Cohesive granular columns collapsing: Numerics questioning failure, cohesion, and friction

Simulations of the failure of cohesive granular steps with varying intensities of the contact adhesive force are presented. The simulations are compared with experimental and numerical studies of wet shear flows [Badetti et al., J. Rheol. 62, 1175-1196 (2018) and Khamseh et al., Phys. Rev. E 92, 022201 (2015)], computing the apparent friction coefficient. We observe consistent behaviors. We reproduce the dependence between the macroscopic cohesion and the contact adhesion [Rumpf, Chem. Ing. Tech. 42, 538-540 (1970) and Richefeu et al., Phys. Rev. E 73(5), 051304 (2006)] observed experimentally for sticky polymer-coated grains, as well as the range of friction explored [Gans et al., Phys. Rev. E 101, 032904 (2020)]. Focusing on the interface between moving and static materials, and assuming a linear failure, we infer the orientation of the failure plane with the horizontal. We disclose a nonmonotonous evolution with the intensity of the contact adhesion. Assuming an ideal Coulomb material allows for proposing an interpretation to this nonmonotonous behavior. Although the systems are past incipient failure, we consider an edge of material at equilibrium, for which the failure angle is related to the internal frictional properties of the material. In this framework, the nonmonotonous evolution of the failure orientation may hint at a cohesion-induced weakening mechanism, by which stronger contact adhesion involve weaker friction.