True triaxial shear mechanical response and fracture surface evolution of granite under constant normal stiffness boundary
HU Liang1, 2, ZHAO Jun1, 2*, GUO Yue1, 2
(1. National Key Laboratory of Intelligent Mining and Equipment for Deep Metal Mines, Northeastern University, Shenyang, Liaoning 110819, China; 2. Liaoning Provincial Key Laboratory of Deep Engineering and Intelligent Technology, Northeastern University, Shenyang, Liaoning 110819, China)
Abstract：In deep true triaxial stress environments, the interaction between rock masses and support structures is intricate. Traditional constant normal load (CNL) tests, which neglect the effect of support stiffness, face challenges in characterizing the stress evolution paths and the genuine shear response governed by rock-support coupling. This study systematically compares the influence of constant normal stress (CNS) and CNL boundaries on the mechanical response of granite through true triaxial shear tests. The research focuses on analyzing the effects of varying lateral stresses (0–50 MPa) and normal stiffnesses (0–70 GPa/m) on true triaxial shear deformation, shear strength, shear failure modes, fracture surface morphology, and energy dissipation characteristics during shear fracture and friction sliding. The experimental results indicate that lateral stress primarily constrains lateral deformation, while its enhancement of normal deformation is an indirect effect. The CNS boundary effectively suppresses the normal dilatancy of granite. Under CNL conditions, increasing lateral stress exhibits a negative correlation with the rate of peak shear strength enhancement; specifically, the strength enhancement rate was 41.9% when lateral stress increased from 0 MPa to 10 MPa, but decreased to 11.36% when lateral stress further increased from 10 MPa to 50 MPa. The CNS boundary condition continuously suppresses normal dilation and enhances both peak and residual shear strength, with a maximum increase of 52.17%. Furthermore, under CNL conditions, lateral stress tends to induce feather-like secondary wing cracks and step-like fracture surfaces. Conversely, the coupling of dynamic normal stress and lateral confinement under the CNS boundary alters the energy dissipation mechanisms during fracture and friction sliding, resulting in a relatively smoother fracture surface morphology. This demonstrates the significant influence of boundary conditions on the failure mode. The study concludes that considering the CNS boundary provides a more realistic representation of the mechanical response of deep rock masses, offering crucial theoretical insights for support design and stability control.
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2026, Vol. 45 
