Snow Toughness Measurements and Possible Applications to Avalanche Triggering

A snow slab avalanche release usually results from the unstable expansion of a basal crack at the interface between an upper layer (slab) and an underlying substrate, followed by crown crack nucleation and propagation. Despite the fact that many models proposed so far to predict this kind of rupture were only based on continuum mechanics, the use of fracture mechanics seems to be more appropriate to deal with the possible unstable propagation of such defects. For this purpose, a precise knowledge of snow fracture toughnesses in both tensile and shear modes is needed. In the present work, we developed an experimental set similar to Kirchner and Michot's, in order to measure mode I toughness. The experimental campaign carried out in the Alps during the 2000-2001 and 2001-2002 winters on homogeneous sintered snow with different densities gave toughness results of the same order of magnitude as Michot's. However, an unexpected and reproducible dependence of toughness on cantilever length was evidenced. Discrete element simulations of the toughness experiment, considering snow as a cohesive granular material, showed that the elastic energy was stored along a branching pattern. These findings suggest that the classical toughness should be replaced by a generalised toughness defined on the basis of the fractal dimension of this force line pattern.