Strength-Load Ratio An Index of Deep Slab Avalanche Conditions

Deep slab avalanches are often initiated by the catastrophic collapse of a weak basal layer under the overlying static load of snow. Three winters of strength and load studies in the Bridger Range of Montana indicate that the Snow Resistograph and core sampling tube can be used effectively to anticipate deep slab avalanche conditions. The method involves measuring the strength-to-load ratio at the base. Slf the pack. Whenever this ratio becomes less than 2 the snowpack is subject to sudden failure resulting from overloads. On a steep slope this often produces a deep slab avalanche. Normal surface snow consolidation is the product of sintering.and may be expressed as a power function of the age of the snow. The data suggest that both the rate of consolidation and the ultimate strength of the snow during the first month is predetermined by the physical conditions at the time of deposition. Consolidation of the basal snow layer is very erratic because of metamorphic interference with the sintering process. In general the strength of the basal snow is guided by the rate of snow accumulation such that the rate of consolidation is approximateiy 1/2 the rate of loading. This pro-duces a declining strength-to-load ratio at the base of the pack and leads in the direction of collapse. Actual collapse therefore, depends in general upon the length of the season of snow accumula-tion, and the initial weakness of the basal layer at the time of burial. In detail the collapse depends upon the delicate interplay of load hardening and metamorphic weakening which occasionally pro-duces short periods of extreme systemic fragility. I