Triggering mechanism of the wet granular avalanches
Résumé
Because of the complexity of the physics that governs granular media, understanding the triggering mechanism of
granular avalanches remains a scientific challenge. Such phenomena are controlled by many factors, including the
grain size and nature, the geometry of the granular structure, but also the environment conditions such as temperature
and moisture which influence is difficult to assess. To this end, we reproduce avalanches at the laboratory
scale under well-controlled conditions and focus our study on the moisture parameter.
Recent studies show that the dynamics of a slowly inclined granular bed present three regimes of destabilization1:
(1) a regime of small independent and localized rearrangements followed by (2) a regime of collective and successive
motions of grains, called ‘precursors’, which appear at quasi-periodic-tilting angles and then (3) the avalanche
which occurs at the maximum stability angle. This avalanche angle strongly depends on the relative humidity2.
The experiments consisted in recording images of the surface of the granular bed, in lateral and top simultaneous
views, while slowly and regularly tilting it from the horizontal to the avalanche angle. Image processing was then
performed to describe the dynamics of the medium for different experimental conditions.
We did experiments at different relative humidities, between 40% and 95%, on different monodisperse spherical
grains: glass beads with diameters 0.875, 0.525 and 0.204 mm, respectively, and polystyrene beads with diameter
0.14 mm. The velocity of the packing inclination was set to 0.17°/s .The two cameras acquired one image per
second, to determine the angle of the surface during the inclination and to detect the precursors, respectively. We
incline the bed of grains from the horizontal to initiate the first avalanche, and then we continue the inclination to
get successive avalanches. The angle of the surface at the avalanche is controlled by the relative humidity, with a
trend in agreement with Gomez et al.2.We also observed a dependency of the precursor dynamics with the relative
humidity which strongly modifies the mechanical properties of the granular pile by making it more cohesive. Other
parameters, such as grain size and nature, height of the granular bed and tilt speed, are also investigated