Item: Snowcover stability tests and the areal variability of snow strength
Title: Snowcover stability tests and the areal variability of snow strength
Proceedings: 1988 International Snow Science Workshop, Whistler, October 12-15
Authors: Paul M.B. Fohn
Abstract: This paper describes in a first part the advantages and drawbacks of the following tests: wedge test, shovel test, "Rutschblock-" test, shear frame test. The theoretical background and the pratical value of each test are discussed. Whereas the wedge and the shovel test require smaller shoveling effort than the others, they both are hampered by a bending mode of failure, which is not easy to compensate for. The "Rutschblock"-test, which simulates a basal shear failure by a skier, is shown to be a useful tool for practitioners as long as certain precautions are taken. The shear frame test, which yields together with previous shovel or "Rutschblock"- tests numerical indices for weak sublayers, is a delicate method, which is rather confined to special investigations. In a second part the paper deals with the representativity of such snowcover stability tests. Recent studies (e.g. Conway and Abrahamson, 1988) claim that a snow slope contains many irregular, small failure spots. Such irregularities wouid render any snowcover test illusory. In order to illuminate these aspects for Alpine conditions several measuring series (shear frame ar.d/or "Rutschblock") along the peripheries of slabs and across extended slopes that had not avalanched are presented. Although stability variations were found - mainly depending on aspect, underground, terrain geometry and sliding layer depth - the variations were explainable and by all means srEaller than the ones reported in previous work. Most of the apparent discrepancies may be explained by a different shear frame procedure and/or by climatological reasons. There are several indications that for a slab release many small or a few large "deficit areas" are needed.
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Keywords: snow strength, stability, wedge test, shovel test, rutschblock test, shear frame test
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