Boundary stresses due to sheared granular mixtures

Models for stress produced by a sheared granular layer indicate stress should scale with particle size (such as the classic model suggested by Bagnold in 1954 where stress scales as particle size squared [1]). However, it is not clear how this particle-size scaling should be modified for a mixture of different-sized particles, important for applications such as debris flows. We investigate external stresses generated by a dense sheared granular mixture flowing in a thin layer over a solid boundary. To do so, we use Distinct Element Method (DEM) simulations based on a soft sphere model and compare the results with large-scale experimental measurements. Based on results from a variety of mixtures of different-sized particles, we have found that the scaling of the stress at the boundary does not depend on a simple metric such as average particle size. Instead, the scaling of the stress appears to have a more complicated dependence on both the relative sizes of the particles in the mixture and the relative concentration of the different species. [1]R.A. Bagnold (1954) Proc. R. Soc. Lond., A 225 pp. 49-63.