Dry snow slab quasi‐brittle fracture initiation and verification from field tests

Abstract

Dry snow slab avalanches initiate from shear fracture in a thin weak layer underneath a planar slab of cohesive snow. In this paper, field data from more than 400 snow shear fracture tests are analysed and applied to the problem of snow slab release and snow pack instability evaluation. The paper contains a new, but simple, analytical model to estimate the critical length for fracture. The model contains the assumption of a finite fracture process zone which may be a significant fraction of the slab depth D or the critical length L for weak layer shear fracture. The results suggest that the ratio L/D is of order 1 for both the data and the model as previously predicted for the snow slab. Using a combination of experimental results from laboratory tests, two sets of field data and the new model, suggests that previous theoretical arguments about snow slab release over almost 30 years are congruent with the field data. Simple extension to explain viscoelastic temperature effects and slope normal weak layer deformation on snow slab instability evaluation is included in the analysis. Since the experimental field data contain viscoelastic effects and slope normal effects, it is imperative to account for these in the model and for snow slab instability evaluation.