Shear strength characteristics of rock-like joints in different control modes and unloading stress paths

In order to study the shear strength characteristics of rock-like material joints in different loading control modes and unloading stress paths, the direct shear tests of rock-like material joints in two loading control modes and three unloading stress paths were carried out by means of RDS-200 rock joint shear test system. The results show that: (1) when the stress control mode is used for shear test, it can be found that the joint shear stress-time curve mostly has the instability drop, and the shear stress-displacement curve is mostly in the form of broken line after instability in each shear stress loading and unloading path. (2) Compared with the results of the joints that experiencing shear loading in a stress control mode, the instability shear strength is reduced when the rock-like material joints are sheared in the following control mode and unloading stress paths including loading shear stress in a displacement control mode, unloading the normal stress in a stress control mode while keeping the shear stress constant, unloading the normal stress while increasing the shear stress in stress control modes, unloading both the normal stress and the shear stress in stress control modes. The average decrease percentage of cohesion is 36.07 %, while the change range of internal friction angle is only 4.12 %. Therefore, the decrease of cohesion is the main reason for the decrease of the joint instability shear strength. (3) Based on Mohr-Coulomb formula for joints experiencing the shear loading in a stress control mode, the average relative error of the instability shear strength of joints experiencing a shear loading in the displacement control mode or experiencing loading and unloading in other stress paths in the stress control mode can reach up to 55.78 %. The maximum value of the average relative error can be reduced to 5.23 % after the cohesion correction coefficient k is introduced to modify the Mohr-Coulomb formula, which shows that the reduction correction of cohesion is necessary and feasible for engineering joints that may experiencing shear stress loading in the displacement control mode or may experiencing loading and unloading in other stress paths in the stress control mode. The research results have a certain reference value for accurately estimating the shear bearing capacity of rock mass joints in different loading control modes and unloading stress paths.