Precursors and triggering mechanisms of granular avalanches

Exploration of the dynamical response of an inclined granular packing before the avalanche starts allows a study of the dynamical transition from a static packing to a flowing one and brings information which may be helpful for the prediction of the occurrence of the avalanche. During inclination, small rearrangements implying only a few grains are first detected. The size and the rate of these rearrangements increase with the inclination angle. At some stage, large amplitude and quasi periodical events are observed. These events, called precursors, consist of collective motions of grains. Other studies showed that the precursors are bulk phenomena and allowed to interpret these events as reorganizations of the weak-contact sub network occurring in the packing. Besides optical imaging techniques which give access to grain motion and rearrangements during the tilting, acoustic methods can be experimentally used to detect precursors and to probe internal rearrangements in the bulk of the granular layer. 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. We give an overview of the acoustic probing methods implemented previously in the experiments of granular layer destabilization. They can be divided in two groups, the passive methods where acoustic sensors are used to listen to the sounds emitted by the destabilized layer itself and the active methods where an acoustic signal with desired properties is generated and detected by transducers in the medium. Systematic experiments of granular layer destabilization for various granular media and external conditions are compared and allow better understanding of the mechanisms responsible for the appearance, periodicity and intensity of precursors.