Experimental study of stress-dependent soil-water characteristics and their applications on numerical analysis of slope stability

Soil water characteristic curve (SWCC) is an important physical property representing the water storage capacity of a soil at different soil suctions. It is a vital characteristic to govern engineering behavior of an unsaturated soil and is an essential hydraulic parameter for transient seepage analysis in unsaturated soils. In this thesis, a new triaxial pressure-plate system equipped with an accurate volume measuring device has been developed for determining stress-dependent SWCC (SDSWCC) under isotropic and deviatoric stress conditions. Three series of tests have been carried out for investigating the influence of stress states under Ko, isotropic and deviatoric conditions on soil-water characteristics of a completely decomposed tuff (CDT) in Hong Kong. Some SWCCs and SDSWCCs have been used as input parameters for simulating transient seepage analysis in an initially unsaturated slope.Under Ko stress condition, both desorption rate and adsorption rate increase with an applied stress, but the size of the hysteresis loop is not significantly affected by the stress level applied. For the test series under isotropic stress, the desorption rate and the adsorption rate remain almost unchanged as an applied stress increases. Moreover, the size of the hysteresis loop decreases with an increase in isotropic stress. Under deviatoric stress of stress ratio, q/p, equal to 1.2, a higher applied stress, a lower desorption rate and a smaller hysteresis loop are. A common phenomenon among the three test series is an increase of an air-entry value with applied stress. Furthermore, it is found that the compression of a specimen under applied load and shrinkage during a drying stage has significant influence on the estimated air-entry value. Measured air-entry values with volume change correction are higher than that without correction for volume changes.Parametric numerical analyses have been carried out to investigate the effects of wetting path and stress-dependency of SWCC (SDSWCC) on pore water pressure distributions and hence factor of safety of an initially unsaturated soil slope when it is subjected to a prolonged 10-day rainfall. Damming effects on pore water pressure distributions due to the presence of pile obstacles are also studied two-dimensionally. During the 10-day prolonged rainfall, the FOS obtained from a conventional analyses using a drying SWCC is the highest, whereas an analysis using wetting SDSWCCs would predict the most adverse initial pore-water pressure distributions with depth, and hence the lowest FOS. For the geometry and ground conditions considered, damming of groundwater due to the presence of pile obstacles is very limited. Thus, there is no significant difference in the computed FOSs with and without considering damming effects.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2004