Item: The starting mechanism of avalanches
Title: The starting mechanism of avalanches
Proceedings: Proceedings of the 1980 International Snow Science Workshop, Vancouver, BC, Canada
Authors: Andre Roch Consultant, Geneva, Switzerland
Abstract: In regard to the question of how slab avalanches are triggered, Robert Haefeli gave one of the first explanations. He stated, following the logic of W. Paulke, that the cause was due to the shock of a 'tensile rupture initiated at a convex portion of the slope. This explanation is only a special case, however, and even Haefeli later modified his thinking. E. Bucher dealt with the stability problem in his doctoral thesis. He defined a stability index as a ratio of strength of a snow layer, Ts ' to the stress applied by body forces to that layer, T. When the ratio of these quantities, S = Ts/T = 1, a snow layer is at the limit of equilibrium and avalanche formation is imminent. Figure 1 depicts a schematic representation of this situation. When S becomes less than 1, either because of an increase of stress or because of a decrease in strength, avalanches are possible. In order to estimate stability, I went on slopes to measure the weight of snow laying above the weakest layer and the strength of this layer with a shear frame. In this way, I found that the stability index appeared to be greater than one, while at the same time I observed that avalanches had occurred on similar slopes where the stability index was obviously less than one.
Keywords: stability, shear frame, slab avalanche, triggered, rupture
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