Item: Uncertainties in assessing the stability of fractured slopes
Title: Uncertainties in assessing the stability of fractured slopes
Proceedings: Proceedings of the 2006 International Snow Science Workshop, Telluride, Colorado
Authors: Karl W. Birkeland, Scott Savage, Simon Trautman, Kalle Kronholm, Spencer Logan, and Jürg Schweizer Forest Service National Avalanche Center, Bozeman, Montana, USA Big Sky Snow Safety, Big Sky, Montana, USA Moonlight Basin Snow Safety, Big Sky, Monta
Abstract: While doing avalanche mitigation work or traveling in the backcountry, occasionally a sizable part of a slope fractures without triggering an avalanche. An example is when a weak layer fractures with a characteristic “whumpf” sound and tensile cracks open up, but no avalanche releases. Disagreement exists among avalanche professionals about the immediate safety of these slopes. Many assume that if the slope does not slide during initial fracture propagation then it is unlikely to slide and is probably safe. Others treat the slopes with extra caution, especially immediately following the event. This paper provides a synopsis of recent research and two case studies that provide insight into this problem. Research shows that shear strength decreases immediately after a collapse, followed by differing strengthening rates. In both case studies, avalanche mitigation work with explosives resulted in the fracturing of some slab boundaries, as evidenced by tensile cracks visible on the surface. Additional explosives applied to the slopes shortly following the initial fractures resulted in sizable avalanches, casting doubt on the idea that fractured slopes are necessarily safe. Over many years and a handful of such experiences, an unofficial policy at Big Sky Ski Area has evolved whereby the snow safety group typically will not open slopes that have deep fractures until the following day. Our paper does not provide definitive answers about the safety of fractured slopes. However, it does point out uncertainties in our knowledge and, as a result, suggests taking a cautious approach toward such slopes.
Keywords: snow stability, fracture, whumpf, avalanche release, avalanche forecasting
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