Experimental investigations of stress or void ratio responses of loess upon confined and vertical unconfined wetting

Many geological disasters in loess areas are related to soil wetting. Previous studies of loess wetting behavior focused mainly on zero vertical net stress or constant vertical net stress, while few studies have been conducted under confined conditions (constant volume). The stress regime and microstructure evolution characteristic of loess subjecting confined wetting remain unclear. This study investigates the intact loess of two burial depths (denoted as HFT10 m and HFT30 m) subjected to controlled-suction wetting under both confined and vertical unconfined conditions. The loess microstructure evolution characteristics before and after loading-confined and loading–unloading-unconfined wetting were evaluated by mercury intrusion porosimetry (MIP) tests. Under vertical unconfined wetting, the results indicated that all specimens exhibited wetting-induced expansion, with denser specimens showing higher swelling strain. Under confined wetting, the vertical stress decreased gradually, and the extent of stress reduction intensified as specimen strain increased. The HFT10 m sample showed a larger wetting-induced stress reduction than the HFT30 m samples. The soil water retention curves (SWRCs) at the degree of saturation (S_r)-void ratio (e)-suction (s) space between confined and unconfined was different, this may be due to the different microstructure evolution characteristic of unconfined wetting and confined wetting. The paths (suction against vertical net stress) were within the newly defined loading-collapse (LC) curve. The experimental results of this study highlight that the vertical stress in the soil beneath the bearing platform decreases upon wetting, which may result in an increase in the axial stress of the pile, even if the loess does not undergo collapse.