Granular avalanches of entangled rigid particles

In granular mechanics, the shape of grains plays a critical role in the overall dynamics and significantly affects the macroscopic properties of the system. Using a dam break setup, granular collapses of nonconvex (cross-shaped) plastic particles assumed quasirigid are conducted experimentally and simulated numerically for a wide range of aspect ratios a=H0/L0, with H0 the initial height of the column and L0 its initial length. We report avalanche dynamics such as the top-driven collapse and the buckling collapse, as well as an intermittent flow behavior where reproducibility is lost and where the stability of the column is determined by the random initial configuration of the assembly of entangled particles. While counterintuitive and despite fundamentally different dynamics, we find that the runout distance L∞ and the final height H∞ of our granular collapses of crosses agree with those of spherical particles both experimentally and numerically. Our discrete element method simulations are able to reproduce all flow behaviors observed experimentally and they show excellent quantitative agreement with the experimental data. In the simulations, extra care is given to adopting a tangential friction force model based on the cumulative tangential displacement at the contact point, critical to represent stable cases, and to determining the contact model parameters. The analysis of (i) the force network via the average probability density function of contact force magnitude and (ii) the fabric anisotropy suggests that the stability of the column is a complex problem determined by mesoscale properties that we could not reliably identify at that point.