In order to understand the hydrological properties of a single fracture, the visualization of the fluid flow through it is very useful. In this study, the visualization is successfully performed by the two approaches. One is a flooding experiment to the artificial single fractures made of transparent acrylic resin and white silicon gum, and another is a three-dimensional numerical flow simulation of the flooding experiment by using the lattice Boltzmann method. The surface of the artificial fracture is numerically generated by the Glover's method based on a fractal theory and carved on a modelling wax by a numerically controlled modelling machine. Therefore, we can control the geometric surface properties of the artificial fracture at will so that the generated fracture surface may have the similar properties to a natural fracture surface. From the flow experiment and the numerical simulation, followings are observed. First, the fluid flows through a fracture selecting the large aperture regions and forms several complex flow channels. Second, the shape of the flow channels becomes more complex with the increase In the contact area. Based on these observation results, the deviation from the cubic law of fracture permeability is estimated quantitatively, and it is concluded that tortuosity of the channel flow most affects the deviation from the cubic law of fracture permeability.

1. INTRODUCTION

To estimate the fluid flow through a rock mass is important for the environmental problems, especially for the geological isolation of nuclear wastes and toxic substances. This is because such the hazardous substances are transported by the fluid flow through the rock mass. Therefore, to Characterize the behaviour of fluid flow through the fractures is one of the most important research subjects. Generally, the fracture permeability has been estimated by the cubic law, that is, volumetric flow rate of a fracture is directly proportional to the cubic of its aperture. This law is valid for the laminar flow between two perfectly smooth parallel plates. However, a fracture surface is rough and some parts on it are contacting. This would make the fracture permeability deviate from the cubic law. Moreover, the fracture permeability is affected by the stress working on the fracture surface. Therefore, many pieces of research have investigated these factors in order to evaluate the fracture permeability more precisely (Witherspoon et al. 1980,Walsh 1981, Tsang & Witherspoon 1981, Barton et al. 1985, etc.). For such the research, visualization of the fluid flow through a fracture is very helpful, because we can observe how and where the fluid flows through the fracture in detail and estimate the contact condition of the fracture surfaces that is one of the most important factors for the fluid flow behaviour. The visualization was usually performed by the numerical flow simulation that solved the simple Reynolds equation (Brown 1987). However, the simulation cannot estimate the three-dimensional fluid flow behaviour, because the Reynolds equation assumes the local cubic law and the normal component of the flow rate vectors to the fracture surface cannot be taken into account.

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