The paper describes a three phase single point MPM formulation of the coupled flow (water 7 and air) -mechanical analysis of geotechnical problems involving unsaturated soils. The 8 governing balance and dynamic momentum equations are discretized and adapted to MPM 9 characteristics: an Eulerian computational mesh and a Lagrangian analysis of material 10 points. General mathematical expressions for the terms of the set of governing equations are 11given. A suction dependent elastoplastic Mohr-Coulomb model, expressed in terms of net 12 stress and suction variables is implemented. The instability of a slope subjected to rain 13 infiltration, inspired from a real case, is solved and discussed. The model shows the 14 development of the initial failure surface in a region of deviatoric strain localization, the 15 evolution of stress and suction states in some characteristic locations, the progressive large 16 strain deformation of the slope and the dynamics of the motion characterized by the history 17 of displacement, velocity and acceleration of the unstable mass. 18 19

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Long runout landslides can cause significant damage and represent one of the most important problems in geotechnical engineering. Understanding the mechanics of the landslide runout process is important for risk assessment and is challenging due to its complexities. This work examines the runout of the 22 March 2014 Oso, Washington, landslide. The Oso landslide is one of the worst landslide disasters in USA history with 43 fatalities. It occurred in multiple failure stages, involving several failure surfaces and significant soil softening, and travelled over 1 km across the valley. It initiated after a period of wet weather in an area prone to landslide movements. The triggering causes of the landslide movement are still under investigation. In this paper, the material point method is used to simulate the runout of the Oso landslide. This numerical tool is capable of modeling large deformation problems. It is used to investigate several hypothetical scenarios to identify key factors that contributed to the Oso landslide long runout distance.

One of the factors causing the acceleration of landslides is the loss of strength of the soil involved in the potential unstable mechanism. The travelled distance and the landslide velocity, a key factor in risk analysis, will be determined by the loss of resistant forces. Brittle behaviour, commonly associated with cemented soils, overconsolidated plastic clay formations and sensitive clays, lead to the progressive failure phenomenon explained by the reduction of the strength with increasing strain. In the present study, this phenomenon has been analysed in the case of a saturated slope which becomes unstable by increasing the boundary pore water pressure. A Mohr-Coulomb model with strain softening behaviour induced by increasing deviatoric plastic strain is used. The paper focusses not only on the stability of the slope but also on the post failure behaviour (run-out and sliding velocity). A coupled hydro-mechanical formulation of the material point method has been used to simulate the whole instability process. The influence of the brittleness of the material on the triggering of instability and run-out is evaluated by means of a parametric study varying peak and residual strength. The onset of the failure and the failure geometry are controlled by both peak and residual values. Good correlations between run-outs and brittleness are found. The decay of the strength determines the acceleration of the landslides and the travelled distance.Peer ReviewedPostprint (author's final draft