For a slab avalanche to release, a weak layer buried below a cohesive snow slab is required, and the system of weak layer and slab must facilitate crack propagation over large distances. This process, called dynamic crack propagation, is still rather poorly understood even though it is highly relevant for avalanche release. While models are nowadays able to simulate crack propagation over increasingly larger distances, validation data from field experiments are not yet available.
We therefore performed a series of flat field Propagation Saw Test (PST) experiments, up to ten meters long, over a period of 10 weeks on the same weak layer. Within this period, PST results evolved from crack arrest to full propagation and back to crack arrest. All PST experiments were analyzed using digital image correlation to derive high-resolution displacement fields. From these we determined the static specific fracture energy at the onset of crack propagation. In addition, we computed a dynamic fracture energy of the weak layer. To do so, we separated the work done in the weak layer during dynamic crack propagation in two parts. One part is the energy required to advance the crack ahead of the crack tip, namely the dissipation of dynamic fracture, the second part is used for weak layer compaction, the elastic-plastic compaction part.
Results showed that in our leveled propagation saw tests, the dissipation due to compaction was around 30 times higher than the dissipation of dynamic fracture. The latter was in the range of 5 mJ m-2 to 0.43 J m-2 and therefore somewhat lower than the static specific fracture energy prior to crack propagation that ranged from 0.1 to 1.5 J m-2. The dissipation of dynamic fracture alone is insufficient for self-sustained crack propagation in leveled terrain. The separation of the two energy dissipations can help distinguish between stable (small whumpfs) and unstable crack growth (remote triggering of avalanches) occurring in layered snowpacks.
Overall, our dataset provides new insight into the dynamics of crack propagation and provides valuable data to validate models used to study this process.