Time-dependent behavior of rock joints considering asperity degradation

The time-dependent behavior of rock joints, specifically creep and stress relaxation, govern the long-term stability of underground structures, but have not been fully understood. The effect of asperity degradation on the time-dependent behavior of rock joints is investigated in this test. Triangular synthetic rock joints with four asperity angles (θ = 0° 10° 30° and 45°) are tested for creep and stress relaxation. It is revealed that the failure mode of a rock joint during creep and stress relaxation shifts from surface sliding (θ = 0°) to a combination of surface sliding and asperity crushing (θ > 10°). Asperity crushing becomes more severe as θ increases. Consequently, a rougher rock joint with higher θ exhibits higher creep and stress relaxation rates due to more asperity crushing. For each θ the creep and stress relaxation rates both increase exponentially with increasing shear stress before the stable long-term strength is reached, suggesting a higher degree of asperity degradation as shear resistance mobilizes. Semi-empirical correlations are proposed to quantify the effects of asperity angles on creep and stress relaxation of rock joints at various shear stresses.