Systematic production and characterization of artificially produced weak layers of depth hoar

Abstract

Buried weak snowpack layers are a prerequisite for dry-snow slab avalanches, which are responsible for most recreational avalanche fatalities. To assess avalanche release probability and size requires detailed knowledge on weak layer mechanical properties. 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 laboratory experiments with artificially produced snow samples containing a weak layer consisting of depth hoar. Growing weak layers artificially allows us to control and investigate the full microstructural parameter range. In addition, the controlled laboratory environment helps improve repeatability and limit the scatter that is inherent in field testing. To evaluate the properties and reproducibility of artificially grown depth hoar samples, we designed a snow-metamorphism box with a regulated heating plate at the bottom to impose a large temperature gradient across our snow sample. We then performed compression tests to measure the strength of the artificial weak layers. We used a mechanical testing machine to measure the peak force at the moment of weak layer failure. With digital image correlation we analyzed the deformation of the sample prior to failure. To establish a link between mechanical properties and microstructure, all samples were additionally characterized with micro-tomography. First findings show that we can produce samples with similar properties with reasonable accuracy and that there is a correlation between the resulting mechanical properties and the applied temperature gradient as well as the duration of the depth hoar metamorphism. Our results will help us improve our understanding of the growth and failure behavior of weak snowpack layers consisting of depth hoar and will ultimately allow us to better forecast avalanche release probability.