Temporal evolution of crack propagation characteristics in a weak snowpack layer: conditions of crack arrest and sustained propagation

. For a slab avalanche to release, the system, consisting of a weak layer below a cohesive slab, must facilitate crack propagation over large distances – a process we call dynamic crack propagation. Field measurements on crack propagation at this scale are very scarce. We therefore performed a series of propagation saw test experiments, up to ten meters long, over a period of 10 weeks and analyzed these using digital image correlation techniques. We derived the elastic modulus of the slab (0.5 to 50 15 MPa), the elastic modulus of the weak layer (50 kPa to 1 MPa) and the specific fracture energy of the weak layer (0.1 to 1.5 J m -2 ) with a homogeneous and a layered slab model. During crack propagation, we measured crack speed, touchdown distance and the energy dissipation due to compaction and dynamic fracture (5 mJ m -2 to 0.43 J m -2 ). Crack speeds were highest for PSTs resulting in full propagation and crack arrest lengths were always shorter than touchdown lengths. Based on these findings, an index for self-sustained crack propagation is proposed. Our data set provides unique insight and valuable 20 data to validate models. of the snowpack to promote self-sustaining crack propagation and, second, to provide an index for evaluating the propensity for self-sustaining 85 dynamic crack propagation that can be derived from manual and simulated snowpack profiles. Therefore, we conducted a series of very long PST experiments (up to 10 m long) over the entire life cycle of a weak layer, involving very different states of the snowpack. Highspeed videos of these experiments were analysed with image correlation techniques to estimate crack propagation characteristics, such as crack speed and crack propagation distance. In addition, mechanical properties of the weak layer and slab, as well as specific fracture energy of the weak layer were estimated using 90 two mechanical models. Furthermore, we suggest an estimate of dynamic fracture energy, which we call the dissipation of dynamic fracture, by separating the work done in the weak layer ahead of and behind the crack tip. In the end, our data set provides insight into which snowpacks promote self-sustained crack propagation and how propagation propensity can be estimated from measured or simulated snowpack profiles. RX100-V, 1920 x 1080 pixel 2 , 50 fps) to capture slab deformation prior to crack propagation, when we cut the weak layer with a 2 mm thick snow saw. The side wall of the PST was speckled with black ink (Indian Ink, Lefranc & Bourgeois) to enhance the contrast for digital image correlation (DIC) analysis. Both cameras were aligned perpendicular to the centre-point of the PST beam. fracture process zone, extending the first bond fractures to the tip. In III, the slab further subsides causing the 240 weak-layer structure to fracture multiple times before closer packing of the weak layer is achieved and the slab comes to rest again.