THESIS
2008
xix, 138 leaves : ill. ; 30 cm
Abstract
One of the most distinct features of unsaturated soil behavior is the potential of collapse upon saturation. Alonso et al. (1987) stated that an unsaturated soil may either swell or collapse upon saturation if the confining stress is sufficiently low or high, and that it is also possible that a soil might experience a reversal in the volumetric behavior during wetting, i.e., an initial swelling followed by collapse. In this kind of study, the wetting-induced soil collapse is considered to uniquely correlate with the change of degree of saturation or suction for a given soil, under the same confining pressure and initial dry density. Moreover, the pore fluid distribution is assumed to be uniform during this variation process. Actually, for a soil element, the spatially graded distributio...[
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One of the most distinct features of unsaturated soil behavior is the potential of collapse upon saturation. Alonso et al. (1987) stated that an unsaturated soil may either swell or collapse upon saturation if the confining stress is sufficiently low or high, and that it is also possible that a soil might experience a reversal in the volumetric behavior during wetting, i.e., an initial swelling followed by collapse. In this kind of study, the wetting-induced soil collapse is considered to uniquely correlate with the change of degree of saturation or suction for a given soil, under the same confining pressure and initial dry density. Moreover, the pore fluid distribution is assumed to be uniform during this variation process. Actually, for a soil element, the spatially graded distribution of pore fluid always exists before a uniform distribution pore fluid state is reached. However the influence of the nonuniform distribution of the pore fluid on the wetting-induced collapse has been overlooked for a long time.
Based on a microscopic study by Li (2003), the uneven distribution of the fluid on the grain surface may lead to the solid grains becoming self-unbalanced, and such microscopically unbalanced fluid pressures will lead to an auxiliary effective stress at the continuum level. This microscopic analysis demonstrates the importance of the specific microstructure of soil constituents in determining the mechanical behavior of the soil skeleton.
In this study, a series of different wetting tests at various rates were conducted to investigate the influence of the spatially graded pore fluid distribution on the wetting-induced soil collapse behavior. It was found that the volume change for any zone increased significantly with the increasing gradient of degree of saturation in this corresponding zone. Moreover, the stabilization of volumetric strain for each zone always lags a few hours behind the arrival of the corresponding peak of degree of saturation gradient. With the reduction of the gradient of degree of saturation after the peak value, the volumetric strain does not change anymore. The reason may be attributed to the fact that the wetting-induced soil collapse is irreversible. The reduction of the gradient of degree of saturation is analogous to the unloading process, in which only the elastic volume change can be recovered.
Through controlling different wetting rates, the gradient of degree of saturation can be changed. It was found that the magnitude of the wetting-induced soil collapse decreases with decreasing gradient of degree of saturation at a reduced rate. As the gradient of degree of saturation reduces to a particular value, its influence on wetting-induced collapse becomes insignificant.
This finding provided encouraging evidence to the important influence of nonuniform distribution of the pore fluid on the wetting-induced soil collapse.
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