GLACIER SNOW BRIDGE MECHANICS 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 end- of-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 1. OBJECTIVES 3) Provide a class outline and mnemonic for teaching snow bridge analysis. Hopefully, this will Crossing a long, narrow snow bridge over a stimulate discussion of heuristics for snow bridge gaping, blue crevasse may be at once the most go/no-go decisions. exciting and hazardous part of glacier travel. Teaching novices how to analyze snow bridges For context, the class outline, “Snow Bridge can be challenging, in part because the mechanics Analysis” is part of broader topic, “Glacier Route- of snow bridges have not been well described. finding,” so an outline for that is included as well. In the spirit of merging theory and practice, this paper has three goals: 2. INTRODUCTION 1) Elucidate snow bridge mechanics at the 2.1 Epidemiology educator level. Hopefully this will demystify for our Two hundred significant falls into a crevasse/moat students how we use probes on snow bridges. were reported to the American Alpine Club from 2) Calculate representative forces generated by 1951-2003 (Williamson 2005). Many more went various modes of locomotion including hiking, unreported. At the National Outdoor Leadership crawling, snowshoeing and skiing. School (NOLS) we track the epidemiology of _____ injuries, illnesses, and near misses. Near misses *Corresponding author: Rick Rochelle are defined as a close call in a dangerous situation NOLS Alaska, PO Box 981,Palmer, Alaska, 99645 that did not result in an injury. Over the period (907) 745-4047 ext. 344 1999-2002, 3.8% of near misses school-wide [email protected] occurred while on glacier—16 of 418 (Leemon and 193 Schimelpfenig 2003). Eleven of those 16 were At 1330, the instructor-in-training had just taken crevasse falls. For context, these occurred over over the lead. He was uphill and out of view of an 630,937 program days, where a program day is instructor, who was leading a second rope team, defined as a student or instructor being in the field when the instructor-in-training came upon a snow for one day. bridge that was 4.5 m long. (We define length as the distance to span the crevasse). The snow At NOLS Alaska the criteria for crevasse near bridge was 2 m thick and 6 m wide (i.e. open to misses are (1) any un-roped step or fall into a each side). He noted it was sagging a little and crevasse, (2) roped falls to head depth or deeper, made the decision to have everyone put on skis, or (3) the rope system does not work as planned. “to spread out the weight.” On NOLS Alaska mountaineering courses from 2004-2006 the most common type of near miss When he reached the middle of the bridge it (43%) was crevasse falls due to snow bridge collapsed catastrophically. Due to the skis, the failure (not (1) or (3) above). Table 1 shows 12 second climber on his rope team had difficulty near misses as well as 2 (shoulder) injuries arresting and was dragged ~5 m uphill. They occurred during the period due to snow bridge made several mistakes but the thing that intrigued failures. For comparison there were 2 avalanche us was the perception that skis would provide a near misses (climber-triggered, but no one caught) significant advantage—enough so that they felt it at NOLS Alaska during that period. These worth the time to put them on. In fact, skis did not incidents occurred over 18,176 program-days. prevent the fall and probably lengthened it. Of the 16 crevasse fall incidents, 12 were This bridge failed in flexure due to the climber attributable to inaccurately assessing the snow inducing a bending moment, which was at its bridge strength. (Four of the near misses were maximum when he briefly reached the center of attributable to inattentive students who stepped on the bridge. The moment was only slightly smaller well-marked snow bridges previously deemed to due to the skis than it would have been on foot. weak to cross.) To reduce injury and near miss Thus the skis gave him a false sense of security rates while glacial mountaineering, we must and reduced his partner’s ability to self-arrest. A understand snow bridge mechanics. lack of understanding of the types of failures and the mechanics involved skewed this climber’s ) a l t cost-benefit analysis towards wearing skis, which e l ) o m k r t a ) a ( s u f m l may have contributed to his decision to attempt to l e ( m i a a l ( m u t a m e f A cross a long, sagging bridge late on a warm day. u d o n u c t e a m s n S i l i m f l g i a L d x t 2.3 Snow Bridge Failure Types o d a n i f i e a s O r i # b % N M d M M Bridge failures, whether made of concrete and Near Miss 12 43 1.3 9.0 15.0 steel or of snow fall into two primary categories— Injury 2 9 chest 3.8 7.5 shear failure or flexure failure. Shear failures can Total 14 28 chest 6.0 15.0 be sub-divided into two sub-categories by location. Table 1. Crevasse falls due to snow bridge failure were One is punching failure, which in a highway bridge a plurality of near misses reported on NOLS Alaska involves failure at the top of a pile-supported pier mountaineering courses from 2004-2006. or of the deck (road) surface. Picture a small portion of the concrete and re-bar collapsing under 2.2 Case study an overweight truck tire. The other type of shear The impetus for this study was a particular incident failure is at the end of a bridge, where loads are wherein an instructor-in-training fell 15 m into a transferred from one structural member to another crevasse. Though he was uninjured, we analyzed (I-beam to abutment). Flexure failure is due to an this near miss closely for lessons learned. In this applied bending moment. On a highway bridge case, the climber was being coached on how to that could be from an overweight vehicle in the route-find and lead in glaciated terrain. This middle of a long span which causes tensile stress occurred on 21 May 2004 in the Chugach underneath the span in excess of the tensile Mountains, Alaska. There had been significant strength. warming according to the course log: “Sunny in In the discussion, we revisit the details of these the AM, cloudy in the PM, +5 C—significantly ° three failure types in the medium of snow. But more melting today than in previous days.” here is a non-scientific way to explain the three types to students using old cartoon characters: 194 1. Shear (punching) failure: Picture Wile E. circumference of each type of footwear and Coyote. In his haste to catch Road Runner, he assumed a bridge thickness of 1 m (for punching steps on an otherwise reasonable looking shear). All calculations of end-of-bridge shears bridge. But this bridge is only thick enough to and moments assumed these surface areas: hold Road Runner, not a coyote. He falls Hiking: through, leaving a perfect outline of his body in 0.027 m2 = 0.30 m x 0.09 m an otherwise intact bridge. 2. Shear (end-of-bridge) failure: Wile E. Coyote Crawling works his way back to the top only to follow 0.018 m2 = 0.10 m x 0.18 m (hand) and Road Runner’s tracks across another bridge.