The effects of snow variability on slab avalanche release

Dry snow slab avalanches are released by the failure of a weak layer underlying a thick cohesive slab. A model is derived which accounts for spatial variability of this weak layer and for stress redistribution between weak and strong regions. The model is applied to the case of randomly varying shear strength of the weak layer. It is demonstrated that strength variations have a dramatic knockdown effect on slope stability. An equation is derived, which relates the critical load supported by the weak layer to its mean shear strength and shear strength variance, and this equation is used to formulate a slope stability index with fluctuation corrections. The nature of the critical flaw is studied, and precursors to failure are investigated. The effect of time healing of damaged regions due to fast metamorphism is introduced into the model, and it is demonstrated that this has a minimal effect on slope failure strength. The internal stresses in the snowpack are evaluated and discussed, and a failure scenario is developed which accounts for the interplay of snowpack heterogeneity, strain softening and internal stresses.