Field measurements of multiaxial strength of weak snowpack layers

Failure initiation within a buried weak snow layer is the first step towards slab avalanche release. Since avalanches occur on inclined terrain, this failure happens under multiaxial loading conditions, particularly compression and shear loading. Like many geomaterials, snow shows different strength depending on the loading conditions. This dependence of strength on the loading conditions is commonly represented using failure envelopes. For snow, the experimental data on failure envelopes is very limited. To address this gap, we designed a displacement-controlled testing device to perform multiaxial strength measurements in the field. In this study we present results from a measurement series conducted on a buried surface hoar layer in February 2024 in Davos, Switzerland. The testing device includes a tilting mechanism that allows us to adjust the loading angle and thus vary the multiaxial stress state within the weak layer. Results show that the strength in compression is about 50% higher than that in shear. Furthermore, our data suggest that for the weak layer tested, the failure envelope had an elliptical shape. Additionally, we also recorded videos of the strength tests and used digital image correlation (DIC) to estimate the strain rate of the sandwiched weak layer to be between 10^-3 and 10^-4. While these preliminary experiments showed that our new measurement set-up can be used to obtain failure envelopes in the field, they also highlighted some shortcomings of the measurement setup. We therefore intend to improve the measurement setup to also investigate the influence of strain rate on the measured strength values. Ultimately, this will allow us to collect a comprehensive set of failure envelopes of common weak snowpack layers.