Failure initiation in snow stratifications containing weak layers: Nucleation of whumpfs and slab avalanches

The failure of snow stratifications containing weak layers is re-investigated as recent experiments indicate that the established assumption of failure in pure shear (initiating a shear band or slip surface) must be reconsidered. Local failure of the weak layer implies, in the general case, simultaneous slope-parallel and slope-normal displacements of the overlying slab, the latter being caused by local volume changes of the weak layer in the course of failure. Considering both slope-parallel and slope-normal deformation, we calculate the change of total energy in response to a disturbance of the weak layer. The critical size of a flaw in the weak layer and the energy barrier for failure propagation are thus evaluated in terms of continuum properties of the stratification. The model is applied to the analysis of critical flaw sizes required for the initiation of snow slab avalanches and/or whumpfs. Unlike previous theories, the model is consistent with a critical crack length that may increase with increasing slope angle, as recently confirmed in field experiments. The present model predicts substantially decreased energy barrier and critical crack length with respect to existing theories. It unifies and extends previous work on shear failure of slopes and on crack nucleation in collapsible stratifications.