Stress effects on soil-water characteristics of unsaturated expansive soils

Soil-water characteristic curve (SWCC) is a key property for unsaturated soils. It descibes the water retention capacity of unsaturated soils. SWCC can be used to predict other useful soil properties such as water permeability and is an essential property for both numerical and analytical transient seepage and slope stability analyses.In the thesis, a newly modified triaxial apparatus has been used to investigate the influence of isotropic (ISO) stress conditions on measured SWCCs of some unsaturated expansive soils from China. A one-dimensional (1D) stress controllable volumetric pressure plate extractor and pressure plate/membrane extractors without applying any external stress are also used in the study. The SWCCs of the expansive soils are found to be stress dependent. The difference between SWCCs tested under zero stress and non-zero stresses is significant. Generally, the expansive soil has a lower water retention ability under stress compared with the results from conventional SWCC tests without external stress. However, the difference between 1D and ISO stresses is small except for the part in the matric suction range of l-10 kPa. There is no difference in soil-water characteristics under stress of 50- and lOO-kPa when the suction is higher than 10 kPa. The test results of Scanning Electron Microscopy (SEM) and Mercury Instrusion Porosimetry (MIP) show that the recompacted ZY expansive soil has a bi-modal distribution of pore sizes. The pore spaces are mostly composed by inter-aggregate and intra-aggregate pores. Because inter-aggregate pores can be compressed by external stresses, the SWCCs exhibit clear difference under the low suction range from 1 to 10 kPa. Intra-aggregate pores are left unchanged, therefore, the soil-water characteristics are more or less the same in high suction range of l0-500 kPa, regardless of different stress conditions.A parametric study has been carried out to investigate the influence of rainfall patterns, amount and duration on groundwater responses in an unsaturated slope of residual soils. As for 24-hr short rainfalls, the advanced rainfall pattern seems to be the most critical rainfall type for the slope stability because it can always induce the highest pore-water pressure distributions along the slope. The higher the rainfall depth is, the higher the pore-water pressure buildup will be. However, in this study, it is found that a further increase in the return period of a rainfall of l00-yr, the pore-water pressure does not necessarily increase. Rainfall patterns are more relevant for short rainfalls, since the rainwater distributes more uniformly for storms with long duration.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2000