Effects of Grain Size, Density, and Contact Angle on the Soil–Water Characteristic Curve of Coarse Granular Materials

The soil–water characteristic curve (SWCC) is essential for understanding hydraulic behavior in geotechnical applications involving coarse granular materials. However, existing models often overlook the coupled effects of key factors. This study systematically investigates the influence of grain size distribution, density, and contact angle on the SWCC using a numerical approach that combines the discrete element method (DEM) with an enhanced pore morphology method incorporating locally variable contact angles (Lvca-PMM). The results show that smaller uniformity coefficients (Cu), larger median grain sizes (D50), higher porosity (φ), and larger contact angles (θ) shift the SWCC to the left, reducing both the air entry value (Ψa) and residual suction (Ψr). Specifically, linear relationships were identified between Ψa, Ψr, Cu, φ, and cos(θ), while a power-law relationship was observed with D50. Furthermore, the interaction of these factors plays a critical role, where a change in one property can amplify or diminish the effects of others. Based on these findings, empirical equations for predicting Ψa and Ψr were developed, offering practical tools for engineers to efficiently estimate the SWCC. This research provides deeper insight into the water retention properties of coarse soils and supports the optimized design of granular fills and drainage systems.