Force signatures of creep in a photoelastic granular medium

Granular creep is the subsurface 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 emergence and 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 form.

In addition, changes in force chain structure manifest 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.