Precursors and triggering mechanisms of granular avalanches

International audienceExploration of the dynamical response of an inclined granular packing before the avalanche startsallows a study of the dynamical transition from a static packing to a flowing one and bringsinformation which may be helpful for the prediction of the occurrence of the avalanche. Duringinclination, small rearrangements implying only a few grains are first detected. The size and the rate ofthese rearrangements increase with the inclination angle. At some stage, large amplitude and quasiperiodical events are observed. These events, called precursors, consist of collective motions ofgrains. Other studies showed that the precursors are bulk phenomena and allowed to interpret theseevents 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 thetilting, acoustic methods can be experimentally used to detect precursors and to probe internalrearrangements in the bulk of the granular layer. Because part of the acoustic wave energytransports through the contact network and the elastic beads of the medium, the probing methodsbased on the monitoring of acoustic signatures are sensitive to changes in the elastic properties of thegranular layer. We give an overview of the acoustic probing methods implemented previously in theexperiments of granular layer destabilization. They can be divided in two groups, the passive methodswhere acoustic sensors are used to listen to the sounds emitted by the destabilized layer itself and theactive methods where an acoustic signal with desired properties is generated and detected bytransducers in the medium. Systematic experiments of granular layer destabilization for variousgranular media and external conditions are compared and allow better understanding of themechanisms responsible for the appearance, periodicity and intensity of precursors