A MODIFIED EXTENDED COLUMN TEST TO REDUCE SLAB DEGRADATION: PRELIMINARY RESULTS

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1 A MODIFIED EXTENDED COLUMN TEST TO REDUCE SLAB DEGRADATION: PRELIMINARY RESULTS Maggi Kraft 1, Matt Wieland 2,1, Jordy Hendrikx 1* 1 Snow and Avalanche Laboratory, Montana State University, Bozeman, Montana, USA. 2 Moonlight Basin Ski Patrol, Big Sky, MT, USA ABSTRACT: Snow profiles and stability tests are commonly used to evaluate the strength of the snowpack. While conducting a stability test such as the extended column test (ECT), degradation of the upper slab during the initial loading steps of the test may influence the energy required for initiation and the propagation potential of a weak layer. In this study the ECT test was modified to examine the effects of upper slab degradation from the initial loading steps. This was examined by using only the loading step (e.g. easy, moderate, hard) that the test failed on, rather than increasing from easy to moderate to hard (as per usual). By modifying the ECT to reduce the degradation of the upper layers we aim to more accurately mimic the affects a skier has on the weak layer. To test this hypothesis, we conducted a small field experiment with twenty paired ECTs, modified and regular, on a sheltered and uniform slope on Mount Ellis in the northern Gallatin Range of southwest Montana. The results of our study were surprising and indicate that there was not a statistically significant difference between our modified and the regular ECT. From this small data set, we thereby infer that degradation of the upper layers during the initial loading steps do not affect the transmission of energy to the weak layer (P = under a 95% significance level). A larger dataset may result in a more normal distribution and more robust analysis. Furthermore, this test was performed in one area under one set of snowpack parameters and our results may be different in other conditions. 1. INTRODUCTION Dry-snow slab avalanches occur when a cohesive slab fails on a weak layer. The triggering of this initial failure may occur in several ways; due to rapid near-surface loading such as people or explosives, gradual uniform loading such as precipitation events, or a no-loading situation where the snowpack changes as in a rapid surface warming event, all of which differ in the rate of snowpack loading (Schweizer et al. 2003). The formation of avalanches is an intricate interaction between terrain, snowpack and weather conditions. Properties of a slab and weak layer developed in these conditions influences failure initiation and fracture propagation and must be considered. Often the layers adjacent the weak layer noticeably differ in grain size and hardness (Schweizer et al. 2003). Harder slab layers may restrict deformation of the weak layer reducing chance of fracture initiation and propagation in the weak layer (Habermann et al. 2008). Harder layers within the snowpack may have a bridging effect in the force of transmission; the impact energy is *Corresponding author address: Jordy Hendrikx, Snow and Avalanche Laboratory, Department of Earth Sciences, Montana State University, P.O. Box , Bozeman, MT, USA 59717; jordy.hendrikx@montana.edu spread out within the harder layer reducing the energy at greater depths (Camponovo and Schweizer, 1996). Snow profiles and stability tests are used to evaluate the strength of the snowpack. While conducting a stability test such as the extended column test (ECT), degradation of the upper slab during the initial loading steps of the test may influence the energy required for initiation and the propagation potential of a weak layer lower in the snowpack. In this study the ECT test was modified to examine the effects of upper slab degradation from the initial loading steps. By modifying the ECT to minimize the degradation of the upper layers we aim to more accurately mimic the affects a skier has on the weak layer. The modified ECT did not include the initial loading steps, instead began from the level the regular ECT ended. Twenty paired ECTs were performed on Mount Ellis in the northern Gallatin Range of southwest Montana. The implications of the modified test for exposing failure initiation and fracture propagation are discussed. 2. METHODS A test site was chosen where fracture propagation regularly occurred on the middle to hard loading

2 steps (i.e ) during an ECT. The study area was located on the northeast aspect of Mount Ellis at an elevation of 2357 meters(7733 ft.) (Figure 1). At this site elevation, slope angle, aspect, air temperature and site characteristics were collected. A test pit was excavated and snow stratigraphy was determined according to Greene et al. (2010). Layer thickness, hand hardness, temperature, grain size/type were recorded. An ECT was performed and the results were recorded along with fracture character as described by Green et al. (2010). Directly uphill (0.5m) of the regular ECT, a second, but modified ECT was performed. This alternating pattern was repeated within two areas of the study area (Figure 2). The modified ECT was adjusted so that the initial loading steps were not included and the test began at the level in which the regular ECT ended. For example, if the regular ECT score was ECTP24 then the modified test would begin with the loading step from the shoulder (i.e. Hard), skipping tap levels 1-20 (i.e. Easy and Moderate). This process with paired tests was performed twenty times. Figure 2: Pit Locations in the study area. Pits were dug 0.5m apart alternating between modified (M) and regular (R) ECTs. 3. RESULTS The snowpack was approximately 1.5 m deep (Figure 3). A persistent pencil hard crust was found near the surface. Snow crystal grain type below the surface hoar layer was observed as faceting rounds, while above the surface hoar it was rounds. Within the study area propagation consistently occurred on the surface hoar layer at 0.95 m up (Figure 3). Figure 1: Google Earth image of the study location on a northeast aspect of Mount Ellis in the Gallatin Range of south-western Montana, USA. Primary attention was given to the layer where fracture propagation consistently occurred. The nonparametric Wilcoxon Signed Rank Test was used to compare the regular and modified tests (P <.05). This nonparametric test was used as it does not require assumptions about the normalcy of the data and is effective in nominal paired analysis. Figure 3: Snow pit profile of the test pit in the study area. Snow depth was 1.55 m. Propagating layer was consistently near 0.95 m above the ground on surface hoar.

3 Table 1 contains the results of the twenty paired regular and modified ECTs and Figure 4 is a graph of each pair. Sixteen pairs propagated during the loading steps from the shoulder (i.e. Hard) and the remaining four were from the elbow (i.e. Moderate). The regular ECTs range is between 14 and 29 while the modified test ranges from 12 to 33.Table 2 provides a summary of the data. Table 1: Regular and modified ECT results Regular Modified ECTP18 ECTP16 ECTP24 ECTP26 ECTP29 ECTP27 ECTP24 ECTP28 ECTP24 ECTP25 ECTP26 ECTP28 ECTP19 ECTP15 ECTP14 ECTP14 ECTP24 ECTP28 ECTP18 ECTP12 ECTP24 ECTP33 ECTP37 ECTP28 ECTP21 ECTP26 ECTP23 ECTP27 ECTP25 ECTP25 ECTP16 ECTP16 ECTP24 ECTP26 Table 2: The five number summary of the distribution of the observations Regular Modified Minimum Quartile Median Quartile Max The data were then plotted as a box and whisker plot, showing the regular and modified tests (Figure 5). The median is the bold line in the center of the box, while the box represents the 25-75th percentiles and the whiskers are the upper and lower range. Both boxplots are skewed to below the median but this is most evident in the regular ECT boxplot. One outlier was apparent in the regular ECT boxplot (with a value of 37), that was not consistent with the other tests and may be the result of testing error or spatial variability. This outlier was not included in the analysis. Number of Taps Regular ECT Modified ECT Test Figure 4: Graph of the paired ECT results showing both the regular and modified ECT results.

4 indicate that there was not a significant difference between the regular and modified tests. An N value of 16 was used because there were three tests where there was no difference between pairs, and one outlier that was removed. Table 3: Results of the regular versus modified ECT Average Taps Regular Average Taps Modified Average Tap Difference 1.75 (Modified Regular) Wilcoxon Number 34.5 P Value Figure 5: Box plots of the regular versus modified ECTs. The median is the bold line in the center of the box, while the box represents the 25-75th percentile and the whiskers are the upper and lower range The nonparametric Wilcoxon Signed Rank Test was calculated and summarized (Table 3). The Wilcoxon s value of 34.5 and P value greater than under a 95% significance level ( P < 0.05) Figure 6 displays the difference in loading taps between the modified and regular tests. Standard deviation bars are displayed for each test. The average difference between the regular and modified ECTs is There was a difference between tests except for in three trials. Out of the twenty trials performed only five needed less taps (i.e. negative values in Figure 6) to fracture on the modified test, the other tests either used the same number of steps or more (i.e. zero, or positive values in Figure 6). Difference in Number of Taps Test Figure 6: Difference between the modified and regular ECTs (modified regular)

5 4. DISCUSSION This analysis was designed to examine whether the deterioration of the upper layers in the snowpack during an extended column test would, or would not affect the results. While on a slope, skiers do not gradually compact the upper snow pack layers but rather apply a direct force. Our modification of the ECT to test this theory did not provide statistically differing results. Based on our small data set, our results indicate that the degradation of the upper layers during the initial loading steps did not affect the transmission of energy to the weak layer (P = under a 95% significance level). No additional or differing bridging effect was evident during the modified test when compared to the regular ECT. During the regular ECT, compaction of soft snow may attenuate the stress to deeper layers within the snowpack but we are unsure of the magnitude of this effect. While there was not a statistical difference between the modified and regular ECT it is important to note that out of the twenty trials performed only five needed less taps to fracture on the modified test, the other tests either used the same number of steps or more. Tremper, B., and Williams, K., Snow, Weather and Avalanches: Observation Guidelines for Avalanche Programs in the United States. American Avalanche Association, Pagosa Springs, CO, Second Printing Fall Habermann, M., Schweizer, J., and Jamieson, J. B., Influence of snowpack layering on human-triggered snow slab avalanche release: Cold Regions Science and Technology, v. 54, no. 3, p Schweizer, J., Jamieson, J. B., and Schneebeli, M., Snow avalanche formation: Reviews of Geophysics, v. 41, no. 4. These results are surprising, as intuitively, we had expected that for example, if a column required 24 taps with a normal ECT that we would require much less than 24 shoulder taps in our modified ECT. However, for the cases examined here, in most instances the opposite was true. We acknowledge that the results may be different under altered circumstances. For instance applying this testing methodology to a propagating fracture lower in the snowpack or with a thicker and/or harder slab above the weak layer may make the bridging affect more evident. Furthermore, a larger dataset to reduce the effects of spatial variability and uncertainty may result in a more normal distribution and different result. REFERENCES Camponovo, C., and Schweizer, J., Measurements on Skier Triggering: Snow Cover Stability, Avalanche Initiation and Forecasting. International Snow Science Workshop, Banff, Canada. Greene, E., Atkins, D., Birkeland, K., Elder, K., Landry, C., Lazar, B., McCammon, I., Moore, M., Sharaf, D., Sternenz, C.,