Rtr Extent of the plastic zone Rvr Extent of Mode A and Mode B of yield initiation are present RL Reduced level r Radial distance from the centreline of a cylindrical vertical shaft S Degree of saturation SA Storativity of Aquifer Se Effective degree of saturation Ssat Saturated... conducted instantaneously, the soil would strain in an undrained condition On the other hand, the soil would strain in a drained condition if this excavation were performed at an infinitely slow rate In reality, the soil will be partially drained as the actual excavation was carried out over a finite period Yong et al (1989) have shown that consolidation phenomenon results in additional movements and changes... that the Old Alluvium could be found lying to the north and north-east of the Kallang River Basin between the central granite and the granite at Changi Similar Old Alluvium deposits, which lie against the Jurong 8 Formation, can be found in the north-west region of the Singapore island in the Buloh Besar area Pitts (1984) highlighted that the area of Old Alluvium in the north-west region of the main ... degree of saturation Ssat Saturated degree of saturation Sres Residual saturation T Temperature TA Transmissivity of aquifer t Thickness of shaft lining V Maximum shear at final excavated depth in. .. environments of deposits of Old Alluvium in Singapore appeared to correlate well with the Alluvial Complex in Singkep and Bangka of Indonesia The maximum recorded depth of the Old Alluvium in Singapore.. .NUMERICAL STUDY OF A LARGE DIAMETER SHAFT IN OLD ALLUVIUM TAN RWE YUN (B Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CIVIL ENGINEERING NATIONAL