Item: Glacier snow bridge mechanics
Title: Glacier snow bridge mechanics
Proceedings: Proceedings of the 2006 International Snow Science Workshop, Telluride, Colorado
Authors: Rick R. Rochelle Assistant Director, National Outdoor Leadership School—Alaska Palmer, Alaska Rodger D. Rochelle, PE State Alternative Delivery Engineer, North Carolina Department of Transportation Raleigh, North Carolina
Abstract: The vocabulary of avalanche mechanics is useful in describing the mechanics of glacier snow bridges. We calculate the forces involved in isotropic snow bridges and present theoretical considerations for anisotropic bridges. We also discuss travel decisions (hike, crawl, snowshoe, ski, or do not cross). Slab avalanches have a large ratio of shear plane area to that exposed to tension, such is not the case with snow bridges. Scientists accustomed to avalanche mechanics must turn their models ninety degrees to consider shear failure across layers (mode II) and tensile failures initiating underneath the bridge (mode I). Calculated snow pressures for various flotation devices are used for calculations of shear and moment. The potential types of failure are (1) shear at the footwear—snow bridge interface (punching failure), (2) shear at the ends of bridges, and (3) flexure (tensile failure). Type (1) falls tend to be the shortest and type (3) falls the longest. Results show that wearing boots creates seven times as much shear stress (at the edge of the footwear) as skiing, making punching failure more likely. But this type of failure is primarily important for short, thin bridges. On bridges longer than skis, the skis only reduce endof- bridge shear and moment modestly. Maximum moment, which is probably the most critical measure, is roughly proportional to the square of the bridge aspect ratio. Since injury potential, self-arrest distance, and rescue complexity increase with skis, knowing how forces vary with bridge geometry will help glacier travelers in their cost-benefit analyses. Probe poles convey information about resistance to penetration, which is directly related to shear strength, tensile strength, and fracture toughness. Anisotropic concerns such as ice lenses and penetration of load pressure are discussed. Finally, mnemonics and heuristics are proposed to assist the glacier-traveling community in analyzing snow bridges.
Keywords: snow bridge, glacier, shear, tension, flexure, toughness
Digital Abstract Not Available