Compaction and Creep in a Photoelastic Granular System

Creep is the subsurface, slow movement and deformation of constituents in a granular packing, such as sand or sediments, due to the applied stress and disordered nature of its grain-scale interactions. The phenomenon of creep in dense granular systems is relatively understudied, leaving many open questions. We explored creep through experiments in which we observed the influence of a controlled disturbance on rearrangements in a granular packing. Our granular system consists of disks that are made from a birefringent material; this allows us to use image acquisition to observe both the movement of the grains and the changing stress distribution within the system. In the experiment, we deliver disturbances via taps of a pendulum to one side of the chamber that contains the granular packing. The tapping strength is measured using an accelerometer. We tilt the chamber to varying slopes to observe changes in system response as we approach the critical slope for more rapid granular flow. Using image analysis, we examine grain rearrangements, changes in packing density, and stress redistribution as the grains creep due to these disturbances.