Stress-dependent water retention behaviour of two intact aeolian soils with multi-modal pore size distributions

The water retention curve (WRC) of unsaturated soils plays a significant role in evaluating rainfall-induced geohazards in geological and geotechnical engineering, such as ground subsidence and slope failure. Stress can affect WRC because loading/unloading alters not only density but also pore size distribution (PSD). Previous studies of stress-dependent WRC focused on soils with mono- and bi-modal PSDs only. In this study, the WRCs of intact paleosol and intact loess with quadri- and tri-modal PSDs were measured using suction- and stress-controlled pressure plate tests. The full-range PSDs of tested specimens were determined by combining X-ray computed tomography (CT) and mercury intrusion porosimetry (MIP). Experimental results show that the multi-modal PSDs and hence WRCs of both soils are strongly affected by stress in a similar approach. For instance, the relationship between degree of hysteresis and suction showed a double-humped pattern, in which the Peak-I and Peak-II were identified at the relatively low (0.1–20 kPa) and high (20–400 kPa) suctions, respectively. The application of stress resulted in a decrease of hysteresis in the Peak-I (about 30%) but an increase in the Peak-II (about 31%). The increase of hysteresis in the Peak-II differs from the behaviour of soils with mono- and bi-modal PSDs, mainly because the reduction of mega- and macro-pore diameters upon loading increases pore non-uniformity in the range corresponding to the Peak-II.