Shear failure of weak snow layers in the first hours after burial

Abstract. In a dry stratified snowcover slab avalanches release following failure in a weak layer below the slab. Typically, such weak layers consist either of persistent grain types or precipitation particles. Experience suggests that non-persistent instabilities often crest during or towards the end of a storm – probably because weak layers of precipitation particles strengthen rapidly. Studies so far have mainly focused on persistent grain types providing only sparse data to describe non-persistent weak layer failure.

To understand differences between persistent and non-persistent weak layers we measured fracture mechanical properties relevant for avalanche release in a temporal series of laboratory tests. At defined lag times we tested small layered samples containing a weak layer of surface hoar, facets or decomposing fragmented particles in shear. Highspeed frames from the failure zone and image correlation analysis confirm that weak layers concentrate the shear strain. Failure consistently occurred after 20–30 % of strain energy was dissipated – despite shear strain rates as high 10⁻² s⁻¹. Our results of shear modulus and shear fracture toughness compare well with published data. The values for surface hoar and decomposing fragmented particles increased due to sintering. In the first hours after burial both weak layers had similarly low values, indicating they are equally fragile. Only for surface hoar and decomposing fragmented particles could we calibrate a formulation which allows for estimating the shear modulus from SMP signals.