In the majority of fatal snow avalanches, skiers, and snowmobilers apply load to the snow cover which triggers the initial failure in a weak layer. Understanding how the stress from skiing and snowmobiling transmit through the snow cover can help people avoid situations where they can trigger avalanches. In this study, capacitive sensors were used to measure this stress within the mountain snow cover. At specific depths, snowmobiles added more stress than skiers, thus increasing the probability of initiating a fracture in a weak layer and releasing a slab avalanche. The increased penetration depth of snowmobiles into the snow cover compared to skiers was the primary reason for this increase in stress.
The upper snow cover was quantified using a bridging index which is the uniform hand hardness index of a particular layer in the snow cover multiplied by the thickness of that layer. The added stress was then related to localized slope stability using stability indices. Bridging index thresholds of 130 to 190 for skiers and 160 to 260 for snowmobiles yielded stability index values greater than 1.5, thus indicting the transitions from an unstable snow cover to a more stable snow cover.
Stress measurements were presented to illustrate the difference between the loading response of an isolated column in common slope stability tests and loading a largely undisturbed snow cover by skiers or snowmobilers. Adjusting the depth of stability tests to account for the penetration depth of snowmobiles loaded the snow cover more similarly to the loading applied by snowmobiling on the undisturbed snow. The stress profile in stability tests matched skiing and snowmobiling more closely when the snow cover was softer compared to when the snow cover was harder. Finally, in the extended column test, a modern snow stability test, stress was only measurable directly below the dynamic loading and not on the opposite side of the column.
The major and novel findings of this research are: 1) the first stress measurements from common stability tests, 2) the first description of how stress transmits through a snow cover quantified by the bridging index, and 3) the first stress measurements beneath snowmobiles.