Formation of riedel shear fractures in granular materials: findings from analogue shear experiments and theoretical analyses

Abstract

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).