Influence of stress state on soil-water characteristics and slope stability

A soil-water characteristic curve defines the relationship between the soil (matric) suction and either the water content or the degree of saturation. Physically, this soil-water characteristic is a measure of the water storage capacity of the soil for a given soil suction. Conventionally, the soil-water characteristic curves (SWCCs) are determined in the laboratory using a pressure plate apparatus in which vertical or confining stress cannot be applied. For investigating the influence on the stress state on the soil-water characteristics, a new stress controllable pressure plate apparatus has been developed. Effects of K-0 stress conditions on the SWCCs of an undisturbed volcanic soil in Hong Kong are determined and illustrated. The net normal stresses considered in the apparatus are 40 and 80 kPa, which are appropriate for many slope failures in Hong Kong. Experimental results show that the soil-water characteristic of the soil specimens is strongly dependent on the confining stress. Numerical analyses of transient seepage in unsaturated soil slopes using the measured stress-dependent soil-water characteristic curves predict that the distributions of pore-water pressure can be significantly different from those predicted by the analyses using the conventional drying SWCC. For the cut slope and the rainfall considered, the former analyses predicted a considerably lower factor of safety than that by the latter analyses. These results suggest that wetting stress-dependent soil-water characteristic curves should be considered for better and safer assessment of slope instability.