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UBC Theses and Dissertations

Effects of void redistribution on liquefaction-induced ground deformations in earthquakes : a numerical investigation Seid-Karbasi, Mahmood

Abstract

Liquefaction-induced ground failure continues to be a major component of earthquake-related damages in many parts of the world. Experience from past earthquakes indicates lateral spreads and flow slides have been widespread in saturated granular soils in coastal and river areas. Movements may exceed several meters even in very gentle slopes. More interestingly, failures have occurred not only during, but also after earthquake shaking. The mechanism involved in large lateral displacements is still poorly understood. Sand deposits often comprise of low permeability sub-layers e.g., silt seams. Such layers form a hydraulic barrier to upward flow of water associated with earthquake-induced pore pressures. This impedance of flow path results in an increase of soil skeleton volume (or void ratio) beneath the barrier. The void redistribution mechanism as the focus of this study explains why residual strengths from failed case histories are generally much lower than that of laboratory data based on undrained condition. A numerical stress-flow coupled procedure based on an effective stress approach has been utilized to investigate void redistribution effects on the seismic behavior of gentle sandy slopes. This study showed that an expansion zone develops at the base of barrier layers in stratified deposits subjected to cyclic loading that can greatly reduce shear strength and results in large deformations. This mechanism can lead to a steady state condition within a thin zone beneath the barrier causing flow slide when a threshold expansion occurs in that zone. It was found that contraction and expansion, respectively at lower parts and upper parts of a liquefiable slope with a barrier layer is a characteristic feature of seismic behavior of such deposits. A key factor is the pattern of deformations localized at the barrier base, and magnitude that takes place with some delay. In this thesis, a framework for understanding the mechanism of large deformations, and a practical approach for numerical modeling of flow slides are presented. The study was extended to investigate factors affecting the seismic response of slopes, including: layer thickness, barrier depth and thickness, ground inclination, permeability contrast, base motion characteristics and soil consistency. Another finding of this study was that a partial saturation condition results in delay in excess pore pressure rise, and this factor may be responsible for the controversial behavior of the Wildlife Liquefaction Array, California (USA) during the 1987 Superstition earthquake. It was demonstrated that seismic drains are a promising measure to mitigate the possible devastating effects of barrier layers.

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