Snow slab avalanches are believed to begin by the gravity-driven shear failure of weak layers in stratified snow. The critical crack length for shear crack propagation along such layers should increase without bound as the slope decreases. However, recent experiments show that the critical length of artificially introduced cracks remains constant or, if anything, slightly decreases with decreasing slope. This surprising observation can be understood in terms of volumetric collapse of the weak layer during failure, resulting in the formation and propagation of mixed-mode anticracks, which are driven simultaneously by slope-parallel and slope-normal components of gravity. Such fractures may propagate even if crack-face friction impedes downhill sliding of the snowpack, indicating a scenario in which two separate conditions have to be met for slab avalanche release.
[1] Dry snow slab avalanches release as a result of the failure of a weakly bonded layer located below a slab-like layer of cohesive snow. Traditionally the failure of the weak layer was attributed to the formation and propagation of volume-conserving simple shear cracks. Over the past decade, however, evidence for slope-normal subsidence associated with fracture propagation in snow has accumulated, pointing toward a new understanding of fracture propagation in snow. The typically very high porosity of weak layers implies a loss of volume during fracture, as the aggregate of ice grains composing the weak layer collapses and rearranges in a tighter packing order. Therefore, in the new models failure is attributed to the formation and propagation of mixed-mode anticracks. In order to experimentally investigate the nature of the deformation field associated with fracture propagation through weak snowpack layers we monitored propagating fractures in field tests on natural snowpacks with a high level of detail using high-speed photography and analyzed the data with particle tracking velocimetry. The collapse of the weak layer was always observed and ranged between 0.3 cm and 4 cm. In all experiments the collapse was found to coincide with the fracture front. A shear crack preceding the collapse front was not detected at any stage of the process. It is concluded that the volumetric collapse of the weak layer, which is characteristic of anticracking, is the rule rather than the exception for the fracture process in stratified snow and therefore for slab avalanche release.Citation: van Herwijnen, A., J. Schweizer, and J. Heierli (2010), Measurement of the deformation field associated with fracture propagation in weak snowpack layers,
[1] In this letter we analyze the frictional contact forces during and immediately after the collapse of a weak snowpack layer, when the sliding plane consists of the freshly collapsed and crushed, but not yet eroded granular debris of the weak layer. The results from thirty-four field experiments show that frictional contact forces per unit area are on the order of 0.6 times the normal stress, equivalent to a friction angle close to 30 degrees. The measurements show that there is a transient, sharp drop in the coefficient of friction during the collapse of the weak layer and relatively constant values afterwards. One implication of our findings is that the minimum angle for avalanche release does not depend on shear strength, as commonly thought, but results from crack-face friction which comes into play only as the fracture through the weak layer is already propagating.
[1] Fracture processes in layered snow have many implications on avalanche hazard and mountain safety. This paper is a contribution for a better understanding of fracture dynamics in a stratified medium with a collapsible layer in terms of a simple analytical model involving only field measurable parameters. For this purpose a simple three-layer snow stratification is considered to consist of a compact basal layer, a collapsible weak layer, and a homogenous top layer. It is shown analytically that the fracture of the weak layer can occur in the form of a localized disturbance zone propagating a collapse with constant velocity and wavelength. Simple analytical expressions describing the disturbance are obtained. The results are in good agreement with observations in the terrain. The presented fracture process is relevant to the release of dry snow slab avalanches and to the propagation of firn quakes and whumpfs but may also apply to other stratified media with appropriate properties.
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