The critical size of macroscopic imperfections in dry snow slab avalanche initiation

Abstract

[1] Dry snow slab avalanches initiate after mode II fracture propagation within a thin, weak layer under a planar slab. Dry alpine snow in which avalanches form is a porous material, typically with volume fraction filled by solids of between 10% and 50%. If snow slab avalanches were caused by small scale flaws, there would be continuous avalanches and alpine snow would not survive on steep slopes. Instead, snow slab avalanches initiate from macroscopic imperfections or weak zones within the weak layer. These are called "sweet spots" by practitioners and deficit zones in the pioneering work by Conway and Abrahamson in the 1980s. In this paper, the results of 750 in situ shear fracture tests from 68 slab-weak layer combinations are summarized in relation to the critical sweet spot length and weak layer crystal form. The results show that the critical sweet spot lengths follow a gamma probability density function with a range from 0.14 to 1.3 m and most probable value 0.5 m. The results also imply that critical sweet spot length depends on weak layer grain type with significantly larger values for persistent forms (surface hoar and facets) than for nonpersistent forms (decomposing and fragmented grains).

The propagation saw test: slope scale validation and alternative test methods

“…However, the objective of the PST is not to measure the true critical length required for propagation in an undisturbed snowpack, but rather to test the general propensity for propagation to begin and the ability of the slab and weak layer to sustain propagation once it begins. In this sense, the standard PST method may not appropriately test natural initiation lengths or realistically replicate slab deformation required for selfpropagating fractures (McClung, 2009(McClung, , 2011, but allows users to target and identify most slab and weak-layer combinations in which a propagating fracture can easily begin and be sustained (Tables 2 and 3). The only consistent exception appears to be in shallow, soft slabs where perhaps the lateral support provided by the 3-D snowpack on a natural slope sustains the slab and weak-layer mechanical balance required for propagation, whereas in the test the soft slab breaks (sf results) more easily.…”

“…More recently, some research has questioned the influence of alternative equipment and methods on the test (e.g. McClung, 2009McClung, , 2011.…”

Numerical simulation of microstructural damage and tensile strength of snow

“…Persistent uncertainties in how initial cracks form in snow require investigation into tensile strength and damage formation in snow at the microstructural level. Current models assume a weak zone when applying fracture mechanics [ McClung , ] to failure predictions for real‐world conditions. How this weak zone initially forms is unknown.…”

Healing of snow surface-to-surface contacts by isothermal sintering

“…It is known that there are at least six mass-transport mechanisms playing a role in the growth of ice bonds (Maeno and Ebinuma, 1983) and many factors that may affect sintering: temperature and its gradient, normal pressure, microstructural properties of grains, and pore-space configuration/geometry (McClung and Schaerer, 2006;Blackford, 2007). Isothermal snow sintering is mainly controlled by vapour diffusion (Hobbs and Mason, 1964), while external pressure intensifies the process through plastic deformation and recrystallization (Blackford, 2007).…”