A THM-coupled MPM framework for modelling rock-ice avalanches

Rock-ice avalanches exhibit hypermobility due to thermo-hydro-mechanical (THM) processes involving ice-water phase transitions, frictional heating, and thermal pressurization. They are typically characterized by high velocities and extended runout distance, posing significant hazards to human life and infrastructure in cold mountainous regions. This study presents a novel three-phase material point method (MPM) framework unifying THM coupling among skeleton deformation, pore water seepage, heat transfer, and ice-water phase change within the sliding rock-ice mixture. It also features a multi-material frictional contact model to describe the interactions of multiphase porous mixture with the basal terrain, accounting for frictional heating and thermal pressurization at the interface. The framework further introduces a shear band scaling technique to resolve sub-grid thermal-hydraulic processes. It is validated against (i) the classical freezing process in a semi-infinite porous medium, (ii) frictional heating in elastic/elastoplastic sliding blocks, and (iii) the 2016 Lamplugh rock-ice avalanche. The predictions demonstrate that thermal pressurization substantially reduces basal friction, reasonably explaining the extreme runout (10.5 km) at Lamplugh. The framework offers a unified prediction tool for simulating THM-driven hypermobility in complex terrains.