Force chain dynamics in a quasistatic granular pile

In nature, granular materials fail in abrupt avalanches, earthquakes, and other hazardous events, and also creep over time. Proposed failure mechanisms for these systems are broadly framed as friction limited. However, mechanical descriptions of friction in granular system vary, including those that consider the nonlinear, heterogeneous dynamics of grain-contact forces. In order to study granular controls on failure and creep, we imaged contact forces between quasi-2D photoelastic disks at 15-min intervals in experiments over weeks- to 1-month-long observation periods. In the experiments, the particles are distributed in a slope below the angle of repose with a concomitant establishment of the force chain network approximately along the principal static stress directions. The discrete force chain shifts are initially described by an age-weakening Weibull distribution in frequency over time, with the most common clusters of events <1000 min apart, but there are long quiescent periods that are not well described by the distribution. Different surface slopes show the same contact aging rate, but the steeper the slope the more likely there is to be a failure event. The postsettling discrete events accompany a longer-term strengthening of the localized stresses. We observe that some events may be related to 2^{∘}C temperature change, but associated ground motions measured by a laboratory-installed seismometer appear to have no correlation with particle displacements or force chain changes. The sum of all ground motion is 4 orders of magnitude smaller than the temperature changes, further ruling out mechanical noise as an appreciable cause of events. The results illustrate that local, grain-scale changes in force chain networks can occur long after a granular pile reaches a mesoscale apparently stable state, sometimes without obvious external forcings or imposed state changes. Such force chain dynamics may underlie transitions in natural granular systems between large-scale failures and creep events.