Precursors and triggering mechanisms of granular avalanches
Résumé
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.