Field measurements of snowpack response to explosive loading

Abstract

Avalanche control by explosives is among the key temporary preventive measures. Hitherto, little is known about wave propagation in a snowpack caused by an explosion. During the winter 2013-2014 we performed field experiments on a flat study site. We triggered slurry explosive charges at different heights above the snow surface. At three different distances from the point of explosion we measured surface air pressure and accelerations of the snowpack at various depths. Cameras were placed in the snow pits for recording weak layer failure and crack propagation. We report empirical relations for the decay of near-surface air pressure, accelerations, displacement velocities and displacement with distance from the explosion and depth within the snowpack. Waves within the snowpack arrived earlier at the sensors than the corresponding air pressure waves at the microphones. Air pressure decayed stronger than accelerations within the snowpack. Weak layer failure mainly happened in the top part of the snowpack. We observed two types of weak layer failure, one caused by the direct impact of the air pressure wave close to the point of observation, the other by failure induced by the air pressure wave closer to the point of explosion and subsequent crack propagation. Our measurements increase the understanding of acoustic wave propagation in snow and can be used for comparison with numerical simulations.

On the issue of defining safe distances and overpressure under impact of shock air blast wave (Magnesitovaya mine)

“…The overpressure is determined at the shock air waves (SAW) front to specify the safe distances of the SAW impact to exclude the people injury, facilities and technological equipment damage during underground mine blasting [2][3][4][5][6][7][8][9][10][11][12] (expression (1)). The calculation was made for the mine section of 15.5 m, two variants of the total section of the mine workings adjacent to the face of 15.…”

Numerical simulation of wave propagation and snow failure from explosive loading

“…Microphones were placed 0.05 m above the undisturbed snow surface next to the snow pits. Three accelerometers were installed in cavities at 0.13 m, 0.48 m, and 0.83 m below the snow surface in each snow pit [35]. Special care was given to fit the diameters of the horizontal holes exactly with the diameters of the accelerometers to warrant the coupling of the sensors to the snowpack.…”

“…Special care was given to fit the diameters of the horizontal holes exactly with the diameters of the accelerometers to warrant the coupling of the sensors to the snowpack. Snowpack failure was recorded with compact cameras [35] The snowpack on the investigated day was 187 cm deep and consisted of a 45 cm thick layer of recently deposited snow (consisting of decomposing and fragmented precipitation particles) including two melt-freeze crusts above a well-consolidated base. The base was composed of layers of small rounded grains interspersed with several melt-freeze crusts and ice layers above hard layers of faceted crystals near the bottom of the snowpack.…”

“…However, many of the more recent studies focused on small range effects [6,40,22]. A more detailed review of the past research within snow and explosions is given by Simioni et al [35]. A model considering the porous character of snow based on Biot's [1962] equations has been proposed by Johnson [21], but has rarely been applied to snow since [2].…”

“…We tested the hypothesis that the stresses induced by the acoustic wave propagating through the snowpack locally exceed the snow strength and lead to failure. In the winter 2013-2014 we performed multiple field experiments with avalanche control explosives triggered at different elevations above the snow surface and measured the air pressure above the snowpack as well as acceleration at different depths within the snowpack and distances from the point of explosion [35]. In addition, we recorded weak layer failure with cameras.…”