Diffusion Level: PU Deliverable no. 1.5.1: Experiments on failure of layered materials Trigs: Triggering Instabilities in Materials and Geosystems Experiments on failure of layered materials

Dry-snow slab avalanches start with a failure in a weak snow layer below a cohesive snow slab of ample thickness (Schweizer et al., 2003). The existence of a weak layer, which has a lower strength than its neighbouring layers, is a prerequisite for the release of natural dry-snow slab avalanches (McClung, 1979). Field measurements of strength in weak snow layers or snow layers in general show large scatter in the results (Föhn et al., 1998; Jamieson and Johnston, 2001) mainly because the mechanical properties of snow depend on microstructural characteristics which are variable both in space and time. This impairs the reproducibility of experiments performed at different times even at the same snowpack location. Therefore, various studies on shear failure of homogeneous snow samples under controlled laboratory conditions have been performed (e.g. Schweizer 1998). Depending on snow type and temperature these studies showed brittle behaviour of snow at deformation rates typically faster than 10-4 s-1 and ductile behaviour at deformation rates typically slower than 10-5 s-1. Up to now, experiments on layered snow samples have only been performed by Fukuzawa and Narita (1993). They found the ductile to brittle transition to take place at a strain rate of about 10-4 s-1, an order of magnitude smaller than what Schweizer (1998) found for homogeneous snow samples under shear loading. We perform shearing experiments with both natural an