Assessing the effect of explosions on the stability of the snowpack

Fig. 1: Stills of an explosion taken with a high speed camera (picture: A. van Herwijnen/SLF).

Avalanche control by explosives is a widely used measure for avalanche protection. The pressure wave caused by the explosion (Fig. 1) penetrates the snow pack and can cause an internal weak layer to fracture. This fracture can then lead to the release of an avalanche, provided the snow cover structure is conducive to fracture propagation. Avalanche control is used to secure dangerous slopes threatening ski areas, roads, railway lines and settlements. Avalanche control complements or replaces permanent and temporary measures such as avalanche defense structures, galleries or long road closures and evacuations. Avalanche control is most often performed with explosives by hand or from a helicopter. However, fixed remotely controlled avalanche control installations have become increasingly popular, due to their safe use in all weather conditions.

What are the processes within the snow cover during avalanche control?

We are especially interested in the processes within the snow cover during an explosion and how far from the explosion an avalanche still is likely to be triggered.

Within this project we will answer the following questions:

• What is the effect of the pressure wave onto the snow cover?
• How does the pressure wave travel within a certain snow cover?
• How strong is the pressure wave affecting the snow surface at a certain distance from the explosion?
• What are the processes that lead to triggering an avalanche by explosives?
• Is it possible to model the main processes within the snow cover caused by explosives?
• Is such a model able to consider topography?
• What is the effectiveness of different avalanche control systems?

Trailing the explosion

Explosions are dynamic processes with very high velocities. To measure the attenuation of the pressure wave with distance we use sensors on and within the snow cover at various distances from the explosion. These sensors consist of high precision microphones and accelerometers (Fig. 2). The tests are performed on a large level field with a spatially homogeneous snow cover (Project Spatial Variability) allowing for reproducible results and to ensure that the sensors are not destroyed by an avalanche.

Fig. 2: Open snow pit with measuring equipment. The diagonally inserted sensors on the right side of the pit wall measure the acceleration within the snow cover during an explosion. The microphone (yellow) on the snow surface measures the air overpressure during the experiment. The black markers at the pit wall are used to follow the deformations within the snow cover (picture: S. Simioni/SLF).

Using a numerical model, we will simulate the effect of explosions on the snow cover. This will allow us to evaluate where the optimal placing for fixed avalanche control installations is and what the spacing between the installations should be.

• Stephan Simioni