Void redistribution in layered soil profiles can significantly affect the residual strength of a liquefied soil and the resulting deformations. Liquefaction-induced seepage may get trapped under lower-permeability layers, leading to a localized increase in void ratio, strength loss and enhanced deformations. Since the transient changes in void ratio cannot be measured in the field or in large physical models, its effects can be inferred from shear strain localizations at permeability interfaces and from delayed failure, based on the theoretical understanding and empirical experience. The mechanisms affecting void redistribution, shear-strain localization, and their inter-relationships are examined in this study.
The effects of void redistribution are typically not directly accounted for in practice, and at best can be explicitly accounted for by using the empirical case-history based relationships for residual strengths. However, physical models have shown that the effects of void redistribution depend on many parameters, such as the layer thickness, slope angle, permeability contrast between the layers, shaking intensity, duration and history, and hence cannot be directly correlated with the pre-earthquake soil properties, as implied from the case-history based relationships.
The purpose of the study presented in this dissertation is two-fold: (1) to continue the characterization and advance the understanding of the void-redistribution strength-loss mechanism and related effects, and (2) to evaluate the ability of the currently available numerical tools to directly account for void-redistribution strength-loss mechanism.
First, an analytical study of void redistribution is presented, based on back-analysis of two dense instrumentation arrays from the centrifuge test SSK01, which experienced lateral spreading and shear strain localization. Shear stress and strain are calculated from accelerometer recordings and volumetric strains are calculated from pore-pressure transducers. The analysis provides insight on the flow patterns and their relation to the observed displacements in the test. Second, the material model PM4Sand is presented and its response to partially-drained conditions is explored through single-element FLAC simulations under different patterns of loading. The PM4Sand model is then used to simulate two boundary-value problems in which void redistribution and shear strain localization were important mechanisms in the overall model response - the centrifuge tests SSK01 and EJM02. The simulations are performed to evaluate the ability of the currently available numerical tools to capture the full observed mechanism, including shear strain localization and delayed failure. Simulation results are compared to the centrifuge tests measurements and observations, as well as with the back analyses on the instrumentation arrays. Parametric studies on cyclic strength, permeability and other uncertain factors are presented. For the simulations of the centrifuge test EJM02, two constitutive models are compared (PM4Sand and UBCSand), both capturing some aspects of the observed response but not all. Finally, the limitations in predicting the effects of void redistribution and shear strain localization are discussed.
|Advisor:||Boulanger, Ross W.|
|Commitee:||Idriss, Izzat M., Kutter, Bruce L.|
|School:||University of California, Davis|
|Department:||Civil and Environmental Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 73/07(E), Dissertation Abstracts International|
|Subjects:||Geotechnology, Civil engineering|
|Keywords:||Flac, Layered soil profiles, Liquefaction, Shear strain localization, Void-redistribution|
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