Experimental Study on the Mechanical Characteristics of Single-Fissure Sandstone Under Triaxial Extension

Generally, after excavation, a part of the rock slope or underground tunnel surrounding rock remains in a triaxial extension state. Meanwhile, the natural fissures in the rock mass are further expanded, which damages and destroys the rock. In this study, to investigate the mechanical properties and failure modes of single-fissured rocks under triaxial extension, single-fissure sandstone specimens with the same fissure length (l = 18 mm) but with different dip angles (alpha = 0 degrees, 30 degrees, 45 degrees, 60 degrees, 90 degrees) are examined under variable confining pressures (P-c = 0, 5, 10, 20, and 40 MPa) using an MTS815 testing machine and a compressive-to-tensile load conversion device. The results reveal that P-c has a significant effect on the failure strength of the single-fissure rock specimens. For a given dip angle, when P-c increases, the failure strength and axial deformation of the specimens increase moderately, and the failure stress gradually changes from tensile stress to compressive stress. Furthermore, the microscopic fracture pattern of the specimens also changes from intergranular fracture to transgranular fracture. The crack initiation angle gradually increases with the dip angle and P-c. The crack inclination of the specimen has a minor effect on the failure strength, except for P-c = 0.