Journal: Computers and Geotechnics

Authors: Guodong Ma a, Annan Zhou a,1,*, Ha H. Bui b.

Affiliations: a: Department of Civil and Infrastructure Engineering, School of Engineering, Royal Melbourne Institute of Technology (RMIT University)

b: Department of Civil and Environmental Engineering, Monash University

Rainfall-induced unsaturated slope failure is one of the most common and destructive forms of landslides worldwide, often causing significant casualties and economic losses. Accurate prediction of such failures requires not only robust numerical methods to capture the coupled hydraulic and mechanical behavior of unsaturated soils, but also an in-depth understanding of how the spatial variability of soil properties affects slope stability. In this study, smoothed particle hydrodynamics (SPH) is combined with an advanced unsaturated constitutive model to investigate, for the first time, the effects of void ratio dependence and variability on rainfall-induced unsaturated slope failure. The model captures the hydro-mechanical coupling behavior of unsaturated soils, accounting for the influence of void ratio on water retention, infiltration, and strength characteristics. A single-layer multiphase SPH method is adopted, in which each particle simultaneously represents the water, air, andsolid phasesenabling efficient and robust simulation of large-deformation problems. The SPH model is applied to a synthetic slope with spatially variable soil properties to investigate howvoid ratioheterogeneity alters failure mechanisms and failure initiation conditions. The results provide new insights into the role of void ratio heterogeneity in rainfall-induced landslides and demonstrate the potential of advanced SPH modeling for practical probabilistic analysis in geotechnical applications.

Fig. 1 Degree of saturation and relative permeability coefficient under different void ratios.

Fig. 2 Comparative evaluation of the current model against the Zhou and Sheng (2015) model and experimental data.

Fig. 3 Effects of initial void ratio and initial suction on the performance of the constitutive model.

Fig. 4 Geometry and boundary conditions of the synthetic embankment.

Fig. 5 Surface profile of slope failure.

Fig. 6 Evolution of pore water pressure and vertical effective stress in the homogeneous slope.

Fig. 7 Evolution of porosity and deformation in the homogeneous slope.

Fig. 8 Evolution of pore water pressure and vertical effective stress in the heterogeneous slope.

This study develops and implements a fully coupled smoothed particle hydrodynamics (SPH) numerical framework to investigate rainfall-induced unsaturated slope failure, with particular focus on the dependence and variability of void ratio in unsaturated soils. An improved unsaturated constitutive model is successfully integrated into the SPH framework to capture the complex hydro-mechanical interactions during slope failure. The performance of the unsaturated constitutive model is first verified at the element level, and then validated through SPH simple shear tests and granular collapse tests. The fully coupled SPH model is applied to a rainfall infiltration scenario on an unsaturated embankment slope, providing important insights into how pore-space heterogeneity affects moisture infiltration patterns, pore pressure development, effective stress, and slope stability. Increasing the spatial variability of void ratio generally enhances the potential instability of the slope, creating more preferential paths for seepage. Consequently, due to significant density variability, the infiltration time is shortened and soil strength deteriorates. The numerical results show that increasing the spatial variability of void ratio enlarges the sliding area at slope failure, but has a limited effect on the sliding distance. It also leads to earlier onset of slope failure.

Overall, the SPH-based approach proposed in this paper provides a robust and effective tool for analyzing complex slope failure mechanisms in unsaturated soils. The findings offer valuable guidance and lay a solid foundation for future probabilistic and performance-based slope stability assessments under rainfall conditions, particularly for heterogeneous geomaterials.

Ma, G., Zhou, A., & Bui, H. H. (2026). Smoothed particle hydrodynamics (SPH) modelling for rainfall-induced unsaturated slope failure considering void ratio dependence and variability. Computers and Geotechnics, 189, 107629.