Hydro-mechanical coupling effects on volume change and water retention behaviour of unsaturated expansive soils during compression

During compression of unsaturated soils, the volume change and water content change interact simultaneously. The hydro-mechanical coupling effects on both of them are comprehensively investigated. For this purpose, tests on water density in soils and suction-controlled (0~1000 kPa) one-dimensional compression tests on saturated and unsaturated soils are carried out for Jingmen expansive soils. The conclusions are drawn as follows: (1) For the volume change behaviour, the yielding point can be observed on the load-compression curve, and the compression ones under higher suction intersect the curves under lower suction sequentially. The unloading curves appear to be linear, and the slopes of which decrease with the increasing suction. The volume change formula is presented, which is capable of predicting the shrinkage due to suction increase, volume compression/expansion due to loading/unloading, yielding, and the variety of compressibility due to suction change. (2) For the water retention behaviour during compression, the water content changes slightly under higher suction. When compressed to the net vertical pressure of 2941.8 kPa, the water contents under the four different suctions are similar. A three-parameter Logistic function is presented for simulating the coupling effects of suction and net vertical stress on water content. (3) The degree of saturation increases during both loading and unloading, both of which mean ‘wetting’. However, the water content decreases during loading, which means ‘drying’. The reason for this discrepancy is that the change of degree of saturation can be affected by change of the water content as well as that of void ratio.