Force chains as the link between particle and bulk friction angles in granular material

From sediment transport in rivers to landslides, predictions of granular motion rely on a Mohr‐Coulomb failure criterion parameterized by a friction angle. Measured friction angles are generally large for single grains, smaller for large numbers of grains, and no theory exists for intermediate numbers of grains. We propose that a continuum of friction angles exists between single‐grain and bulk friction angles due to grain‐to‐grain force chains. Physical experiments, probabilistic modeling, and discrete element modeling demonstrate that friction angles decrease by up to 15° as the number of potentially mobile grains increases from 1 to ~20. Decreased stability occurs as longer force chains more effectively dislodge downslope “keystone” grains, implying that bulk friction angles are set by the statistics of single‐grain friction angles. Both angles are distinct from and generally larger than grain contact‐point friction, with implications for a variety of sediment transport processes involving small clusters of grains.