Triaxial Testing to Determine the Shear Strength and the Shear and Normal Stiffness of Rock Joints

The mechanical properties of rock joints, including shear strength, shear stiffness, and normal stiffness, are usually determined through direct shear tests. Essentially, these properties can also be evaluated using triaxial tests performed on core specimens. One advantage of triaxial testing is that specimen deformations can be measured more readily and accurately, thereby simplifying the data acquisition process. In this study, a series of triaxial tests were performed on core rock specimens containing natural joints to evaluate the potential of triaxial testing as an alternative method for determining rock joint mechanical properties. The work included theoretical development for measuring normal and shear displacements, execution of triaxial tests, calculation of normal and shear stiffness, and analysis of the resulting data. Normal stiffness was obtained from hydrostatic triaxial tests, while the feasibility of estimating it from differential triaxial tests was also investigated. Shear stiffness and shear strength were determined from differential triaxial tests conducted under multiple confining stress levels. The results show that both normal and shear stiffness can be measured reliably using triaxial testing. The measured shear strengths align closely with Patton’s bilinear failure envelope. Furthermore, the rock joint friction criterion and Patton’s envelope were examined in light of the test results. These two criteria were united by deriving a relationship between the roughness angle in the friction criterion and the cohesion and residual friction angle in Patton’s bilinear failure envelope.