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

AM Booth and R Hurley and MP Lamb and JE Andrade, GEOPHYSICAL RESEARCH LETTERS, 41, 8862-8869 (2014).

DOI: 10.1002/2014GL061981

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 degrees 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.

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