An Experimental Study on the Effects of True Triaxial Loading and Unloading Stress Paths on the Mechanical Properties of Red Sandstone

Loading and unloading stress paths play critical roles in investigating the deformation and failure of roadway excavation. In this study, tests under four different loading and unloading stress paths were conducted on red sandstone samples, with the aid of a self-developed true triaxial test system. Meanwhile, the deformation and failure characteristics of the samples were monitored during the tests. The following research conclusions were obtained: The octahedral shear stress is linearly correlated with the average effective stress, and the correlation coefficient R2 is 0.9825. The Mogi–Coulomb strength criterion is superior to the Drucker–Prager strength criterion in reflecting strength failure characteristics of red sandstone during loading and unloading. Shear failure tends to occur under uniaxial compression, whereas shear–tensile composite failure occurs under loading and unloading conditions. Compared with the true triaxial loading test, loading and unloading tests produce a larger strain in the unloading direction. Under loading and unloading stress paths, with the increase in intermediate principal stress (IPS), the strain in the direction of IPS gradually changes from expansion to compression, and the peak strength gradually increases. The state of IPS affects the failure strength of the sample and reflects the strengthening effect of IPS. This paper boasts a certain value and significance for research on the deformation and failure characteristics of sandstone in the actual in situ stress environment with triaxial dynamic changes