Influence of snow microstructure on the compressive strength of weak layers

Slab avalanches result from the failure of a weak snowpack layer buried underneath a cohesive slab. Determining the material properties of different weak layer morphologies is therefore necessary to better understand and model slab avalanche formation. Natural weak layers exhibit a variety of different microstructures and densities, and thus show different mechanical behavior. Up to now, mechanical properties of snow have been mainly evaluated based on bulk proxies such as snow density, while relevant microstructural characteristics have not been accounted for. To establish a link between the microstructure of weak layers and their mechanical properties, we performed displacement-controlled laboratory experiments using a uniaxial testing machine. The compression experiments were recorded using a high-speed camera, allowing us to derive the strain within the weak layer. The microstructure of each batch of specimens was analyzed using micro-tomography to obtain density, specific surface area, anisotropy and correlation lengths. As testing a wide range of microstructural morphologies is difficult due to seasonal availability and the need to transport the fragile samples to the laboratory, we used both natural and artificially grown weak layers. We tested weak layers composed of facetted grains, depth hoar, surface hoar, precipitation particles and rounded grains.   The compressive strength of more than 200 tested samples covered two orders of magnitude (0.5 kPa to 150 kPa) for weak layer densities ranging from 110 kg/m3 to 380 kg/m3. As expected, our results show a strong correlation between weak layer density and compressive strength, but also a dependence on other microstructural quantities. These results will help us improve our understanding of the mechanical properties of weak snowpack layers and will ultimately allow us to better forecast avalanche release probability.