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Show 2 Swiss Federal Institute. The histogram of Fig. 1 shows that the distribution of the width of snow slabs released in the region of the Parsenn has a net maximum between 20 and 40 m. The second maximum for the slabs larger than 100 meters is probably not realistic since these large slabs are often composed of several smaller slabs. Slab thickness is generally unknown, but it may be estimated to average about 0.5 meters. Thus the ratio of width to thickness usually exceeds 40. In ten cases where the stratigraphic profile of the slab was actually examined, this ratio was always higher than 50. In addition, the relative importance of the basal force depends on the snow strength at the base in comparison to snow strength throughout the thickness of the slab; here also, numerical data is rare. Rammsonde measurements in certain instances show ratios of up to 5 to 1 between the strength of the base and the slab profile. Thus it seems that in the majority of cases basal conditions determine stability, the periphery only intervening when a critical state exists at the base or in certain cases conditioned by the terrain ( couloirs). Therefore the following analysis will be limited to slabs which extend infinitely in two dimensions and fail only along the basal plane. 2. STATIC CONDITIONS Let us consider an infinite slab of thickness, h, sloped at an angle, p . We adopt a system of rectangular coordinates with axes x, y, and z ( see Fig. 2). This is the case of plane stress studied by Haefeli ( 1942) and is defined in part by the following equations a$ ax 33 ( o where 0 is the density of the snow. Assume that the velocity components, V*, Vy, aw V ^ depend on stress according to an extension of the plasticity equations of Lame |