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- Type of Document: M.Sc. Thesis
- Language: Farsi
- Document No: 40193 (09)
- University: Sharif University of Technology
- Department: Civil Engineering
- Advisor(s): Pak, Ali
- Abstract:
- The behavior of pile foundations under earthquake loading is an important issue that affects the performance of structures. Design procedures have been developed for evaluating pile behavior under earthquake loading; however, the application of these procedures to cases involving liquefiable ground is uncertain. The performance of piles in liquefied soil layers is much more complex than that of non-liquefying soil layers because not only the superstructure and the surrounding soil exert different dynamic loads on pile, but also the stiffness and shear strength of surrounding soil diminishes over time due to both non-linear behavior of soil and pore water pressure generation. In this research, fully coupled three-dimensional dynamic analysis is carried out to investigate the dynamic behavior of pile foundations in liquefied ground. The numerical model consists of two layers of liquefiable and non-liquefiable soils, a pile and a superstructure on the pile head. The critical state bounding surface elastic-plastic sand model, Dafalias and Manzari(2004), which possesses the simulative ability to model the behavior of drained or undrained saturated sands under monotonic and cyclic loadings, is used, while a fully coupled (u-P) formulation is employed to analyze soil displacements and pore water pressures. Furthermore, in this thesis, variation of permeability coefficient during liquefaction is taken into account. The permeability coefficient is related to excess pore water pressure ratio; it increases by generation of pore water pressure and decreases by dissipation of pore pressure. Results obtained from the numerical model are first compared with some simple benchmark problems in which a single pile is modeled in non-liquefying soil layer under static vertical or lateral loading. Then, results of a centrifuge test on pile foundations are used to demonstrate the capability of the model for reliable analysis of piles under dynamic loading. For this purpose, seismic response of a single pile in liquefiable soil layers is compared with the results of centrifuge test. Finally, the verified model is used for parametric study. The parametric study is carried out by varying boundary condition of pile head, pile length, thickness of liquefying soil layer, relative density of liquefying soil layer and frequency of input motion. Three different soil profiles have been considered in this study. In the first profile, the ground consists of one homogenous and liquefiable soil layer; in the second profile, the ground is two-layered: the upper layer is liquefiable while the lower layer is not, and in the third one, the ground is two-layered: the upper layer is dry and the lower layer is saturated and liquefiable. The obtained results indicate that thickness of liquefiable layer and its relative density have a little influence on pile head settlement if the liquefied layer is located far from the pile toe. Also, pile length’s influence on the maximum lateral displacements of pile during excitation is negligible; however, it significantly decreases the magnitude of maximum lateral displacement after the input motion finishes. In general, the parametric study demonstrates that boundary condition of pile head, thickness of liquefying soil layer and frequency of input motion are the most critical parameters which considerably affect piles performance in liquefied grounds. Therefore, these parameters should be of higher value during designing pile foundations in liquefiable grounds.
- Keywords:
- Liquefaction ; Piles Foundation ; Fully Coupled Three Dimensional Dynamic Analysis ; Dynamic Behavior Pile ; Excess Pore Water Pressure
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