Prediction of the transitional normal stress of rock joints under shear

The transitional normal stress of a rock joint subject to shear refers to the critical normal stress under which the normal dilation is completely suppressed. The transitional normal stress is involved in many shear strength/ constitutive models of joints as a key material constant; however, this parameter is poorly constrained and its determination is mostly empirical. Here we propose a simple formulation to predict the transitional normal stress based on the systematic, well-calibrated PFC2D (two-dimensional particle flow code) simulation of the shear characteristics of both sawtooth and JRC-profiled rock joints. In the PFC2D modelling, rock joints are confined by low to high normal stresses approximating the uniaxial compressive strength (joint wall compressive strength of fresh, dry and closely matched joints) of the simulated rock. The formulation can satisfactorily quantify the transitional normal stress of regular and irregular joint surfaces as a function of the rock strength and the joint surface roughness, as validated by the laboratory data. Therefore, it can be readily introduced to the shear strength criteria and constitutive models of rock joints, which could significantly promote the accurate quanti-fication of rock joint shear behaviour.