Stress Concentrations in Weak Snowpack Layers and Conditions for Slab Avalanche Release

Dry‐snow slab avalanches release due to the formation of a crack in a weak layer buried below cohesive snow slabs, followed by rapid crack propagation. The onset of rapid crack propagation occurs if stresses at the crack tip in the weak layer overcome its strength. In this study, we use the finite element method to evaluate the maximum shear stress τmax induced by a preexisting crack in a weak snow layer allowing for the bending of the overlaying slab. It is shown that τmax increases with increasing crack length, slab thickness, slab density, weak layer elastic modulus, and slope angle. In contrast, τmax decreases with increasing elastic modulus of the slab. Assuming a realistic failure envelope, we computed the critical crack length ac for the onset of crack propagation. The model allows for remote triggering from flat (or low angle) terrain. Yet it shows that the critical crack length decreases with increasing slope angle. Plain Language Summary Dry‐snow slab avalanches release due to the formation of a crack in a weak layer buried below cohesive snow slabs, followed by rapid crack propagation. Characterizing conditions for the onset of crack propagation in snow is a great challenge and has been the subject of several investigations. Yet there is still no consensus about the nature of the initial failure in the weak layer, whether it occurs in shear only or if the collapse of the weak layer helps to drive crack propagation. Here, to investigate this question, we employed a numerical model to study stress concentrations in the weak layer in the presence of a preexisting crack, allowing the bending of the overlaying slab. We computed the maximum shear stress close to the crack tip for different system configurations and mechanical properties. We showed that steeper slopes promote crack propagation as predicted by classical shear models. However, the collapse of the weak layer is essential for crack propagation from flat terrain and thus remote avalanche triggering. Key Points We evaluate the effect of slab deformation on the onset of crack propagation in buried weak snow layers using the finite element method The critical crack length for the onset of crack propagation decreases with increasing slope angle Slab bending, induced by weak layer collapse, is essential for crack propagation from flat terrain and thus remote avalanche triggering