Jamming transition of a granular pile below the angle of repose

We experimentally study the relaxation towards mechanical equilibrium following the surface avalanche of a granular pile and discuss it in the general context of glassy materials and jamming transition. Two coexisting dynamics are observed in the surface layer: a short time exponential decay related to rapid isolated grains displacements, and intermittent reactivation bursts consisting in spatially correlated moves. The competition of both dynamics results in long-lived intermittent transients, the total duration of which can last more than a thousand of seconds. The typical time-scales, and the two-time relaxation function of these transients are measured. Analyzing the response of the surface layer to localized perturbations provides new evidence for spatial correlations. We further analyse the relaxation function in a probabilistic framework, which allows to discuss different possible scalings according to the choice of specific hypothesis. Following some considerations on the observed time-scales, we finally discuss the similarities with aging systems which are suggested by our description of the observed dynamics.