Effect of strain localization on frictional behavior of sheared granular materials

We performed laboratory experiments to investigate shear localization and the evolution of frictional behavior as a function of shear strain. Experiments were conducted on water‐saturated layers, 0.3–1 cm thick, of Caesar till, a granular material analogous to fault gouge. We used the double‐direct shear configuration at normal stresses ranging from 0.5 to 5 MPa and shearing velocities of 10–100 μ m/s. Shear localization was assessed via strain markers and two proxies: (1) macroscopic layer dilation in response to perturbations in shear stress and (2) rate/state friction response to shear velocity perturbations. In creep mode experiments, at constant shear stress, we monitored dilation for perturbations in shear stress. In standard friction tests, we measured the coefficient of friction during perturbations in macroscopic strain rate. We find evidence of strain localization beginning at shear strain γ ≈ 0.15 and continuing until γ ≈ 1. Analysis of strain markers supports interpretations based on the proxies for localization and shows that strain is localized in zones of finite thickness. We also investigate symmetry of the friction response to step changes in imposed slip velocity and find that the behavior is symmetric. Our results favor the Ruina law for friction state evolution in which slip is the fundamental variable rather than the Dieterich law. The critical slip distance for friction evolution, D c , is ∼140 μ m. The Dieterich state evolution law requires different values of D c for velocity increases/decreases, 100 μ m versus 175 μ m, respectively, and would imply strain localization/delocalization associated with shear in a finite zone.