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Show STABILITY OF SNOW SLABS C. JACCARD Swiss Federal Institute for snow and avalanche research Weissfluhjoch, Davos ( Translated by Angie Thacker, Alta Avalanche Study Center) ABSTRACT The comparison of stresses in the weakest layer with the corresponding critical curve in the mohr plane leads to a definition of primary stability. Investigation of this definition as a function of slope angle and distributed load, shows it to be in certain cases, independent of load. Secondary stability characterizes the absence of sensitivity to perturbations, especially to a localized rupture of the critical layer. Here, consideration is given to different shapes of the fracture zone and also the influence of a vertical concentrated force applied to the surface. 1. INTRODUCTION The rupture of snow slabs seems to be a principal cause of avalanche release and may occur any time during the winter season so long as the conditions of snow, temperature, wind, and topography are favorable. When the temperature is sufficiently high, avalanches may release alternatively in a pear- shaped fashion; however, in most cases this does not shift large masses. It is therefore of particular interest to study the stability of slabs. A slab is in equilibrium when its weight is balanced by the forces which act around its top, side, bottom and base. The first question that comes to mind is the relative importance of the basal forces in relation to the non- basal forces. This depends first of all on the ratio between the dimensions of the slab parallel to the slope and its thickness. No detailed statistics exist on this ratio, however some indication may be deduced from the 1959- 1963 reports of the |