Field Study on Fracture Propagation in Weak Snowpack Layers

Instabilities in snow slopes generally involve the progressive fracture along a weak snowpack layer. Recent field observations on fractures in weak layers have shown that this process results vertical displacement due to crushing of the weak layer. Long beam specimens containing a natural weak layer were used to study the propagation of fracture. Multiple black markers were inserted in the snow above and below the weak layer. The deformation of the slab during fracture propagation was recorded with a portable digital high speed camera. A technique to reconstruct the trajectories of the markers was developed. This resulted in a detailed visualization of the deformation of the slab during fracture propagation. Results show the propagation of a wave through the slab associated with the collapse of the weak layer. Propagation velocities were relatively constant in the middle of the beam and increased towards the edges. The propagation speed in the middle of the beam was 20 ms, in agreement with theoretical predictions of a new theory for fracture propagation in weak snowpack layers.