Measurement of crack-face friction in collapsed weak snow layers

In this letter we analyze the frictional contact forces during and immediately after the collapse of a weak snowpack layer, when the sliding plane consists of the freshly collapsed and crushed, but not yet eroded granular debris of the weak layer. The results from thirty‐four field experiments show that frictional contact forces per unit area are on the order of 0.6 times the normal stress, equivalent to a friction angle close to 30 degrees. The measurements show that there is a transient, sharp drop in the coefficient of friction during the collapse of the weak layer and relatively constant values afterwards. One implication of our findings is that the minimum angle for avalanche release does not depend on shear strength, as commonly thought, but results from crack‐face friction which comes into play only as the fracture through the weak layer is already propagating.