Abstract
Granular avalanches are typical threshold-type phenomena implying the loss of stability of granular packings when their slope exceeds a critical angle. Detection of avalanche precursors is important for both basic studies and numerous applications. In this respect, only surface observations are presently available, whereas other known techniques are not suitable to detect internal pre-avalanche rearrangements expected from physical considerations and numerical simulations. Here, we report the first experimental evidence of avalanche precursors and the long-lasting post-avalanche rearrangements in the bulk of 3D granular packings. We use an original nonlinear-acoustic probing methodology which is selectively sensitive to the state of weakest intergrain contacts. This methodology allowed us to clearly detect transient pre-avalanche rearrangements of the weak-contact network. Those rearrangements get stronger and exhibit quasi-periodicity closer to the avalanche triggering, whereas the statistics of the sounding signal amplitude shows clear transition from Gaussian to power law behavior.