Oger Luc

 Observation of the avalanche precursors under cycling or wet conditions
Abstract:

The observation of some precursors before avalanches is a quite recent discovery [1,2] but understanding the physical characteristics which impulses the apparition of these precursors of granular avalanches is important for the prediction of some critical events. As pan of the dynamics leading to the avalanche, precursors are identified as collective motions of grains visible on the free surface. When a granular pile is tilted at a constant angular velocity, precursors appear quasi-periodically.

In one hand, our experiments conducted with monodisperse glass beads packings submitted to an automatic symmetric oscillation show well defined periodic precursors along the successive cycles. We simultaneously characterize precursors on the free surface with an optical method(surface 2D probe) and in the bulk with acoustic methods with and amplitude modulated pulse(3D volumetric probe).
In other hand, experiments with glass beads of 0.2, 0.5 and 0.75 mm diameter, and  polystyrene beads of 0.14 mm diameter inside an ambient relative humidity ranging between 40 and 94 % highlight that cohesive forces induced by this humidity have a large impact on the stability of the grain piles. These effects are clearly visible on the precursor and avalanche events.
[1] Nerone N, Aguirre M, Calvo A, Bideau D and Ippolito I 2003 Phys. Rev. E 67 011302
[2] de Richter S K, Ca er G L and Delannay R 2012 J. Stat. Mech. 04013
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