An MPM-FDM coupled method for landslide analysis considering surface–subsurface conjugated water flow

The material point method (MPM) can effectively simulate large deformation problems involving hydro-mechanical coupling, such as rainfall-induced landslides. Current MPM formulations simulate rainfall boundaries by applying either pore water pressure or velocity boundaries directly. This method does not incorporate the effects of surface water ponding and runoff during heavy rainfall. To address this problem, this study proposes a coupled method that integrates the MPM with the finite difference method (FDM) for hydro-mechanical analysis. Underground water flow is modelled using a two-phase, two-point MPM with the Richards equation, while surface water flow is computed by FDM based on shallow water equations. The two models are coupled: the FDM provides the surface water flow velocity and pore water pressure for subsurface flow simulation in the MPM, while the MPM supplies the surface infiltration rate for surface water flow simulation in the FDM. The new method was validated against existing numerical simulations and centrifuge tests. It was found that the new method can effectively capture the interactions between surface and subsurface flows, as well as the shallow landslide involving surface erosion or washout, which existing MPM codes cannot simulate. Parametric studies further reveal that neglecting the coupling effects of surface–subsurface flow predicts deeper sliding surfaces and longer rainfall durations to failure due to the ignorance of surface ponding and positive pore water pressure at the ground surface. Considering surface water flow tends to shift the failure mode from “slide-to-flow” to “flow-like”, especially when slope angle is larger and soil permeability is lower.