Temporal changes in the mechanical properties of snow related to crack propagation after loading

Abstract

Most dry slab avalanches occur during or immediately following loading by snowfall or wind deposition. In the absence of further loading avalanche activity decreases over time. This suggests that loading favorably changes snow cover properties for avalanche release over short time scales (e.g., minutes and hours), and that changes over longer time scales (days or longer) help to stabilize the snowpack. This study quantifies both: 1) the effect of increasing load on the interaction of the slab and weak layer over short time scales, and 2) the longer term stabilizing changes following loading. We developed a field method to rapidly increase the load on existing weak layers, and conducted two different sets of experiments. For the first set of experiments we used a cardboard frame the dimensions of a standard Propagation Saw Test (PST) and added 5, 10, 15, or 20 cm of disaggregated snow on top of PST columns on 11 sampling days. We allowed the added snow to sinter for approximately 30 to 60 min before completely isolating the block and performing a PST. In the second set of experiments we used the same technique to add 10 cm of disaggregated snow on over 30 isolated columns. We then conducted PSTs in the minutes, hours and days following isolation, with tests ranging from 15 min to 4 days. For both experiments we filmed each test at 120 fps for particle tracking velocimetry analysis. We also utilized a model simulating the experiments to better interpret our results. In the first set of experiments, critical crack lengths dramatically decreased with increasing load while crack propagation speed increased, a finding consistent with previous work. In the second set of experiments, we found that critical crack lengths increased rapidly at first and then more slowly over time. Simulations of the experiments suggest that changes in critical crack length over time are caused by an increase in slab elastic modulus in the first hours following loading, and then caused by both increasing slab elastic modulus and weak layer specific fracture energy in days following loading. Overall, our results help to illustrate changes in critical crack lengths immediately after and in the days following loading. Our results are consistent with field observations of increasing avalanche activity during and immediately following loading events and decreasing avalanche activity afterwards.