ABSTRACT:

An experimental apparatus has been constructed to allow hydrological measurements to be made on a rock fracture under normal and shear loading. An epoxy replica of a rock fracture is used in the experiments. Analytical solutions for a point source in a rectangular region with constant pressure on the outer boundary is used to invert the measured flow rates and head drops to find the conductivities in the directions parallel and transverse to the shear. The methods developed in this study can be adapted to use in the field to measure the directional conductivities of a rock fracture in situ.

RÉSUMÉ:

Un dispositif experimental a ete construit pour permettre des mesures hydrologiques sur une fracture de roche sous chargement normal et de cisaillement. Une maquette d'une fracture de roche en epoxyresine a ete utilisee dans ces experiences. Des solutions analytiques pour une source punctuelle dans une region rectangulaire à pression constante sur les limites externcs sont utilisees pour trouver les conductivites dans les directions parallele et perpendiculaire au cisaillement à l'aide des mesures d'ecoulement d'eau par unite de temps, et des variations de pression. Les methodes developpees dans cette etude peuvent être adaptees il l'Utilisation sur Ie terrain, pour mesurer in-situ les conducrivites directionelles d'une fracture de roche.

ZUSAMMENFASSUNG:

Es ist ein Experimcntalaufbau entworfen warden, der hydrologische Messungen auf einer Felskluft unter Normal- und Scherbeanspruchung erlaubt. Ein Epoxi-Harz Abdruck einer Felskluft wird fuer diese Experimente benutzt. Fuer ein Modell ist eine Punkt-Quelle in einer rechteckigen Region unter konstantem Druck auf der außeren Begrenzung vorgeschen, um die gemessenen Fließgeschwindigkeiten und Druckverluste in Durchlassigkeitswerte in Richtungen parallel und senkrecht zur Scherung ZLI ueberfuehren. Die Methoden, die in dieser Studie entwickelt werden, können an Gegebenheiten im Gelande angepaßt werden, um In situ richtungsabhangige Durchlassigkeiten einer Felskluft zu bestimmen.

1. INTRODUCTION

FIow through rock fractures has traditionally been described by the cubic law, which follows from the assumption that the fracture consists of the region bounded by two smooth, parallel plates (Witherspoon et al., 1980). Real rock fractures, however, have rough walls and variable apertures, as well as asperity regions where the two opposing faces of the fracture walls are in contact with each other. The hydraulic conductivity of a rock fracture depends on the aperture distribution, surface roughness, and contact area, each of which are sensitive functions of the stress that acts on the fracture plane. In recent years, numerous, theoretical, numerical and experimentaI studies have investigated the effect of irreguIar aperture and asperity geometry on fluid flow through the fracture (Tsang, 1984; Brown. 1987: Piggott and Elsworth, 1990: Zimmerman et al., /991: Amadei and lllangasekare, 1992). The effect of stress variations on the conductivity of rock fractures has been studied by, among others, Walsh (1981). Barton et al. (1985), Pyrak-Nolte et al. (1990), and Olsson and Brown (1993). Despite much recent work, a complete understanding of the relationship between void space geometry, applied stresses, and hydraulic conductivity has yet to be achieved. The difficulties lie in the intractability of the Navier-Stokes equations that govern fluid flow through the fracture, incomplete knowledge of the details of the fracture geometry, and a lack of realistic constitutive relations governing the mechanical deformation of rough rock surfaces in partial contact.

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