Radial fluid flow regime in a single fracture under high hydraulic pressure during shear process.

• Coupled shear-flow tests with hydraulic pressure up to 0.8 MPa were conducted. • The influence of joint surface destruction and gouge material was discussed. • The anisotropy of flow regime caused by anisotropic joint surface was discussed. • Modify cubic law with hydraulic gradient, mechanical and hydraulic aperture. The single-fracture seepage model is important for fracture network seepage models and underground engineering, but the existing knowledge is insufficient for the roles of the high hydraulic pressure, joint surface destruction, gouge material, and the anisotropy of flow regime in the single-fracture seepage model during shear process. In this study, coupled shear-flow tests were conducted at 0.2, 0.4, 0.6 and 0.8 MPa hydraulic pressure with radial fluid flow, and then 3D simulation models were established by using COMSOL Multiphysics 5.2a with the Reynolds-averaged Navier–Stokes k- ε turbulence model for initial and peak shear conditions. Results showed that the radial flow cubic law diverged from experimental results seriously, and non-Darcy flow behaviour was remarkable during tests. High hydraulic pressure decreased the accuracy of the radial flow cubic law dramatically by increasing the influence of inertia force, and the accuracy was sensitive to the increase in lower hydraulic pressure. The flow regime and velocity were varied and influenced by the anisotropic tortuosity ratio, maximum inclination angle, and the uneven distribution of aperture. The absolute values of velocity vertical to the radial distance reduced with decreasing roughness. The roughness, tortuosity ratio, and maximum inclination angle of the fracture joint reduced dynamically with destruction of the joint surface and formation of gouge material, leading to a significant change in the relationship between mechanical and hydraulic apertures. Moreover, the radial flow cubic law was modified on the basis of the relationship of mechanical and hydraulic aperture with hydraulic gradient. This study may serve as a basis to establish fracture network seepage model and other similar tests. [ABSTRACT FROM AUTHOR]