Expansive soil will appear volume expansion under moisture adsorption condition. It is very important to study the soil-water characteristics of expansive soil under moisture adsorption and expansive conditions. Considering nine Hebei expansive soils as the study objects

soil-water characteristic curve (SWCC) tests by osmotic method and filter paper method are carried out. Filter paper method is used to measure the lower suction range

while osmotic method is used to measure the higher suction range. Moisture adsorption tests are carried out by steam humidification method. Based on soil-water characteristics tests and moisture adsorption tests

the volume change rules of expansive soils under wetting condition can be obtained. The experiment data are then curve fitting by using Matlab

and the gravity water content-suction curves (w-ψ curves) and the volumetric water content-suction curves (θw3-ψ curves) are obtained. The influence of the volume change of expansive soil under moisture adsorption condition on the θw3-ψ curves is analyzed; and the variability and correlation between curve fitting parameters of w-ψ curves and θw3-ψ curves are also studied. The results show that the steam humidification method is a good approach for moisture adsorption test. During the steam humidification process

the moisture is adsorbed slowly and uniformly

and the shape of soil sample remains unchanged. The void ratio?gravity water content curve can be divided into four stages

which are initial stage

approximate linear stage

nearly horizontal stage

and ascent stage along the saturation line. The difference between the two types of SWCCs (w-ψ curves and θw3-ψ curves) is due to the volume change of expansive soil samples during the wetting process

and the extent of volume change depends heavily on the water content of expansive soil samples after the moisture adsorption process. The variability of w-ψ curves is less than that of θw3-ψ curves. For both of the two types of SWCCs

the variability of parameter n is relatively small

which means that the slopes of SWCCs are close to each other for different soils. The variabilites of parameters a and m are larger than that of parameter n. The corresponding curve fitting parameters of w-ψ curves and θw3-ψ curves are linearly correlated. Therefore

the curve fitting parameters of θw3-ψ curves can be preliminary estimated by those of w-ψ curves.