International Snow Science Workshop Proceedings - Montana State University Library

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  [ Forest Service National Avalanche Center, Bozeman, Montana, USA ]
    • Scott Savage  [ Big Sky Snow Safety, Big Sky, Montana, USA ]
    • Simon Trautman  [ Moonlight Basin Snow Safety, Big Sky, Montana, USA ]
    • Kalle Kronholm  [ Norwegian Geotechnical Institute, Oslo, Norway ]
    • Spencer Logan  [ Colorado Avalanche Information Center, Boulder, Colorado, USA ]
    • Jürg Schweizer  [ WSL Swiss Federal Institute for Snow and Avalanche Research SLF, Davos, Switzerland ]

    Date: 2006

    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. Additional Authors: 1Forest Service National Avalanche Center, Bozeman, Montana, USA 2Big Sky Snow Safety, Big Sky, Montana, USA 3Moonlight Basin Snow Safety, Big Sky, Montana, USA 4Norwegian Geotechnical Institute, Oslo, Norway 5Colorado Avalanche Information Center, Boulder, Colorado, USA 6 WSL Swiss Federal Institute for Snow and Avalanche Research SLF, Davos, Switzerland

    Object ID: issw-2006-029-037.pdf

    Language of Article: English

    Presenter(s): Unknown

    Keywords: snow stability, fracture, whumpf, avalanche release, avalanche forecasting

    Page Number(s): 29-37

    Subjects: fractured slopes snow stability avalanche

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