Measurements at recent deep slab avalanches

ABSTRACT: Hard-to-forecast deep slab avalanches can release unpredictably under diverse conditions ranging from storms to clear days to locally induced stress to the snowpack. For the formation of many natural avalanches, a point is reached where the mass loading of overlying snow overpowers the mechanical properties of the weak layer. This can occur from additional loading above the weak layer, such as from precipitation or wind loading. Furthermore, natural failure can occur from solar warming and temperature variations. External stresses applied to the snowpack from skiers, snowmobilers, and other forces can also trigger deep slab avalanches. We collected field measurements of the properties of the failure layers and slab load to determine trends and correlations between such variables. The failure planes were analyzed using the deep tap test, propagation saw test, shear frame tests, and hand hardness and the overlying loads were calculated using density measurements. Spatial variability across the crowns was also assessed by use of multiple profiles and tests. Deep tap tests consistently yielded sudden (Q1) fractures and the cut length in the PST was usually less than 60 % of the column length when the fracture propagated to the end. Preliminary results on spatial variability indicate that DT and shear frame results tended to increase with slab depth at some deep slab locations and crown thickness typically varied substantially. Locations with a thin snowpack, such as near rocky cliffs, were likely trigger points for some of the deep slab avalanches