A great number of experimental investigations and numerical simulations have been performed in the past, regarding coupled shear-flow behavior of rock fractures. However, some of the key issues such as shear-induced complexity of void geometry during large shear displacements remain unresolved. In this study, coupled shear-flow test was conducted on an artificial rock specimen with natural rock fracture characteristics. 3-D numerical simulation was then used to simulate the fluid flow through the rock fracture with void geometry obtained from test. The results show that transmissivity of fracture increases during shear process due mainly to the dilation of fracture, and the total process exhibits a two-stage behavior in terms of the change character of transmissivity and contact area distribution. The transmissivity also varies with water head in terms that transmissivity decreases with the increase of water head. The contact ratio and distribution as well as the surface roughness have large effect on the transmissivity. The numerical simulation by solving 3-D Navier-Stokes equations provides more precise predictions to the flow behaviors than some simplified models neglecting nonlinearity of fluid flow.
Fluid flow through fractured rock is of considerable interest in several domains of rock engineering, such as dam foundation, underground storage and radioactive waste repository. The performance and safety of such rock structures depend on the knowledge of transmissivity of rock masses, which varies with the in-situ stresses around the structure and the hydro-mechanical behaviors of rock fractures. In recent years, the studies considering both normal and shear stresses on fractures with fluid flow, the so-called coupled shear-flow tests, have been extensively conducted [1-5]. However, some of the key issues such as shear-induced fluid flow anisotropy during large shear displacements, evolution of aperture, validity of cubic law at various boundary conditions remain unresolved. For steady state, isothermal, laminar flow between parallel plates, the transmissivity is proportional to the cubic power of fracture aperture, which is well known as the cubic law. A typical natural rock fracture consists of void space and contact areas. Void space provides the flow channels for fluid, which bypasses the contact areas where the two faces of fracture are in contact, with tortuosity. Since natural rock fracture surfaces are much more irregular than parallel plates, cubic law is not accurate for estimating the transmissivity of natural fractures especially when the surface of fracture is very rough [6]. In most conditions, cubic law overestimates the transmissivity of natural fracture. Neglecting the nonlinearity of fluid flow, the transmissivity is considered constant for a certain void geometry, while the flow rate changes proportionally with the water head. In fact, the fluid flow in a complex void geometry is nonlinear, which is governed by 3-D nonlinear Navier-Stokes equations. Reynolds number (defined as Re=?VD/µ; where ? is the density of fluid, V is the mean velocity of flow, D is the characteristic dimension and µ is the dynamic viscosity) is a dimensionless measure to the degree of nonlinearity, which represents the relative strength of inertial force to viscous force.
