ABSTRACT:

The paper shows a coupled stress/energy criterion for spontaneous and artificial triggering of dry snow slab avalanches, starting from a defect in the weak layer of the snow-pack. The model takes into account both the tensional and the energetic approach: a comparison between these two approaches is suggested. Considering the specific energy of different kinds of explosives and evaluating their transmittance capacity through different snow layers, the method is applied to the artificial detachment of avalanches by explosive. Finally, thanks to the computation of the equivalent load and of the energy induced by the shock wave, the effectiveness of the model is tried out for the artificial detachment of snow slab avalanches.

1. INTRODUCTION

Due to the increasing anthropisation of mountain areas, the protection against natural phenomena gained top priority, and snow avalanches stopping or intentional triggering represent a challenge for engineers, as well as spontaneous avalanches release forecasting, including scientific (the knowledge and the modeling of the physical processes) and technical (the design, setting up and/or operation of defenses and/or triggering systems). The paper deals with both aspect, referring to the snow slab avalanches; a model is presented of avalanche triggering, suitable to the spontaneous release likelihood prediction (through the evaluation of the critical snow slab and weak layer thickness values) and to the design of artificial triggering systems. Referring to the latter subject, the effect of shock waves induced in the snow cover by the explosion is then analyzed and discussed. The model is based on a coupled stress-energy failure criterion. Field observation on the snow-pack have confirmed that its typical structure, to produce slab avalanches, is layered: a thin and weak layer under a relatively thick, strong and stiff slab and a cohesive basal layer (soil or previously fallen snow). To test the snow-pack stability, the shear stress in the weak layer is defined as:

(mathematical equation available in full paper)

in which H is the height of the snow slab, a the slope (Fig. 1) and ¿ is the volumic mass. tN is defined as the nominal tangential stress in the absence of defects.

Fig. 1. The snow-pack: snow slab, weak layer and super-weak zone. The origin of the reference system is at the top of the super-weak zone.(available in full paper)

It is nowadays (see, e.g., [1]) accepted that avalanche triggering is caused by the presence of flaws (called super-weak zones, using the terminology adopted by [2]) inside the weak layer. So, a schematic snow-pack is composed by the slab (bonded and unbonded parts), the weak layer and the super-weak zone (Fig. 1). The latter acts as stress concentrator, inducing relevant normal stresses in the slope direction in the unbonded part of the snow slab. Furthermore, stress concentration takes place in the weak layer surrounding the super-weak zone, since the intact weak layer must transfer the component parallel to slope of the dead load the unbonded part. When, for some reason (i.e. snow fall), the super-weak zone widens (in Mode II and Mode III) along the weak layer, a rapid-self propagation of the shear fracture occurs.

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