New Insights for Stress Conditions of Laboratory Shear Tests

ABSTRACT: Numerous laboratory tests have been performed to investigate the shear behavior of rock discontinuities using various experimental setups. Stress heterogeneity on rock discontinuities can significantly influence the shear behavior, however, it has seldom been specified clearly. Instead, the macroscopic or average stress on the joints other than the local stress distribution was usually analyzed in most laboratory shear tests. In this study, numerical simulations considering a simple linear velocity weakening law are conducted on smooth rock joints in various laboratory setups to analyze the stress distribution before and during slip. The results show nonuniform normal and shear stress distribution in almost all the laboratory tests and significant local stress concentration near the edges of the shear surface. The estimated sample-average friction coefficient is slightly higher than the dynamic friction coefficient but lower than the static friction coefficient. Furthermore, a sudden release of the shear stress, especially near the stress-concentrated edges, can be observed in the slip phase during a stick-slip cycle. This study provides a critical review of laboratory shear tests on rock joints and contributes to our understanding of stress distribution and variation during stick-slip shear. 1. INTRODUCTION Rock discontinuities including joints, fractures, faults, and bedding planes commonly exist in rock masses (Elsworth et al. 2016; Marone 1998). Shear failure on rock discontinuities can occur in various engineering cases, such as underground excavations including tunneling, carven and mining (Chai et al. 2023; Wu et al. 2023), slopes and dams (Chai 2023), fluid-injection projects including oil and gas extraction (Elsworth et al. 2016), enhanced geothermal systems (Zhao et al. 2020), and CO2 or waste storage (Elsworth et al. 2016), etc. The shearing of rock discontinuities has also been reported to affect earthquake nucleation (Kaproth & Marone 2013). Various laboratory shear setups have been developed to investigate the shear behavior of rock discontinuities. They include direct shear (DS) tests (Petit 1988), true triaxial direct shear tests (Zhao et al. 2023), single direct shear tests (Rubinstein et al. 2004), double direct shear tests (Marone 1998), triaxial direct shear tests (Frash et al. 2016), triaxial shear (TS) tests (Thompson et al. 2005), biaxial shear tests (Buijze et al. 2020), axial shear tests (Rubino et al. 2022), oblique shear tests (Ghazvinian et al. 2013), and rotary shear (RS) tests (Zhao et al. 2018). Among these test setups, the DS, TS, and RS tests are commonly used by the rock mechanics community.