• Fundamental knowledge about weak layers: New tests have revealed how great the compressive and shear forces must be for a fracture to occur in a weak layer.
• Laboratory analysis: The tests refine the constitutive law – the basis of all physical calculations – for weak layers and could improve avalanche forecasting.

Like a house of cards that collapses when the load becomes too great, weak layers in snow can fail, potentially resulting in slab avalanches. Until now, the forces that cause these layers to fracture have not been adequately explained. New findings from researchers at the WSL Institute for Snow and Avalanche Research SLF are now providing an up-to-date experimental basis for use in avalanche models. The researchers recreated the effect of gravity on a sloping snowpack in a laboratory setting: firstly, the normal force acting perpendicular to the slope and, secondly, the shear force acting parallel to the slope, which increases with the slope angle.
In avalanche research, there have been two opposing views on this issue since the 1970s, and 20 studies have already been published on the subject. One theory states that, as the perpendicular force increases, the required force parallel to the slope also increases. The other holds that, where there is greater pressure from above, fracture will already occur under a lower shear load. Jakob Schöttner's findings have now lent weight to the latter theory. Schöttner, who is studying for a PhD in snow mechanics at the SLF, explains: "Pressure doesn't make the snowpack more stable; rather, both forces together lead to failure of the weak layer."

Snow under pressure

To find out how these components of compressive and shear forces interact, Schöttner examined 63 samples from three natural weak layers in snow-covered surface hoar under combined compressive and shear load in the cold laboratory. Using a high-speed camera, he recorded when the layers fractured. His findings refine the constitutive law – the basis of all physical calculations – for weak layers. This law can be used for model calculations, which in turn could lead to advances in avalanche forecasting.

Cooperation partners

• Technical University of Darmstadt, Germany
• University of Rostock, Germany
• University of Porto, Portugal

Funding

• Swiss National Science Foundation (SNSF)