Abstract: The processes of dehydration shrinkage and water-swelling of expansive soil correspond to the drying and wetting stages of the soil-water characteristic curve (SWCC). The SWCC plays an important role in defining the hydro-mechanical behavior of unsaturated soils. In order to study the SWCC of Nanyang expansive soil in full suction range, three suction control or measurement methods are used: the pressure plate method for low suction range of 0-1.5 MPa, the filter paper method for suction range of 0-40 MPa, and the vapor equilibrium technique for high suction range of 3-368 MPa. The results show that the SWCC has obvious hysteresis phenomenon during the drying and wetting in the low suction range. When the suction is greater than 300 MPa, the hysteresis effect of SWCC disappears; and the drying and wetting curves are almost overlapped. Test data of soil-water characteristic by the filter paper method are located between the main drying and wetting curves of the SWCC. When the suction of is 367.54 MPa, the soil water content is only 0.325%, and it almost approximates to zero. Void ratio not only changes with suction, but also it is influenced by the suction history and suction path. In the relation between void ratio and suction, the curve of wetting path is above that of the drying path; and they become the same at low suction. There is a clear hysteresis of the relation between void ratio and saturation degree, due to the different suction paths; and they will be consistent when close to saturation. The saturation degree increases with the increasing void ratio when suction changes. The experimental data obtained by the vapor equilibrium method have an obvious linear relationship between void ratio and saturation degree; while the test data obtained by the pressure plate method are no an obvious linear relationship in the low suction range.

Key words: soil-water characteristic curve(SWCC), pressure plate method, filter paper method, vapor equilibrium method, full suction range