Identification of avalanche precursors by acoustic probing in the bulk of tilted granular layers

Abstract

International audienceUnderstanding the precursors of granular avalanches is important for the prediction of critical events. As part of the dynamics leading to the avalanche, precursors are identified as collective motions of grains on the free surface. When a granular pile is tilted at a constant angular velocity, precursors appear quasi-periodically. In this paper we simultaneously caracterize precursors on the free surface with an optical method and in the bulk with acoustic methods (nonlinear and linear). Surprisingly, the use on nonlinear acoustic method is not necessary to probe rearrangements in the bulk of the granular material. A linear method can also be used provided that the frequency region is the one where the acoustic propagation is sensitive to the solid skeleton formed by the bead-contact network. Our experiments conducted with monodisperse glass beads show that their surface features are by far the most important for the precursor propreties. Our results allow to probe with a few millisecond time resolution (less than 10−2 degree of inclination) the relaxation phenomena associated to each precursor event. Interpretations of different precursors and different experiments provide an interesting train of thought for the understanding of destabilization mechanisms in granular systems

Precursors and triggering mechanisms of granular avalanches

“…In this case, strong correlations between surface and bulk rearrangements can be revealed (Fig. 3) [27,28]. They show strong variations of the weak contact network during the tilt especially just before avalanche triggering (Fig.…”

“…In this case, strong correlations between surface and bulk rearrangements can be revealed (Fig. 3) [27,28]. The surface rearrangements (Fig.…”

“…Acoustic methods can be experimentally used to detect precursors [25,26,27] and to probe internal rearrangements in the bulk of the granular layer [12,24,28,27,29]. Because part of the acoustic wave energy transports through the contact network and the elastic beads of the medium, the probing methods based on the monitoring of acoustic signatures are sensitive to changes in the elastic properties of the granular layer.…”

Dynamic behavior of humid granular avalanches: Optical measurements to characterize the precursor activity

“…Duranteau el al. [20] have demonstrated the strong correlations between optical and acoustical detections of precursors. They have shown that precursors are both of geometrical nature (at least at the surface) and of elastic nature in the bulk of the layer (at least down to a certain depth).…”

“…But all these first experiments [14,16,24,35] and more recent studies [20,25,36,37] dedicated to the identification of the precursors have been essentially performed in a dry atmosphere, with a relative humidity up to about 50 %.…”