The Rutschblock Test – Procedure and Application in Switzerland

The rutschblock test is one of the most reliable snow stability tests. It is a mini slab avalanche that is tested with the appropriate dynamic load: the skier or snowboarder. The test method and its limitations are reviewed. The application of the rutschblock test as the standard stability test in the observational network of the Swiss Avalanche Warning Service is described. Introduction The rutschblock test was developed in the seventies in Switzerland, established in the eighties by Paul Föhn (Föhn, 1987) and popularised in the form of the wedge (called “rutschkeil”) by Swiss mountain guide Werner Munter (Munter, 1991), who later on discarded the test. Finally, in the nineties, it was taken to North America, where it was intensively used for field studies and refined by Bruce Jamieson (Jamieson and Johnston, 1993a,b; Jamieson, 1995). The rutschblock is the test that is best related to human triggering and therefore the best test to find critical weaknesses in the snowpack. It is a true stability test since it integrates weak layer strength, weak layer depth, slab properties and load characteristics. The sample size is larger compared to other snow stability tests, which makes the rutschblock test more reliable. The test area of 3m is believed to be larger than the critical initial failure area needed for slab release. Therefore, fracture characteristics are likely indicative of fracture propagation potential. The rutschblock test is quantifiable and relatively easy to interpret. However, it is the stability test requiring the most time, though only an additional 10-15 minutes if done in combination with a snow profile – which is highly recommended. As the rutschblock test is essentially a mini slab avalanche, it is also good teaching tool. Procedure As for any stability test, site selection is crucial. The test is best done in avalanche starting zones, but can be done on safer, less steep slopes as shallow as 25°. In that case, during loading the smooth wall needs to be observed carefully to detect fractures, since the block will likely not slide. On a slope, the ideal location is where snow distribution is believed to be relatively uniform and snow depth rather below average. Therefore, the location should not be too close to a ridge or crest where the wind strongly influences snow deposition. As the test is best done in avalanche starting zones, it is obvious that the observers need substantial experience in assessing avalanche hazard and in safely travelling in the backcountry, and that standard safety and additional precautionary measures are indispensable. Preferred sites are small slopes with a smooth profile. Of course, slopes with terrain traps, cliffs or large rocks at the bottom of the slope should be avoided. As McClung and Schaerer (1993) state, the principal difficulty in applying the test is finding a slope that is, at the same time, steep enough, sufficiently safe and representative of starting zones. After choosing a site and taking a snow profile, testers isolate a snow block of 2 m x 1.5 m from the surrounding snow cover. Besides shovels no additional material is needed except a thin cord for cutting the back of the block. The isolated block is then loaded in five steps (Table 1) by a skier, but it also works if loaded by a snowboarder (Gleason, 1998). The loading steps as given in Table 1 are the ones given by Föhn (1987) and used by avalanche professionals in Switzerland. There exist slight differences in the loading steps (CAA, 1995; Jamieson, 2000; Tremper, 2001) compared to the originally proposed scheme by Föhn (1987). The Canadian procedure (CAA, 1995) is very close to the Swiss one, but takes into account peculiarities of the deep snow country. The interpretation and limitations given below are based on studies done with either the Swiss or Canadian procedure, and accordingly only valid, if these procedures are followed. In view of the overall accuracy of the test, the differences are relatively minor. However, within an observational network, consistent procedures are essential. During the loading, a second person should carefully watch what is happening. When a fracture occurs, the testers should note or record the fracture depth, the corresponding layer in the snow profile, and the rutschblock score (1 ... 7). The testers should also record and communicate what portion of the block slid (whole block, part of the block [usually below skis], only an edge) and the characteristics of the fracture plane (smooth, rough, irregular) (Tables 2 and 3) (Schweizer et al., 1995; Schweizer and Wiesinger, 2001). This information is important for assessing whether a slab avalanche can occur, i.e. whether fracture propagation is likely. If only part of the block releases this is usually associated with rather stable conditions, whereas the release of the whole block with smooth fracture plane at rutschblock scores 4 to 5 is an indication of triggering potential. It is therefore proposed to formally integrate some shear quality rating into stability test results (Johnson and Birkeland, 2002). Interpretation Table 1: Rutschblock loading steps as used in Switzerland Rutschblock score Loading step that causes fracture 1 Isolating the block, during digging or sawing 2 Gently approaching or stepping onto the block 3 Pushing downwards by dropping from straight legs to bent knees (weighting) 4 First jump from above with skis/board 5 Second or third jump from above with ski/board 6 Jump from above without skis or board 7 Block does not slide Table 2: Rutschblock test: release type Release type: portion of block that did slide