Slow-to-Fast Transition and Shear Localization in Accelerating Creep of Clayey Soil

Accelerating creep before catastrophic failure commonly follows a power-law velocity-acceleration relationship, with the exponent typically near 2 but often evolving from 1 to 2 at a certain point, indicating a dynamic transition. The underlying mechanisms, however, remain unclear. Here we investigate this transition by monitoring the slip displacement of clayey soil during fluid-injection creep experiments. This transition is discontinuous in the first run but becomes continuous in the initially pre-sheared sample. Using a regularized rate-and-state friction model, we explicitly examine the relationship between the exponent and the frictional properties of the soil. This model describes the dynamic transition, with the exponent evolving from 1 to 2 across a broad range of frictional parameters. Furthermore, by incorporating idealized shear localization processes, the model qualitatively reproduces the shear-history-dependent transition. Our study demonstrates that a combination of structural evolutions and frictional properties may explain slow and fast slips observed in various shear systems.