ABSTRACT:

We report on a numerical study of the effect of fracture surface geometry and aperture distributions on the hydro-mechanical behaviour of a single rock fracture in terms of normal and shear behaviours. Single rock fractures were simulated by a statistical method, and the finite difference approximation was used to solve the Reynolds equation governing fluid flow through single rock fractures. Fracture normal deformation was simulated based on Hooke's law and the Boussinesq solution, and fracture shear behaviour was simulated by considering elastic ?formation of asperities with respect to geometricharacteristics of fracture surfaces for constant normaload and constant normal stiffness conditions. For the coupled hydro-mechanical system of a fluid-filled rock fracture, hydraulic pressure inside a rock fracture was used to calculate fracture deformation. It is shown that the geometricharacteristics of a single rock fracture are the main factors influencing the hydro-mechanical behaviour of a rock fracture.

INTRODUCTION

The primary methods of modelling fluid flow in fractured rock masses can be categorized into three different strategies depending on scale: individual fracture modelling; fracture network modelling; and equivalent continuum modelling. Although each of these models is widely used in many problems of fluid flow of fractured rock masses, fluid flow through single rock fractures has not yet been fully explained. Moreover, a better understanding of hydraulic behaviour of single rock fractures is of primary importance because this is the basic element of realistic models for fluid flow of fractured rock masses.

Rock fractures are well known for their effect on mechanical properties of rock masses. Much experimental work (Bandis et al. 1983, Barton et al. 1985, Goodman 1976, Raven & Gales 1985) has shown the mechanical behaviour of a rock fracture subjected to normal load to be both highly nonlinear and dependent on the geometry of voids between the fracture surfaces. Since the water permeability of most rock material is much lower than that of rock fractures, rock fractures also strongly influence the permeability of crystalline and tight sedimentary rocks. The mechanical conditions of rock fractures are, therefore, coupled to the hydraulic conditions.

The work presented here deals with numerical experiments, based on statistically-generated models of a single rock fracture, for fluid flow under various mechanical and hydraulic conditions. Flow anisotropy of single rock fractures with different geometfie characteristics has been examined in terms of the correlation lengths and the anisotropy ratio of aperture distributions. Both normal behaviour and shear behaviour of single rock fractures have been simulated for various fractures of different statistical features. Finally, the effect of the hydraulic pressure on normal and shear behaviours of a single rock fracture has been examined.

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