Anticrack Avalanche Release System

This model, also known as the mixed-mode anticrack model, has been developed because of one observed phenomenon. A person skiing or even walking on a slope can trigger a slab avalanche on that slope, sometimes several hundred meters further uphill. Those avalanches are also called remotely triggered avalanches.

Sommaire 1- What’s an anticrack ? 2 2- The anticrack model: a two-stage process 3 2.1- First stage 3 2.2- Second stage 4 2.3- The danger of this phenomenon 4 3- Determination of the difficulty of triggering fracture 5 3.1- Field experiments 5 3.2- Analytical determination of fracture propagation risk 7 4- Influence of skiers on fracture propagation 8

1- What’s an anticrack ?

In fracture mechanism we distinguish 3 different modes: * Mode I, or opening mode, refers to the creation of an opening between two layers under a tensile loading. As in the snowpack there are no forces that could lift a slab and create an open space between the upper and the lower slab, this model isn’t relevant in terms of avalanches. * Modes II and III, also called shear modes, refer to two layers slipping over each other. According to this theory, the propagation of a crack in the snowpack depends on the steepness of the slope and, therefore cracks cannot propagate on flat grounds because of the lake of driving forces.
Those 3 different modes cannot explain the remote triggering phenomenon. So there must be another reason: the anticrack model. This mode is a counterpart to the mode I (anti-mode I), and refers to the fact that a crack propagates under compression. In many materials this is not possible. It can only happen when the fracture goes along with a reduction in volume.

Generally all of those modes can occur simultaneously. In this case we speak of the mixed-mode anticrack.

2- The anticrack model: a two-stage process
2.1- First stage

Anticrack triggering: slab weak layer (depth hoar)