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Experimental Mapping of Elastoplastic Surfaces for Sand and Cyclic Failure of Low-Plasticity Fine-Grained Soils

Abstract

Part I of this dissertation describes a simple experimental technique that can be used to obtain the slopes of the plastic potential and yield functions during shear based on the deformation theory of plasticity. The method imposes small perturbations in the direction of the stress increment by closing the drainage valve, thereby abruptly switching from drained to undrained loading conditions during plastic loading. Elastoplastic moduli are obtained immediately before and after the perturbation from the measured deviatoric stress, mean effective stress, deviatoric strains, and volumetric strains for the stress paths immediately before and after closing the drain valve. During drained shear, samples were sheared while the mean effective stress was maintained constant. Combining tests performed at several confining stresses, the proposed method can map yield and plastic potential surfaces and predict their evolution for a wide range of stresses.

Part II of the dissertation focuses on providing insights into the effects of changes in clay mineralogy and pore-fluid chemistry on cyclic behavior of low-plasticity fine-grained soils. A series of cyclic and monotonic direct simple shear experiments was conducted on three low-plasticity fine-grained mixtures of non-plastic silt with either bentonite or kaolinite clay minerals blended with fresh deionized water or saline water. The clay fractions were adjusted to achieve a plasticity index of PI = 9 for all three mixtures to study differences or similarities in their behavior and to examine the effectiveness of index properties in identifying the observed differences in lab results.

Even though all three blends have the same plasticity index, significant differences in their cyclic response were observed. Results indicate that plasticity index is an insufficient indicator of the cyclic behavior of low-plasticity fine-grained soils, and corrections for pore fluid chemistry and clay minerology may be needed for future liquefaction susceptibility and cyclic softening assessment procedures. Site-specific cyclic and monotonic testing for important projects remains to be the recommended approach for properly characterizing the seismic behavior of such soils with the current level of understating of their complex behavior.

The collected experimental data, visualization and post-processing tools, as well as documented experimental procedures of Part I are curated and published for public access on http:www.DesignSafe-ci.org. The data from Part II is being curated and will be published online in the near future.

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