Velocity-Weakening Tendency in Rate-Dependent Strength Behavior of Soft Rock-Like Joints

This paper investigates the effect of different experimental and sample conditions on the velocity-weakening tendency of model rock joints. A total of 100 triaxial tests were conducted on fabricated jointed samples under displacement rates of 0.05-25.00 mm/min by varying the sample porosity (55%-65%), confining stress (100-800 kPa), and overconsolidation ratio [OCR (1.34-4.00)]. The strength of the joints reduced with the displacement rate, and this reduction became larger with increasing sample porosity and confining stress. However, the increase in the OCR reduced this velocity-weakening tendency along the joints. To explain these findings, a set of indentation creep tests was designed, and the creep behavior of a model rock against the indenter was observed under different conditions, which simulated the triaxial test conditions. The insights from the indentation creep experiments are supported by the asperity creep theory to understand the reasons for this behavior. Finally, a strength model is developed based on the experimental results to take this parametric rate-dependency of joints into consideration when estimating the jointed rock strength under different rates.