Two field tests of snow slab stability, the shear frame test and the rutschblock test, were studied at avalanche forecasting areas in British Columbia and Alberta during the winters of 1992-93 to 1994-95. Field work focused on persistent weak snowpack layers consisting of surface hoar or faceted crystals that are the failure planes for most fatal slab avalanche accidents in Canada.

The shear frame test was refined through field and finite element studies. Effects of different frame designs were identified. Shear strength measurements were shown to decrease as the distance between the frame and the weak layer decreased. Field studies of the effect of loading rate and shear frame area on shear strength confirmed previous studies. Using different shear frame operators did not affect the resulting strength measurements provided the operators maintained consistent technique. One particular shape of fracture surface was associated with significantly higher strength measurements. The strength measurements from the first two tests proved to be more variable than measurements from subsequent tests on the same weak layer.

Shear frame stability indices for natural avalanches and for skier-triggered dry slab avalanches were refined by incorporating an adjustment for normal load that depended on microstructure of the weak layer. The stability index for skier triggering was further refined by adjusting for the distance the skis penetrate the snow surface. Skier stability indices based on shear frames tests at both avalanche slopes and safe study sites were correlated with skier-triggered dry slab avalanches. When compared with other forecasting variables, the skier-stability index based on study site tests ranked first or second in predictive value.

Closely spaced rutschblocks on nine avalanche slopes were used to identify snowpack and terrain factors that affect rutschblock results. The frequency of skier-triggered avalanches for common rutschblock scores in the avalanche start zones was determined and shown to be similar to a Swiss study in a different snowpack. For a given rutschblock score, persistent slabs were triggered more frequently than non-persistent slabs.

Limitations of shear frame stability indices and rutschblock tests related to slope inclination and terrain were identified.