Experimental study on lateral unloading mechanical properties of siltstone considering intermediate principal stress

By using the rock true triaxial test system to carry out the lateral unloading test of siltstone true triaxial, unloading failure and deformation feature of siltstone were studied under different intermediate principal stress coefficients b, and the strength feature of rock mass was discussed based on the strength criteria of Mogi-Coulomb. The results show that under the condition of true triaxial lateral unloading stress, with the increase of the intermediate principal stress, the unloading failure mode of the siltstone sample changes from shear-tension composite failure to fracture failure, and the shear fracture angle increases linearly with the increase of b. The stress-strain curve exists an obvious stage. As the intermediate principal stress increases, the maximum principal strain ε1 and the minimum principal strain ε3 increase, and the volume strain εV decreases during failure. The trend of expansion is obvious. During the unloading process, the deformation modulus decreases with the increase of unloading amount in a negative exponential relationship. The Mogi-Coulomb criterion is used to analyze the true triaxial lateral unloading test intensity, and the regression equations passes the F-test at a higher significance level of 0.01. The Mogi-Coulomb strength criterion can be applied to describe the lateral unloading strength relationship of siltstone under different middle main stress conditions. © 2019, Central South University Press. All right reserved.