Mechanics of Transient Friction in Sheared Granular Layers: Where Rate-and-State Descriptions Succeed and Fail

Transient friction in granular shear layers at shallow effective normal stress is governed by evolution of the load-bearing contact network and granular fabric, yet rate-and-state friction (RSF) parameters in this regime remain poorly constrained. We quantify RSF behavior in a ring-shear rheometer at sn = 10–20 kPa using three idealized materials selected to isolate grain-shape and grain-size effects: a fine angular silica powder and small and large spherical glass-bead packs. Velocity-stepping and slide–hold–slide tests constrain steady-state rate dependence, direct and evolution effects, characteristic evolution distance, and frictional healing, while varying layer thickness tests geometric control of state evolution. Under high humidity, both glass-bead systems show resolvable RSF transients and velocity-weakening behavior, with micron-scale Dc that increases modestly with grain size. In contrast, the angular silica powder is nearly velocity-neutral and yields a larger effective Dc despite its finer grains, indicating that Dc depends on localization style and fabric/contact-network evolution rather than grain size alone; Dc is also insensitive to imposed shear-layer thickness over a three-fold range. For the silica powder, velocity-step transients and hold-phase stress relaxation are captured by Ruina slip-law evolution, whereas the magnitude of log-time healing is better described by the Dieterich aging law. These results provide a reproducible benchmark for low-stress transient friction and show how particle morphology, grain size, humidity, and layer geometry shape RSF behavior relevant to near-surface shear zones.