New insights on avalanche release mechanics based on large-scale elastoplastic simulations

The release of snow slab avalanches starts with the failure of highly porous weak snow layer buried beneath a cohesive snow slab leading to mixed-mode crack propagation along the slope. The first modelling attempt of the process date back to 1979 with a pure shear weak layer fracture assumption proposed by McClung. Later, Heierli extended the concept of anticrack, to account for weak layer volumetric collapse and subsequent slab bending. Recent advances reconciled these different approaches and have shown the existence of a supershear crack propagation regime leading to intersonic crack propagation speeds in the up and down-slope directions.
    
    In this work, based on the Material Point Method, finite strain elastoplasticy and critical state theory, we report a transition from sub-Rayleigh anticrack to supershear crack propagation involving the Burridge--Andrews mechanism. The existence of this transition is further confirmed by full-scale avalanche analyses. By accounting for slab fracture, we highlight that soft slabs can prevent supershear transitions to occur. In addition, it is shown that crack branching in the slab can either occur from top to bottom in the case of slow propagating anticracks or from bottom to top for supershear cracks. Through a sensitivity analysis, we investigate the conditions for crack arrest or for the so-called 'en échelon' slab fracture mechanism. Finally, full 3D simulations reveal interesting propagation and release patterns related to the interplay between cross-slope and down/up-slope propagation as well as slab tensile failure. This enables to analyse slab fracture modes at crown, flanks and staunchwall of the avalanche. These new findings allow us to reach a next step in our understanding of the avalanche release mechanics in order to predict both avalanche release shapes and sizes.