Item: Characteristics of weak snow layers or interfaces
Title: Characteristics of weak snow layers or interfaces
Proceedings: Proceedings of the 1992 International Snow Science Workshop, Breckenridge, Colorado, USA
Authors: Paul M. B. Fohn
Abstract: Weak layers or interfaces are a necessary condition for slab formation. In order to gain more insight into the characteristics of such weak layers or interfaces roughly 300 snow pit and shear measurrnent data sets have been analysed. Detection of weak layers/interfaces happened by slab inspection or executing "Rutschblock"tests On potential avalanche slopes. 40 % of the total were weak layers of thickness 1-60 rnrn, 60 % weak interfaces, where no distinct layer texture could be found. Weak layers consisted most often of aged surface hoar (40 %), faceted particles (25 %) and depth hoar (15 %). The other basic particle shapes contributed to a minor extent. The frequency . distribution of the weak layer ages displays a exponential form with a time constant of 15 to 20 days, i.e. the main part of such layer is "younger" than 20 days, but a certain percentage keeps their danger pontential up to several months. The mechanical parameters as cohesion and shear strength of these layers depend slightly on age and overburden pressure, however various grain types show controversial behaviour. The frequency distribution of calculated S' (stability with ski loading) for an "a priori" unstable group of weak layers peaks at 1.3, the one of a more stabler group at 1.8, which yields a glimpse on the predictive potential of "in situ"-measurements and concurrent stability evaluations.
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Keywords: weak layer, interfaces, avalanche slopes
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