Item: The extended column test: a field test for fracture initiation and propagation
Title: The extended column test: a field test for fracture initiation and propagation
Proceedings: Proceedings of the 2006 International Snow Science Workshop, Telluride, Colorado
Authors: Ron Simenhois, and Karl Birkeland, Copper Mountain Ski Area, Copper Mountain, Colorado, Mount Hutt Ski Area, Canterbury, New Zealand, U.S.D.A. Forest Service National Avalanche Center, Bozeman, Montana
Abstract: For dry slab avalanches, fractures initiate and propagate in a weak layer or along an interface. Current field tests like compression or stuffblock tests are designed for assessing fracture initiation; however, these tests may not be as useful for assessing fracture propagation. Furthermore, in some cases these tests may identify layers that are most likely to initiate a fracture under stress, but not necessarily those layers that initiate and propagate the fracture as well, thereby occasionally “masking” those layers of real concern in a weak snowpack. This paper describes the development of a new field test that aims to assess both fracture initiation and propagation in an isolated column. Tested during the winters of 2005-06 in Colorado and 2006 in New Zealand, this test is a variation of the compression test and can be used in the same manner with the stuffblock. By tapping on one side of an extended column of 30 cm downslope by 90 cm in the cross slope direction, the extended column allows a slab to transmit stress across the width of the column. The fracture initiation results are collected as well as the results of the fracture propagation across the extended column. Out of 68 tests of unstable snowpacks (where avalanches recently occurred, or there was whumphing or shooting cracks) the fracture propagated across the entire block in 1 or 2 loading steps every time (100%) and 63 times (93%) it fractured with a compression test load of easy to moderate. Conversely, out of 256 pits where the snowpack was stable, only 4 cases (1.6%) propagated across the entire extended column through a single layer or interface. Thus, in stable snowpacks a fracture may be initiated, but it typically does not propagate across the column. For the snowpacks we tested the extended column test effectively discriminated between stable and unstable slopes.
Keywords: stability test, avalanche forecasting, stability assessment, fracture propagation
Digital Abstract Not Available