The failure of a weak snow layer is the first in a series of processes involved in dry‐snow slab avalanche release. The nature of the initial failure within the weak layer is not yet fully understood but widely debated. The knowledge of the failure criterion is essential for developing avalanche release models and hence for avalanche hazard assessment. Yet different release models assume contradictory criteria as input parameters. We analyzed loading experiments on snow failure performed in a cold laboratory with samples containing a persistent weak snow layer of either faceted crystal, depth hoar, or buried surface hoar. The failure behavior of these layers can be described well with a modified Mohr‐Coulomb model accounting for the possible compressive failure of snow. We consequently propose a new mixed‐mode shear‐compression failure criterion that can be used in avalanche release models.

Dry-snow slab avalanches initiate from a failure in a weak snow layer below a cohesive slab. Snow is considered as a porous ice structure, and the strength distribution of the single elements of this structure, i.e. grains and bonds between grains, shows a high degree of disorder. On the bond or microstructural level, the failure process is believed to start if the fracturing of bonds between snow grains is not balanced by the formation of new bonds. We use a statistical fracture model – a fibre bundle model – to study the failure process in a weak snow layer. The model consists of fibres of various strengths representing single snow grains between two rigid plates which represent the slab above and the substratum below the weak layer. The fibres deform in a linear elastic manner and break instantly at their rupture strength. Broken fibres may sinter (re-bond) and regain strength after a finite sintering time. We show that the different characteristic times for breaking and sintering lead to the rate dependence of snow strength. This is, to our knowledge, the first statistical model to reproduce the ductile-to-brittle transition which snow exhibits with increasing strain rate. When the model is applied to simulate experimental stress–strain curves for different strain rates, the model and experimental results are in fair agreement.

Monitoring acoustic emissions (AE) prior to imminent failure is considered a promising technique for assessing snow slope instability. Gaps in elastic wave propagation characteristics in snow hinder quantitative interpretation of AE signals. Our study focuses on characterizing the propagation of acoustic reference signals in the ultrasonic range across cylindrical snow samples with varying density (240–450kgm−3). We deduced the acoustic attenuation coefficient within snow by performing experiments with different column lengths to eliminate possible influences of the snow-sensor coupling. The attenuation coefficient was measured for the entire burst signal and for single frequency components in the range of 8 to 35kHz. The acoustic wave propagation speed, calculated from the travel time of the acoustic signal, varied between 300ms−1 and 950ms−1, depending on the density and hardness of snow. From the sound speed we also estimated the Young's modulus of our snow samples; the values of the modulus ranged from 30 to 340MPa for densities between 240 and 450kgm−3. In addition, we modeled the sound propagation for our experimental setup using Biot's model for wave propagation in a porous medium. The model results were in good agreement with our experimental results and suggest that our acoustic signals consisted of Biot's slow and fast waves. Our results can be used to improve the identification and localization of acoustic emission sources within snow in view of assessing snow slope instability

In order to study the formation of the initial failure leading to dry‐snow slab avalanche release, we performed loading experiments in a cold laboratory with natural samples including a layer of buried surface hoar. The experimental setup was such that the layered snow samples were loaded continuously for various tilt (slope) angles; loading rates varied between 1 and 20 Pa s⁻¹. The stress at fracture decreased with increasing loading rate and increasing slope angle, i.e., increasing shear component of the load. The latter result means that the layer was stronger in compression than in shear which is attributed to the particularly anisotropic nature of layers of buried surface hoar. Particle image velocimetry revealed that almost 90% of the sample's global deformation was concentrated in the weak layer. For avalanche release our results imply that the shear component of deformation is of particular importance for failure initiation.