Time-dependent behavior of rock joints considering asperity degradation

Abstract

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.