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. Furthermore, the model provides detailed insight into the micro-mechanical processes and stresses within the weak layer and allows us to identify the main drivers of crack propagation. Our model reproduced the experimentally observed displacement field, accelerations and crack propagation speed well. ![]() We first present a method to evaluate the location of the crack tip, which is particularly challenging due to the closure of crack faces during propagation. Our aim is therefore to numerically simulate an exemplary experimental PST with a 3-D DEM model to better understand the micromechanics involved during dynamical snow fracture. 15 prevented a detailed analysis of the internal stresses during crack propagation. However, the oversimplified shape (triangular structure) and the 2-D character of the weak layer employed by Gaume et al. ![]() DEM allows the generation of highly porous samples crucial to model snow failure and was, for instance, used to perform 2-D simulations of a PST yielding good agreement with field experiments. Therefore, DEM is an appealing method to study the effect of the complex and highly porous snow microstructure on the dynamics of crack propagation, which does not require the assumption of a complex macroscopic constitutive model. While their direct observation is so far not feasible, the discrete element method (DEM) has previously been successfully used to study the influence of snow microstructure on the mechanical behavior of snow 18, 19, 20, 21 and crack propagation in weak layers 15, 22. These models provided new insight into key parameters and driving forces, however, the micro-mechanical processes involved during dynamical crack propagation are still essentially unknown. In addition, analytical and numerical models based on fracture and/or continuum mechanics were developed to investigate crack propagation and avalanche release 3, 12, 13, 14, 15, 16, 17. The PST allows analyzing the onset and dynamics of crack propagation and deriving mechanical properties using displacement-tracking techniques 10, 11. The PST involves isolating a snow column, initiating a crack by a sawing in a pre-identified weak layer until a critical crack length is reached, and crack propagation occurs without additional loading. During the past decade, our understanding of fracture processes in snow has greatly improved by the development of a fracture mechanical field test known as the propagation saw test (PST) 7, 8, 9. When buried below a cohesive snow slab, the snowpack becomes metastable and a small perturbation may lead to crack propagation in the weak layer across the slope and subsequent snow slab avalanche release 5, 6. In snow, this process-also 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 metamorphism 4. Highly 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 1, 2, 3. 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. 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. ![]() Results highlight the occurrence of a steady state in crack speed and stress conditions for sufficiently long crack propagation distances (> 4 m). 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. Using a novel discrete micro-mechanical model, we reproduced crack propagation dynamics observed in field experiments, which employ the propagation saw test. 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. Dry-snow slab avalanches result from crack propagation in a highly porous weak layer buried within a stratified and metastable snowpack.
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