Stress measurements in the snow cover below localized dynamic loads

- Snowmobile measurements of dynamic stress in the snow cover quantified with a bridging index.
- Skier measurements of dynamic stress in the snow cover quantified with a bridging index.
- Visualizations of dynamic stress beneath snowmobiles for three simplified hardness profiles.
- Bridging index thresholds of 130 to 190 for skiers yielded stability index greater than 1.5.
- Bridging index thresholds of 160 to 260 for snowmobiles yielded stability index greater than 1.5.
- Bridging index values for 77 skier-triggered avalanches were computed.

In the majority of fatal snow avalanches, skiers and snowmobilers trigger the initial failure in a weak layer by applying load to the snow cover. Understanding how stresses from dynamic surface loads transmit through the snow cover can help people avoid situations where they can trigger avalanches. We build on previous studies that provided more qualitative statements to a quantitative analysis about how the snow cover affects stress transmission. We quantified the upper snow cover for the experiments according to 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. Capacitive sensors were used to measure peak stress below dynamic surface loads within the mountain snow cover. The sensors were used on 33 separate field days to collect over 1420 measurements of loading by skiers and snowmobiles. In order to visualize this measured stress, static stress calculations for an elastic homogeneous 2D snow cover were calibrated to show measured stress values of loading caused by the passage of a snowmobile. The change in 2D shape and magnitude of the induced stress was plotted for three “typical” snow cover structures. Relatively soft snow allowed the specific levels of dynamic stress to penetrate deeper into the snow cover, thus increasing the probability of initiating a failure in a weak layer. Whereas, supportive snow limited the depth that the dynamic stress penetrated by spreading it out laterally. 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. Due to the high variability in the stress measurements, bridging index values for 77 skier-triggered avalanche slopes were computed and compared to the values found from the stability index analysis.