臺灣博碩士論文加值系統

The flow of water through soils in the retaining system may induce some alterations on the magnitude of the working stresses in soils, especially the effective overburden stress. Sand boiling is one of the stability issues related to the change of effective stress due to groundwater flow, but only a few researchers investigate the effect from the effective stress point of view. Accordingly, the primary objective of this study is to investigate the impact of effective stress change inside the excavation zone subject to seepage flow and to evaluate the factors affecting the safety against sand boiling. The study was carried out on 23 experimental failure cases with various excavation geometries and soil densities by utilizing PLAXIS and SEEP/W finite element programs. The results indicate that the failure progression of sand boiling is initiated from the excavation surface, then propagates to the area close to the pile. In addition to the failure propagation, the excavation geometry has a visible effect in establishing the failure mechanism and the governing the computed hydraulic gradient. Concerning the requirement of a reliable safety adequacy, the factor of safety adequacy is proposed in this study by adopting the equivalency theory between reliability and factor of safety. Eventually, two actual failure case histories are introduced to validate the applicability of the proposed factor of safety. The results indicate that the application of the proposed method can describe well the failure condition of an excavation with respect to sand boiling. Note also the application of the proposed factor of safety should obey the applied constraints in this study.

The flow of water through soils in the retaining system may induce some alterations on the magnitude of the working stresses in soils, especially the effective overburden stress. Sand boiling is one of the stability issues related to the change of effective stress due to groundwater flow, but only a few researchers investigate the effect from the effective stress point of view. Accordingly, the primary objective of this study is to investigate the impact of effective stress change inside the excavation zone subject to seepage flow and to evaluate the factors affecting the safety against sand boiling. The study was carried out on 23 experimental failure cases with various excavation geometries and soil densities by utilizing PLAXIS and SEEP/W finite element programs. The results indicate that the failure progression of sand boiling is initiated from the excavation surface, then propagates to the area close to the pile. In addition to the failure propagation, the excavation geometry has a visible effect in establishing the failure mechanism and the governing the computed hydraulic gradient. Concerning the requirement of a reliable safety adequacy, the factor of safety adequacy is proposed in this study by adopting the equivalency theory between reliability and factor of safety. Eventually, two actual failure case histories are introduced to validate the applicability of the proposed factor of safety. The results indicate that the application of the proposed method can describe well the failure condition of an excavation with respect to sand boiling. Note also the application of the proposed factor of safety should obey the applied constraints in this study.

ABSTRACT
ACKNOWLEDGEMENTS
TABLE OF CONTENT
LIST OF TABLES
LIST OF FIGURES
LIST OF SYMBOLS AND ABBREVIATIONS
CHAPTER 1 INTRODUCTION
1.1 Research Background
1.2 Research Objectives
1.3 Thesis Structure
CHAPTER 2 LITERATURE REVIEW
2.1 Groundwater Flow and Failures due to Seepage Flow
2.2 Overview of the Current Approaches in Quantifying Safety against Failures due to Seepage Flow
2.3 Finite Difference Method in Steady-State Flow Analysis
2.4 Model Uncertainty
2.5 Reliability Analysis
2.5.1 Monte Carlo Simulation (MCS)
2.5.2 Subset Simulation (SubSim)
CHAPTER 3 NUMERICAL ANALYSIS ON THE EFFECT OF SEEPAGE TO THE STABILITY OF A SHEETED EXCAVATION
3.1 Analysis of Marsland (1953) Model Test Results
3.1.1 Introduction of Model Tests
3.1.2 Estimation of Ham River Sand (HRS) Soil Parameter
3.2 Assessment of the Predicted Factor of Safety in Failure Cases
3.2.1 Comparison of Sand Boiling Factor of Safety
3.2.2 Assessment of Terzaghi Postulate on the Width of Soil Prism
3.3 Effective Stress Development with Respect to Sand Boiling
3.4 Heave and Factor of Safety Relationship Associated with Sand Boiling
3.5 Summary and Discussions
CHAPTER 4 CALIBRATION OF THE CURRENT SAFETY ASSESSMENTS OF SAND BOILING
4.1 Calibration of Model Uncertainties
4.2 Analysis of the Required Factor of Safety by the Reliability Approach
4.2.1 Theorem of Equivalency between Reliability and Factor of Safety
4.2.2 Probability of Failure and Factor of Safety Relationship
4.3 Summary and Conclusion
CHAPTER 5 EVALUATION AND VALIDATION PROCEDURES ON THE PROPOSED FACTOR OF SAFETY
5.1 Case Study 1: Excavation in Tokushima Prefecture
5.1.1 Introduction
5.1.2 Soil Parameters and Numerical Model
5.1.3 Evaluation of the Case Study
5.2 Case Study 2: Excavation in Shiga Prefecture
5.2.1 Introduction
5.2.2 Soil Parameters and Numerical Model
5.2.3 Evaluation of the Case Study
5.3 Validation on the Applicability of the Calibrated Factor of Safety Adequacy
5.4 Summary and Discussion
CHAPTER 6 CONCLUSIONS AND FUTURE STUDY
6.1 Conclusions
6.1.1 Effective Stress Based-Analysis on the Failure Mechanism of Sand Boiling
6.1.2 Required Factor of Safety and Reliability Approach
6.1.3 Validation of the Proposed Factor of Safety
6.2 Future Study
REFERENCES
APPENDIX 1
APPENDIX 2
APPENDIX 3

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