The Extended Column Test (ECT) is a new stability test that aims to assess the fracture propagation potential across a 90 cm wide isolated column. Initial results with a dataset consisting of over 300 tests collected by one observer demonstrated the ECT's effectiveness for differentiating between stable and unstable slopes. Further, we have received positive feedback on the test from a world-wide network of observers. This paper: 1) presents new recording standards for the test, 2) uses the SnowPilot dataset to further assess the effectiveness of the test by analyzing over 300 tests performed by several observers in different snow climates, 3) looks at the spatial variability of ECT results from several test grids, and 4) compares side-by-side results between the ECT and the Propagation Saw Test on stable and unstable slopes. Our results indicate that the ECT is an effective stability test, with a false stability ratio generally less than other standard snow stability tests. Results are sometimes quite spatially uniform, though occasionally slopes may exhibit variable ECT results. In comparison to the PST, our data suggest the ECT has a lower false stability rate, but a higher false instability rate. No test is perfect and all tests must be used in conjunction with additional data, but our results show the ECT is valuable additional tool for assessing snow stability.

Snow slab avalanches, characterized by a distinct, broad fracture line, are released following anticrack propagation in highly porous weak snow layers buried below cohesive slabs. The anticrack mechanism is driven by the volumetric collapse of the weak layer, which leads to the closure of crack faces and to the onset of frictional contact. Here, on the basis of snow fracture experiments, full-scale avalanche measurements and numerical simulations, we report the existence of a transition from sub-Rayleigh anticrack to supershear crack propagation. This transition follows the Burridge–Andrews mechanism, in which a supershear daughter crack nucleates ahead of the main fracture front and eventually propagates faster than the shear wave speed. Furthermore, we show that the supershear propagation regime can exist even if the shear-to-normal stress ratio is lower than the static friction coefficient as a result of the loss of frictional resistance during collapse. This finding shows that snow slab avalanches have fundamental similarities with strike-slip earthquakes.

Abstract. The Extended Column Test (ECT) and the Propagation Saw Test (PST) are two commonly used tests to assess the likelihood of crack propagation in a snowpack. Guidelines suggest beams with lengths of around 1 m, yet little is known about how test length affects propagation. Thus, we performed 163 ECTs and PSTs 1.0-10.0 m long. On days with full crack propagation in 1.0-1.5 m tests, we then made videos of tests 2.0-10.0 m long. We inserted markers for particle tracking to measure collapse amplitude, propagation speed, and wavelength. We also used a finite element (FE) model to simulate the strain energy release rate at fixed crack lengths. We find that (1) the proportion of tests with full propagation decreased with test length; (2) collapse was greater at the ends of the beams than in the centers; (3) collapse amplitude was independent of beam length and did not reach a constant value; (4) collapse wavelengths in the longer tests were around 3 m, two times greater than what is predicted by the anticrack model. We also confirmed the prediction that centered PSTs had double the critical length of edge PSTs. Based on our results, we conclude that cracks propagated more frequently in the shorter tests because of increased stress concentration from the far edge. The FE model suggests this edge effect occurs for PSTs of up to 2 m long or a crack to beam length ratio ≥ 0.20. Our results suggest that ECT and PST length guidelines may need to be revisited.

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