Volume change behaviour of unsaturated compacted weakly expansive soils

In order to investigate volume change behaviour of unsaturated weakly expansive soils with different initial densities and water contents, a series of wetting tests were performed on compacted specimens at different vertical pressures. The unsaturated compression line, consisting of the compression equilibrium points in an unsaturated state under different vertical pressures, and the swell-under-load line, consisting of the swelling or compression equilibrium points after saturation under different vertical pressures, can all be obtained from the wetting tests at different vertical pressures. From intersections of the unsaturated compression lines and the swell-under-load lines with different initial dry densities, a unique line called the demarcation line (DL) is obtained. The physical meaning of DL is that the specimen volume remains unchanged during wetting. Therefore, the volume change behaviour of expansive soils in the plane of void ratio versus logarithm of net vertical pressure can be divided in two regions: the swelling region and the compression region. In the swelling region, if the initial state point is below the DL, the specimen will swell during wetting. In the compression region, if the initial state point is above the DL, the specimen will compress during wetting. Finally, the swelling and compression mechanisms of weakly expansive soils are discussed based on the pore-size distribution from the results of the Mercury Intrusion Porosimetry (MIP) tests on weakly expansive soil.