Characteristic Stress Levels and Brittle Fracturing of Hard Rocks Subjected to True Triaxial Compression with Low Minimum Principal Stress

In this study, true triaxial compression tests were carried out on three types of hard rocks (i.e., granite, marble and sandstone) using rectangular prismatic specimens (50 × 50 × 100 mm³) with low minimum principal stress σ₃, and various intermediate principal stresses σ₂. The main purposes were to establish the relationship between the characteristic stress levels (i.e., crack initiation stress, crack damage stress and peak stress) and the corresponding principal stresses and to investigate the brittle fracturing process of hard rocks near excavation boundaries. The test results indicated that the stress–strain curves were primarily characterized by the linear-elastic–brittle behavior. The failure planes of the specimens in the tests were found to be adjacent to the σ₃ loading surface, and almost parallel to the σ₁–σ₂ plane, which were analogous to the spalling of the surrounding rock. With the aid of scanning electron microscopy, it was shown that cleavage fractures accounted for the majority of the fracture morphology in the sandstone specimens. Two revised methods were developed to determine the crack initiation stress of hard rocks under true triaxial compression, and these characteristic stress levels could be appropriately fitted by utilizing both the parabolic and power functions. Although the power function achieved better fitting results, the parameters in the parabolic function could be associated with the tensile cracks induced during the loading process. The influence of intermediate principal stress on the strength, deformation and failure was significant. In addition, the brittle fracturing process could be illustrated by the crack-induced strains in three principal stress directions.