  (Shortened abstract) Snow slab avalanches release after the failure and
collapse of a weak layer buried below a cohesive snow slab. This results in the
propagation of a subsidence, known as a collapse wave or anticrack. The slab
may eventually break and detach from the rest of the snowpack, provided that
the slope is steep enough to enable gravity to overcome the friction. The only
theoretical framework to date (Heierli 2005) proposed an explicit solution for
the propagation speed in steady state which could not account for weak layer
properties, was not mathematically bounded for certain values of the physical
constants involved, and could not explain the newly uncovered supershear
transition for steep slopes. Here, a new model for the stationary propagation
of anticracks is set up, so as to account for the anticrack speed regime on the
one hand, and the supershear regime on the other hand, the existence of which
has been recently revealed and ascertained by numerical simulations. The
results presented here seem consistent with most of the available data, and
highlight the role that the compaction of the weak layer can play in reducing
the anticrack speed. On the contrary, by storing energy upon failure and
suddenly releasing it at the crack tip, the weak layer elasticity could help
justify the higher speeds sometimes observed in both regimes. Finally, a more
accurate model is proposed, based on the modelling of both the slab and the
weak layer as Timoshenko beams; although its complexity prevents us from
solving it analytically, it provides enlightening insights into the mechanical
processes at work at the interface between both layers, from a
strength-of-materials perspective. This analysis is a first step towards a
better understanding of the underlying mechanisms of propagation of cracks in
slab avalanches, and towards more accurate avalanche size and occurrence
predictions.
