Effects of the water retention curve and infiltration process in progressive landslides using the Material Point Method (MPM)

This study investigates the influence of infiltration processes and the soil-water retention curve (SWRC) on the initiation and evolution of progressive landslides in unsaturated soils using the Material Point Method (MPM). An unsaturated biphasic formulation was adopted, incorporating suction effects and strain- softening behavior through a Mohr-Coulomb model with continuous degradation. The hydraulic response was defined by the van Genuchten model, representing the nonlinear relationship between saturation and matric suction. Simulations under different SWRC conditions evaluated their effects on failure initiation, run-out distance, and post-failure retrogression. The results confirm MPM's ability to capture large-deformation phenomena—including failure propagation and secondary displacements—not addressed by traditional small-strain methods. The retention curve parameters directly influenced failure timing, total displacements, and saturation evolution, highlighting the importance of accurate SWRC calibration. Hysteresis effects were not considered, as the focus was on wetting conditions under sustained infiltration. These findings provide insight into slope instability processes under hydrological forcing and support the application of MPM to unsaturated soil problems in geotechnical risk assessment and design.