Numerical Investigation of Triaxial Compressive Rock Tests Under CNL and CNS Boundary Conditions Using PFC2D

Compared to the boundary condition of constant normal load (CNL), constant normal stiffness (CNS) is conventionally considered to represent the mechanical behaviours of rock mass in underground excavations and bolted joints. Previous studies mainly focus on the differences between direct shear strength parameters under CNL and CNS boundary conditions. Comparison study between triaxial compressive test results under CNL and CNS condition has been rarely reported. Concerning the typical rocks, such as Jinping marble and Lac du Bonnet (LdB) granite, this work prepares numerical triaxial compressive tests using PFC2D to investigate the strength and mechanical behaviours under CNL and CNS boundary conditions. The numerical results reveal that the mechanical characteristics vary significantly between two boundary conditions in the triaxial compressive test, especially when the variation of CNS is up to 5 GPa/m. The enhancement effect of confining pressure (EECP) with the CNS modulating from 5 to 15 GPa/m restricts the dilation of rock specimens. Accordingly, compared to the CNL condition, the peak, and the residual strength present growth linearly while the brittle index decreases. Also, the impact of EECP relies on rock type and initial confining pressure. The results indicate that rock mass with low strength under high in-situ stress may suffer more impact of the EECP.