Two-phase fully-coupled smoothed particle hydrodynamics (SPH) model for unsaturated soils and its application to rainfall-induced slope collapse

In this paper, a new fully-coupled Smoothed Particle Hydrodynamics (SPH) formulation for unsaturated soils is developed to study the influence of rainfall infiltration on slope stability. The single-layer two-phase formulation is investigated in SPH for the first time to simulate the response of unsaturated soils. The use of a single set of particles improves the computational efficiency and facilitates the implementation of infiltration boundary conditions. The Drucker–Prager strain-softening model, with the use of Bishop's effective stress, is adopted as the soil’s constitutive model. New extensions of a first-order consistent wall boundary treatment are proposed for the coupled hydro-mechanical problem to enforce non-slip/free-slip conditions for the soil phase and water phase. A novel stress diffusion algorithm for general application is introduced to smooth out the numerical noise in the stress field under large deformation. The accuracy of the formulated SPH model is examined with available analytical solutions and experimental data. The proposed numerical scheme is finally applied to the simulation of rainfall-induced slope collapse of an unsaturated slope with two different bedrock geometries. Results demonstrate that the geometry of the bedrock is shown to play an important role in the failure initiation and propagation of the collapse. It is found that the proposed model allows the investigation of both triggering and post-failure mechanism, providing a smooth stress field even at large deformations.