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

Abstract

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.

Modeling snow slab avalanches caused by weak layer failure – Part II: Coupled mixed-mode criterion for skier-triggered anticracks

“…In these cases, the strain rates are much lower (< 10 −7 s −1 ) (McClung, 1979) and the effect of temporal effects like sintering must be accounted for in an extended failure criterion (van Herwijnen and Miller, 2013;Birkeland et al, 2019a). Since, material properties of snow strongly depend on strain rate (Reuter et al, 2019), they must be chosen to correctly represent the rapid loading by a skier and the corresponding 10 brittle failure behavior.…”

Modeling snow slab avalanches caused by weak-layer failure – Part 2: Coupled mixed-mode criterion for skier-triggered anticracks

“…Both are only applicable when the mode I and mode II toughnesses are of the same order. As discussed in Part 1 (Rosendahl and Weißgraeber, 2020), the tearing mode I (G + Ic ) and shear mode II (G IIc ) fracture toughnesses are of the same order of magnitude. However, the collapse mode I fracture toughness G − Ic is up to 2 orders of magnitude larger than G + Ic .…”

“…Mixed-mode energy criteria describe the interaction of crack opening modes I, II and III. Mode I corresponds to crack opening normal to the crack faces, which comprises both tearing and collapse each associated with a distinct fracture toughness, G + Ic and G − Ic , respectively (see Part 1, Rosendahl and Weißgraeber, 2020). Modes II and III are shear crack modes and correspond to displacements tangential to the crack faces.…”

“…The criterion accounts for mixed-mode shear and compression loading. We employ closed-form analytical expressions for weak-layer stresses and energy release rates of cracks within the weak layer given in Part 1 of this series (Rosendahl and Weißgraeber, 2020). P. L. Rosendahl and P. Weißgraeber: Modeling snow slab avalanches caused by weak-layer failure -Part 2 133 2 Finite fracture mechanics criterion for skier-triggered anticrack nucleation Dry-snow slab avalanche release is typically preceded by the nucleation of an anticrack within the weak layer.…”

Response to referee

“…(29) in our manuscript). We use this equation to compute data that can be compared to the results of many other analyses, also very recent works, that use the same concept [3][4][5][6][7][8][9][10][11].…”