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Assessment of Effectivness of Compaction Improvement for Liquefaction Mitigation of Saturated Sand Deposits Based on Performance Levels of Shallow Foundations

Shahir, Hadi | 2009

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 40124 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Pak, Ali; Ghannad, Mohammad Ali
  7. Abstract:
  8. Settlement and tilt of structures due to liquefaction of subsoil layers is a major cause of damage during earthquake. One of the main approaches to reduce terrible effects of liquefaction on shallow foundations is ground improvement. Among the variety of ground improvement methods, the densification-based methods have been more paid attention for liquefaction mitigation. For liquefaction hazard mitigation using densification methods, three basic values should be determined i.e. depth, lateral extent, and degree of improvement (percent increase in Dr). In the current design practice, the need for ground improvement is usually decided based on the semi-empirical methods for assessment of liquefaction triggering potential of soil profile in level ground neglecting any influence of the structure. If ground improvement has to be carried out, and densification is selected as the most appropriate method, the degree of improvement is obtained based on the preferred safety factor against liquefaction. It is conventionally suggested that the densified zone should extend vertically up to the bottom of the layer susceptible to liquefaction and laterally by a distance equal to the densification depth from the edge of footing. In this design procedure, it is not known that how the treated soil-foundation system will respond to the earthquake shaking and how effective the improvement will be in reducing foundation settlement. In the recent decades, geotechnical design codes are being shifted from classical limit analysis approach toward the performance-based approach. The basic philosophy of performance based design relies on control of performance targets through a displacement-based design procedure. Therefore, any performance-based design procedure for soil improvement by densification method should address the three values of depth, lateral extent, and degree of improvement based upon the limiting amounts of foundation settlement and/or tilt that are deemed acceptable. For this purpose, it is necessary to utilize enhanced procedures, i.e. using 3D analytical procedures calibrated by field and model test observations, for evaluating the degree and spatial extent of improvement required to obtain desired levels of performance. The numerical study presented in this research, addresses the effects of ground improvement by densification on mitigation of liquefaction in saturated sand deposits underlying foundation of structures. In this regard, a 3D finite element model for fully coupled dynamic analysis of saturated porous media has been utilized. For constitutive modeling of sand behavior, a well-calibrated bounding-surface plasticity model which is capable of accounting for the monotonic and cyclic response of saturated sand in a wide range of densities and confining pressures, has been used. Another main feature of the proposed numerical model is the taking of the variation of permeability into account during liquefaction. The numerical simulations of this study have been performed using OpenSees which is an open-source software framework. In this research, two 3D u-p elements with novel variable permeability functions have been implemented in OpenSees. The numerical model has been verified by simulation of a series of centrifuge experiments performed on models of footing on liquefiable and improved subsoils and the analysis results are compared with the experimental measurements. After verification of the numerical model, comprehensive parametric studies have been conducted and based on the obtained results; practical relationships for estimation of settlement of rigid footings on liquefied and improved subsoils and an algorithm for design of ground improvement based on the performance of foundation are proposed.
  9. Keywords:
  10. Liquefaction ; Ground Improvement ; Congestion ; Shallow Footing ; Seismic Performance ; Coupled Analysis ; Variable Permeability

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