Effect of fracture roughness on permeability coefficient under uniaxial compression

The surface roughness of natural rock-fractures is an important factor affecting the fractured rock mass flow characteristics and further complicating the flow process in the natural fractures. To further study the influence of the fracture surface roughness on the permeability coefficient under uniaxial compression and different hydraulic pressures, 3D printing technology and digital modeling were utilized to prepare the fracture specimens with different fracture surface roughnesses and laboratory permeability tests were conducted through a self-made testing device under different normal pressures and different hydraulic pressures. The experimental results show that in the absence of normal pressure, the rough fracture specimens permeability decreases in a negative exponential form with the increase in the fracture surface roughness. The Forchheimer equation is used to quantitatively describe the nonlinear relationship between seepage flow rate and hydraulic gradients. The regression analyses of the experimental data indicate that the Forchheimer equation provides a good description of the flow process through the rough fracture surface. With the increase in the fracture surface roughness, the linear term coefficient decreases, while the nonlinear term coefficient increases. Under the conditions of fixed normal pressure and normal pressure greater than hydraulic pressure, the fracture specimens permeability decreases linearly with the increase in the fracture surface roughness, and the influence of the fracture surface roughness on the permeability increases with the increase in the hydraulic pressure. The coefficient \delta was used to analyze the difference between the influences of fracture surface roughness on the permeability under normal pressure and without normal pressure. The coefficient \delta increases with the increase in the hydraulic gradients and decreases with the increase in the normal pressure. The results can further clarify the fluid flow through rough fracture surfaces and provide a solid foundation for further research in the fields of rock mass flow characteristics.