Abstract
Quasi-periodic collective displacements of grains at the free surface of a tilted grain packing constitute precursors of granular avalanches. Laboratory experiments are commonly performed by slowly tilting the packing from 0° to the maximal stability angle θA. In these conditions, the number of precursors is too small to assess reproducible and robust statistical analyses of the precursor activity. To go beyond this limitation, we have developed a specific experimental protocol consisting of tilting the packing with successive oscillation cycles. We use a high-resolution optical camera and process the images of the packing free surface to identify precursory events during many consecutive cycles of a single packing. We observe the same behavior for all half-cycles, forth and back: appearance of the first precursors after the same variation of inclination, exponential evolution of the weak surface activity for the first precursors and linear growth of stronger surface activity for the following ones. The experimental protocol provides both reproducible precursor measurements based on large sample statistical inferences and a quasi-stationary state after one full-cycle. This approach is very promising for highlighting the effects of external parameters, including humidity and packing geometry.
Highlights
Grain packing fabricationTo achieve the so-called "same history" of a packing fabrication [21], we have followed the approach introduced by [20] which consists in a four-steps protocol
1 Introduction quired to predict the onset of the avalanche
Laboratory experiments are performed to tification have been performed by direct tilting experistudy gravity destabilization of a packing of glass beads by ments of a grain packing from the horizontal position to gently inclining it, assuming a quasi-static behavior
Summary
To achieve the so-called "same history" of a packing fabrication [21], we have followed the approach introduced by [20] which consists in a four-steps protocol. The number of grain layers is about 90, which is high enough to avoid any influence on the maximal stability angle of the packing, the well-known wall effect in granular packings [22]. Note that this number is much larger that the limit of 10 layers observed by Aguirre et al [13]. The average initial packing fraction (ratio between the volume of sphere material with the volume of the whole packing) evaluated in [20] is 0.594 ± 0.001 but the last step of the packing fabrication may compact the superficial layer of grains by only few tenths but enough to result in a "scrapped surface" of the packing
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