Down-hill creep of a granular material under expansion/contraction cycles

We investigate the down-hill creep of an inclined layer of granular material caused by quasi-static oscillatory variations of the size of the particles. The size variation is taken to be maximum at the surface and decreasing with depth, as it may be argued to occur in the case of a granular soil affected by atmospheric conditions. The material is modeled as an athermal two dimensional polydisperse system of soft disks under the action of gravity. The slope angle is below the angle of repose and therefore the system reaches an equilibrium configuration under static external conditions. However, under a protocol in which particles slowly change size in a quasistatic oscillatory way, the system is observed to creep down in a synchronized way with the oscillation. We measure the creep advance per cycle as a function of the slope angle and the degree of change in particle size. We also find that the creep rate is maximum at the surface and smoothly decreases with depth, as it is observed to occur in the field.

Creep motion of a model frictional system

We report on the dynamics of a model frictional system submitted to minute external perturbations. The system consists of a chain of sliders connected through elastic springs that rest on an incline. By introducing cyclic expansions and contractions of the springs we observe a reptation of the chain. We account for the average reptation velocity theoretically. The velocity of small systems exhibits a series of plateaus as a function of the incline angle. Due to elastic effects, there exists a critical amplitude below which the reptation is expected to cease. However, rather than a full stop of the creep, we observe in numerical simulations a transition between a continuous-creep and an irregular-creep regime when the critical amplitude is approached. The latter transition is reminiscent of the transition between the continuous and the irregular compaction of granular matter submitted to periodic temperature changes.