Force chain dynamics in a quasi-static 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 non-linear, heterogeneous dynamics of grain-contact forces. In order to study granular controls on failure and creep, we imaged contact forces between quasi-2D photoelastic discs at 15-minute intervals in experiments over weeks- to one-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 $<$1000 minutes apart, but there are long quiescent periods that are not well described by the distribution. The post-settling discrete events accompany a longer term strengthening of the localized stresses. We observe that some events may be related to 2$^o$C temperature change, but associated ground motions measured by a seismometer appear to have no correlation with particle displacements or force-chain changes. 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.