Study on the evolution law of rock joint shear stiffness during shearing process through loading-unloading tests

Tunnels are usually located in highly jointed rock masses and most of them are in the area with frequent seismic activities. The joints have a great impact on the strength and failure mode of the rock mass. The jointed rock mass is subjected to frequent loading and unloading under complex geological and engineering conditions. Shear stiffness is a key parameter to reflect the shear deformation properties of the discontinuities. It is usually considered a constant value in the traditional calculation, but this assumption may not conform to engineering practice in practical engineering. Loading-unloading shear tests were conducted to study the evolution law of rock joint shear stiffness during the shearing process. The rock-like specimens with different joint roughness coefficients were used to simulate the natural rocks. The results show that the shear stiffness will gradually decrease with the increase of shear displacement. The shear stiffness of rock joints increases with the increase of joint roughness coefficient. According to the test results, a shear stiffness prediction model related to shear strain was derived. The prediction results were in close agreement with the laboratory test results, and the error range was within 12%. Which indicated that the established prediction model can better reproduce the shear stiffness change behavior of rock joints in different shear stages. In tunnel engineering design, the evolution of the shear stiffness of rock joints needs to be considered. More accurate results can be obtained by inserting the prediction model into the numerical simulation programs.