Abstract: The morphology of deep rock fractures continuously changes during shear, leading to complex flow characteristics that affect the stability of deep rock engineering. To clarify the shear and flow characteristics of rock fractures under constant normal stiffness (CNS) boundary conditions, three-dimensional self-affine fracture surfaces with different roughness coefficients were constructed using fractal theory. A numerical method accounting for roughness degradation due to shear was employed to analyze the evolution of fracture morphology parameters under different CNS conditions. Subsequently, COMSOL software was utilized to calculate the flow of fractures after shear. The effects of normal stiffness, shear displacement and fractal dimension on nonlinear flow characteristics of fractures were studied. The results show: (1) The mechanical aperture of the fracture increases with the fractal dimension, but the increase in normal stiffness slows down its growth rate. The contact ratio is primarily controlled by normal stiffness and increases as it increases. (2) The relationship between the pressure gradient and flow rate of fracture seepage can be well described by Forchheimer law. The fitting coefficients A and B show a power function decrease trend with the increase of shear displacement, increase with the increase of normal stiffness, and decrease with the increase of fracture fractal dimension. (3) The hydraulic aperture of the fracture increases with the increase of fractal dimension, and decreases with the increase of normal stiffness. A hydraulic aperture model with mechanical aperture and aperture standard deviation as independent variables has been established. (4) The critical Reynolds number for the fracture flow initially decreases and then increases with the increase of shear displacement, and decreases with the increase of fractal dimension and normal stiffness. A formula for calculating the critical Reynolds number of sheared fractures under CNS conditions has been established based on fracture fractal dimension, contact ratio, and hydraulic aperture model.

Key words: constant normal stiffness, shear flow, hydraulic aperture, critical Reynolds number