Micro-mechanical insights into the dynamics of crack propagation in snow fracture experiments

Grégoire Bobillier,Jürg Schweizer,Alec Van Herwijnen,Jürg Dual,Bastian Bergfeld,Johan Gaume

doi:10.1038/s41598-021-90910-3

Grégoire Bobillier, Jürg Schweizer + Show 4 more

Open Access

https://doi.org/10.1038/s41598-021-90910-3

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Abstract

Dry-snow slab avalanches result from crack propagation in a highly porous weak layer buried within a stratified and metastable snowpack. While our understanding of slab avalanche mechanisms improved with recent experimental and numerical advances, fundamental micro-mechanical processes remain poorly understood due to a lack of non-invasive monitoring techniques. Using a novel discrete micro-mechanical model, we reproduced crack propagation dynamics observed in field experiments, which employ the propagation saw test. The detailed microscopic analysis of weak layer stresses and bond breaking allowed us to define the crack tip location of closing crack faces, analyze its spatio-temporal characteristics and monitor the evolution of stress concentrations and the fracture process zone both in transient and steady-state regimes. Results highlight the occurrence of a steady state in crack speed and stress conditions for sufficiently long crack propagation distances (> 4 m). Crack propagation without external driving force except gravity is possible due to the local mixed-mode shear-compression stress nature at the crack tip induced by slab bending and weak layer volumetric collapse. Our result shed light into the microscopic origin of dynamic crack propagation in snow slab avalanche release that eventually will improve the evaluation of avalanche release sizes and thus hazard management and forecasting in mountainous regions.

Highlights

Dry-snow slab avalanches result from crack propagation in a highly porous weak layer buried within a stratified and metastable snowpack
The experimental data we use for model development were obtained with a field experiment, a propagation saw test (PST), on a flat and uniform site close to Davos, Switzerland; the experimental procedures are described in detail by Bergfeld et al
The dynamics of key measures observed in the PST field experiment were very well reproduced by the discrete element method (DEM) simulation

Summary

Introduction

Dry-snow slab avalanches result from crack propagation in a highly porous weak layer buried within a stratified and metastable snowpack. Porous brittle materials subject to mixed mode (compressive-shear) loading exhibit localized progressive failure resulting in the nucleation of a closing crack that may propagate dynamically. Porous brittle materials subject to mixed mode (compressive-shear) loading exhibit localized progressive failure resulting in the nucleation of a closing crack that may propagate dynamically1–3 In snow, this process— known as anticrack—is known to occur in weak snowpack layers, which have a peculiar and highly anisotropic structure related to their formation mechanism through temperature m etamorphism. Analytical and numerical models based on fracture and/or continuum mechanics were developed to investigate crack propagation and avalanche release3,12–17 These models provided new insight into key parameters and driving forces, the micro-mechanical processes involved during dynamical crack propagation are still essentially unknown. The oversimplified shape (triangular structure) and the 2-D character of the weak layer employed by Gaume et al. prevented a detailed analysis of the internal stresses during crack propagation

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