The behaviour of snow under the effect of combined compressive and shear loading

The failure of a weak layer which lies underneath a more cohesive planar slab, is responsible for slab avalanche initiation. The weak layer may fail under combined effect of shear and compressive loading. Therefore, a preliminary study was carried out to understand the effect of combined compressive and shear loading on a weak snow layer. Since it is difficult to transport snow samples containing a weak layer into an environmental chamber, the mechanical measurements were carried out in situ. A digital force gauge attached to shear frame of area 0.01 m2, was used to measure the shear strength. The compressive pressure was applied on the weak layer with the help of dead weights. The dynamic shear force at high loading rate (average around 25 N s-1) was applied for less than 1 sec and it was observed that the samples failed in a brittle manner. Experiments were carried out on round grain snow layer (RGsr), faceted snow (FCso) and near-surface faceted particles (FCsf) weak layer. Multi-axial stress response for FCsf snow layer was found to be entirely different from RGsr and FCso weak snow layers. Positive correlation between shear strength and compressive pressure was observed for RGsr and FCso snow layers. The experimental study revealed that there exists an elliptical envelope of instability for FCsf snow layer. The elliptical envelope for FCsf indicates that if the state of stress in the weak layer lies on the boundary or out of the elliptical envelope, the snowpack is unstable.