Stress-Dependent Soil Water Characteristic Curves of Compacted Sand–Gypsum Mixtures

The presence of calcium sulfate (CaSO4) impacts soil structure, bonding, pore size distribution, and water-holding characteristics, altering both compaction characteristics and soil water characteristic curves (SWCC). In this study, compacted sand with different amounts of gypsum (0–60%) were tested under controlled stresses, focusing on assessing the combined influence of gypsum (Gc) and net-normal stress (σn) on the SWCC characteristics under drying–wetting conditions. The results demonstrate that adding gypsum to sand makes the SWCC shape flatter, and the position is shifted upward to the left side. The SWCC-suction parameters at air entry and residual point for drying and wetting curves (ψa, ψr, ψae, and ψwe) increase when gypsum content increases, whereas the corresponding SWCC-water content parameters (θa, θr, θae, and θwe) generally decrease first and then increase beyond 20% gypsum content in drying and 40% in wetting. The SWCC-suction parameters (ψa and ψae) increase first and then decrease as net-normal stress increases while the (ψr and ψwe) express a decrease rate. The water content parameters (θa and θae) show a reduction trend with increasing net-normal stress, and the influence is insignificant on the parameters (θr and θwe). The hysteresis area (HA) trend was affected by the combined effect of both Gc and σn. Water holding capacity (WHC) generally increases with Gc; however, at certain Gc, it decreases with an increasing σn. Fitting the classical SWCC models shows that it is possible to use these models for estimating SWCC of soils containing relatively low gypsum content (Gc ≤ 20%). For gypsum rich soils, both SSR and RMSE exhibit an increase trend indicating poor prediction because of the emerging dual-porosity behavior that is not entirely captured by the models. In general, the SWCC behavior was dominated by sand skeleton, gypsum cementation, and pore-filling mechanisms in low gypsum mixtures, whereas the evolved microstructural alteration governed the high gypsum mixtures.