Avalanche precursor behavior during cyclic-tilting experiments.

Abstract

Multicycle experiments performed by slowly tilting granular packings in the range ±θ_{M} are analyzed for θ_{M}=21, 22, 23, and 23.5^{∘}. Based on optical measurements of the free surface activity, a method is developed to detect numerous bead rearrangements associated to almost regular peaks of activity: on average, 1800 events are detected during an experiment. Occurring 11^{∘} after the beginning of the inclination, precursors represent 75% of these events and the interprecursor angle tends to decrease when the surface activity increases. The evolution of the cumulative activity with the half-cycle number can be approximated by a stretched exponential function with an exponent β. The characteristic time for the system to reach a steady state of activity linearly increases with θ_{M}. The residual activity also increases but reaches a plateau. The energy injected into the system by successive gravity destabilizations decreases with θ_{M}: when θ_{M}≤22^{∘}, β≤1 suggests relaxation processes and compaction induced by the precursors themselves increases the stability of the packing. An in-depth force sensor measures the precursor-induced activity at the wall of the grain container: a linear dependency relates wall and surface activities. In-depth acoustic measurements are used to trigger surface high-speed optical signals: during a precursor, which duration is of about 0.18 s, the elastic wave velocity inside the packing drops to few m.s^{-1} because of weak intergrain contacts that allow beads to rearrange.