Scale Effects on Shear Strength of Rough Rock Joints Caused by Normal Stress Conditions

Scale effects on the mechanical behavior of rock joints have been extensively studied in rocks and rock-like materials. However, limited attention has been paid to understanding scale effects on the shear strength of rock joints in relation to normal stress s(n) applied to rock samples under direct shear tests. In this research, a two-dimensional particle flow code (PFC2D) is adopted to build a synthetic sandstone rock model with a standard joint roughness coefficient (JRC) profile. The manufactured rock model, which is adjusted by the experiment data and tested by the empirical Barton's shear strength criterion, is then used to research scale effects on the shear strength of rock joints caused by normal stresses. It is found that the failure type can be affected by JRC and s(n). Therefore, a scale effect index (SEI) that is equal to JRC plus two times s(n) (MPa) is proposed to identify the types of shear failure. Overall, shearing off asperities is the main failure mechanism for rock samples with SEI > 14, which leads to negative scale effects. It is also found that the degree of scale effects on the shear strength of rock joints is more obvious at low normal stress conditions, where s(n) < 2 MPa.