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Snow deformation and acoustic emissions accompany failures in the snowpack

 

Avalanches are triggered by the breaking of individual bonds between the grains of ice in a weak zone of the snowpack. If the damage caused is large enough, a crack occurs which swiftly propagates within the weak layer and releases the overlaying slab as a dry-snow slab avalanche.

In the case of spontaneous avalanches, very little is known about how and when these failures occur. The SLF is thus conducting snow-load experiments in a cold laboratory testing snow samples that include an artificially created weak layer. With the aid of a shear apparatus constructed specifically for snow-testing purposes, the researchers can continuously increase the force being exerted on the sample until the weak layer fails. Using video cameras, they study the deformation of the snowpack before the failure occurs. They also record the sounds generated by the breaking of bonds between the grains of snow. The acoustic emissions permit conclusions to be drawn about the fracture processes going on inside the snow sample.

shear apparatus
Fig. 1: Operating principle of the shear apparatus. Illustration: Raffinerie

Project details

Project duration

2014 - 2017

 
cracks in the snowpack generate ultrasonic noises
Fig. 2: Before slab avalanches release, friction between snow grains and small cracks in the snowpack generate ultrasonic noises which can be captured by acoustic sensors.
Schematic representation
Fig. 3: Schematic representation (left) and photograph (right) of an artificially created weak layer. Natural snow is passed through a sieve to produce a sample with a thin, low-density (ρ) layer at its centre. A temperature gradient is applied to transform this layer into a fragile, weak layer consisting of coarse grains.
 
Load (force) and energy rate of the acoustic emissions prior to the fracture
Fig. 4: Load (force) and energy rate of the acoustic emissions prior to the fracture.

Initial results reveal that the weak layer in the snow sample is severely deformed just before failure takes place. Acoustic signals likewise indicate that the failure does not occur suddenly, but is the result of a gradual damage process within the sample. As soon as the load is applied to the sample, tiny cracks form and early signals are discernible and can be measured. Number and strength of the acoustic signals increase continuously until the catastrophic failure occurs. Measurements demonstrate that the damage process tends to accelerate before the failure occurs; and that crack size steadily increases. Thus, an indication of imminent catastrophic failure of the snow sample is acoustically provided. It is hoped that further investigations will reveal whether these results can be replicated on a natural avalanche slope, and whether acoustic signals can provide an early warning of an impending spontaneous avalanche. In natural surroundings, one major difficulty arises from the severe attenuation of high-frequency acoustic signals in snow, limiting their detection to short distances.