Force signatures of creep in a photoelastic granular medium

Creep is the subsurface, slow movement of constituents in a granular packing due to applied stress and the disordered nature of its grain-scale interactions, as in the example of slowly evolving sloped hillsides. We explore creep through experiments of quasi two-dimensional piles of disks that are made from a birefringent material, which allows us to use image acquisition to observe both grain movements and grain-scale force networks. Controlled disturbances to the pile are used to instigate creep events. We investigate the evolution of the force network and particle rearrangements to illuminate signatures of these events. We find that shifts in force chains provide a precursor to larger, avalanche-scale disruptions that can predict where an avalanche will occur. In addition, changes in force chain structure manifested at greater depth than any noticeable particle shifts, suggesting that there is a distinct "flowing" layer that transitions to creep behavior deeper in the pile.