Crack propagation speeds in weak snowpack layers

Abstract

For the release of a slab avalanche, crack propagation within a weak snowpack layer below a cohesive snow slab is required. As crack speed measurements can give insight into underlying processes, we analysed three crack propagation events that occurred in similar snowpacks and covered all scales relevant for avalanche release. For the largest scale, up to 400 m, we estimated crack speed from an avalanche movie; for scales between 5 and 25 m, we used accelerometers placed on the snow surface and for scales below 5 m, we performed a propagation saw test. The mean crack speeds ranged from 36 ± 6 to 49 ± 5 m s−1, and did not exhibit scale dependence. Using the discrete element method and the material point method, we reproduced the measured crack speeds reasonably well, in particular the terminal crack speed observed at smaller scales. Finally, we used a finite element model to assess the speed of different elastic waves in a layered snowpack. Results suggest that the observed cracks propagated as mixed mode closing cracks and that the flexural wave of the slab is responsible for the energy transfer to the crack tip.

A Depth‐Averaged Material Point Method for Shallow Landslides: Applications to Snow Slab Avalanche Release

“…• On the one hand, for short propagation distances, typically below 2 m, the failure in the weak layer occurs as a so called mixed-mode anticrack (Heierli et al, 2008), in line with field measurements (Bergfeld et al, 2022; as well as recent numerical experiments using the DEM (Gaume, van Herwijnen, et al (2015) and Bobillier et al (2021)). The anticrack mechanism, originally introduced to explain deep earthquakes (Fletcher & Pollard, 1981) refers to a mode of compressive fracture driven by the volumetric collapse of the highly porous weak layer, which leads to the closure of crack faces and to the onset of frictional contact (Gaume et al, 2018).…”

A Depth-Averaged Material Point Method for Shallow Landslides: Applications to Snow Slab Avalanche Release

“…12). Interestingly, this simulation shows a race between crack propagation in the weak layer and slab fractures, leading to lateral en-echelon failures, a process often reported in the field (Bergfeld et al, 2022). The overall shape differs from the pictures in Fig.…”

“…This is why in our case, we have a smooth increase of the speed towards the longitudinal wave speed c P . In 3D simulations, a daughter supershear crack nucleates ahead of the main fracture with a speed which continuously increases towards c P , while crossing the forbidden region between c R and c S (Gaume et al, 2019;Bergfeld et al, 2022;Bizzarri & Das, 2012). We obviously cannot reproduce the discontinuity reported in 3D simulations, but the temporal evolution of the supershear crack speed is in line with the latter 3D simulation.…”

Temporal evolution of crack propagation characteristics in a weak snowpack layer: conditions of crack arrest and sustained propagation

“…However, the typically 2-3 m long PST experiments are rather small tests, pos-sibly not representative of slope scale crack propagation in avalanches. While the one-dimensionality of crack propagation along PST columns seems not be altering crack propagation speed (Bergfeld et al, 2022), crack propagation in PSTs of typical size is affected by edge effects Bair et al, 2014).…”

“…Based on videos of avalanches, Hamre et al (2014) reported widely varying crack speed estimates ranging from 18 to 428 m s −1 . Bergfeld et al (2022) improved the methodology and estimated crack speeds by evaluating 14 crack paths in an avalanche video recording, resulting in crack speeds between 23 and 44 m s −1 (mean: 36 ± 6 m s −1 ) covering distances from 26 to 440 m. In addition to these experimental studies, Trottet et al (2022) performed numerical simulations based on the material point method (Gaume et al, 2018b) and reported the existence of a transition from sub-Rayleigh anticrack to supershear crack propagation. While sub-Rayleigh anticrack propagation can explain crack speeds below 100 m s −1 , the transition to supershear crack propagation (a crack propagating faster than the shear wave speed in the slab) potentially also explains the high crack speeds reported by Hamre et al (2014).…”