Force chain dynamics in a quasi-static granular pile (2504.20248v1)

Abstract: 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.