State-Dependent Soil-Water Characteristic Curves (SDSWCCs) of Weathered Soils

It is generally recognized that unsaturated soil behaviour and properties are governed by two stress state variables: soil suction and net stress. Although a soil-water characteristic curve (SWCC) can provide a measurement of the water retention ability of an unsaturated soil, very often only a single stress state variable, i.e. soil suction, is considered in experimental programs. In addition, the volume changes of soil specimens during the determination of SWCC have often been ignored. Some recent experimental studies have demonstrated that the other stress state variable, i.e. net stress, is also important and needs to be considered. Ignoring the effects of net stress and volume changes on measuring any soil property of an unsaturated soil may impose severe constraints in the development and verification of advanced constitutive soil models and hence may affect the accuracy of numerical predictions significantly. In this study, a modified volumetric pressure plate extractor was used to determine the state-dependent soil-water characteristic curve (SDSWCC) of a completely decomposed granite (CDG) with due consideration of volume changes. Intact block samples of CDG were obtained from a fill slope in Hong Kong. In the laboratory, both intact and recompacted specimens were tested and subjected to different stress levels, together with one cycle of drying and wetting paths. The measured results from the CDG specimens are compared with the previous test results of a completely decomposed volcanic tuff (CDV) in Hong Kong. Differences in the observed soil-water characteristics are examined and discussed.