Abstract
Abstract. Understanding failure initiation within weak snow layers is essential for modeling and predicting dry-snow slab avalanches. We therefore performed laboratory experiments with snow samples containing a weak layer consisting of either faceted crystals or depth hoar. During these experiments the samples were loaded with different loading rates and at various tilt angles until fracture. The strength of the samples decreased with increasing loading rate and increasing tilt angle. Additionally, we took pictures of the side of four samples with a high-speed video camera and calculated the displacement using a particle image velocimetry (PIV) algorithm. The fracture process within the weak layer could thus be observed in detail. Catastrophic failure started due to a shear fracture just above the interface between the depth hoar layer and the underlying crust.
Highlights
Most skier-triggered dry-snow slab avalanches release due to the failure of a weak layer consisting of depth hoar, faceted crystals, or surface hoar crystals (Schweizer and Jamieson, 2001)
In the experiments we performed with samples containing a weak layer of surface hoar (Reiweger and Schweizer, 2010) we found that at least this kind of weak layer failed in shear more likely than in compression
Sample strength decreased with increasing loading rate and tilt angle
Summary
Most skier-triggered dry-snow slab avalanches release due to the failure of a weak layer consisting of depth hoar, faceted crystals, or surface hoar crystals (Schweizer and Jamieson, 2001). A macroscopic crack (O(10 cm) or more) in the weak layer underlying a cohesive slab might lead to crack propagation and eventually to the release of a slab avalanche (Schweizer et al, 2003). Heierli et al (2008) suggested avalanche initiation to be modeled as a mixed-mode anticrack where the main mechanism behind weak layer failure is collapse of the weak layer. Other authors such as McClung (1979, 2009) or Bažant et al (2003) suggest that the layer first fails in shear. Previous laboratory studies on the mechanical behavior of snow mainly used displacement-controlled shear experiments. It was shown that snow strength decreases with increasing rate of displacement (Narita, 1980)
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