Formation of Riedel shear fractures in granular materials: Findings from analogue shear experiments and theoretical analyses

We performed simple shear experiments to investigate the development of low- (R1) and high- (R2) angle Riedel shear localization in wet sand–talc mixtures with varying volume proportions. With increasing talc content, the granular medium underwent rheological changes, showing larger homogeneous ductile strains prior to brittle failure. Talc-rich models developed a perceptible penetrative planar fabric of flaky talc grains in response to the ductile strains. The relative growth of R1 and R2 also varies consistently with talc content. Both R1 and R2 formed equally at angles of ~ 15° and ~ 75° to the bulk shear direction, respectively, when the medium was of pure sand. In contrast, a talc-rich (90% by volume) medium produced only R1 shear fractures. The rheological changes and the presence of a shape fabric in the medium appear to be the potential factors resulting in the variation of R1 versus R2 growth in the experiments. We present a theoretical analysis to show possible effects of the penetrative fabric independently, considering mechanical anisotropy in the medium. This analysis takes into account two anisotropic factors, m: ratio of shear and Young's modulii, and n: ratio of Young's modulii along and across the fabric. The shear failure is assumed to follow a Coulomb–Navier criterion. Theoretical calculations show that the Coulomb stress factor (F) for isotropic materials (m = 0.33 and n = 1) reaches maximum values on planes oriented at angles of 15° and 75° to the bulk shear plane, leading to shear failure along both R1 and R2. In the case of anisotropic materials (m < 0.33 or n > 1), the stress factor is characterized by a single maximum of F within the range of 0° to 90°, corresponding to planes oriented at a low angle to the bulk shear plane (R1).