ABSTRACT:

An experimental study was carried out in granite joints coming from an underground hydroelectric power scheme, with the purpose of obtaining values of the parameters that describe their hydromechanical behaviour under normal stress. The paper starts with a brief critical review of the hydromechanical idealisations that have been used for the joints, followed by the description of the testing equipment and testing procedure. The test results are then presented and analysed, and the parameters obtained are discussed.

RESUME:

Une etude experimentalle est ete realisee dans des surfaces de discontinuite vennant d'une usine hydro-electrique souterraine, avec I'objectif d'obtenir les valeurs des parametres qui decrient leur comportement hydraumecanique sous contrainte normale. Cette comunication commence avec une brève revision critique des idealisations hydromecaniques utilisees pour les surfaces de discontinuite, suivie de la description de l'equipement d'essai et du protocol des essais. Les resultats sont presentes et analyses, et les parametres obtenus sont discutes.

ZUSAMMENFASSUNG:

Die vorliegende Arbeit beschreibt die Durchfuehrung einer Versuchsstudie an Granittrennflachen aus einem unterirdischen Wasserkraftwerk, in der Absicht Werte der Parameter zu erhalten, die ihr hydromechanisches Verhalten bei normaler Belastung beschreiben. Die Studie beginnt mit einer kritischen Kurzuebersicht der fuer Trennflachen gebrauchlichen hydromechanischen Idealisierungen, gefolgt von einer Beschreibung des Testaufbaus und -verlaufs. Es schlieβt sich eine Darstellung und Analyse der Ergebnisse an, sowie eine Diskussion der erhaltenen Parameter.

1
INTRODUCTION

The hydromechanical behaviour of rock masses is influenced in a decisive way by the presence of the joints. They are largely responsible for the seepage occurring through the rock mass and are the most sensitive elements with respect to deformation under stress changes. Hydromechanical studies of fractured rock masses must therefore pay special attention to the role of joints and to correct assessment of their properties. Having this in mind, hydromechanical tests were performed in laboratory on samples of granite joints coming from the underground power station of the Alto Lindoso hydroelectric scheme, in the North of Portugal. They permitted to study the influence of the normal stress on the joint on its deformation and on the fluid flow. These tests were part of a more vast testing programme of the same rock mass which focused primarily on the behaviour of the joints and included the geometrical characterisation of the jointing system and the study of the topography of the joint surfaces, with determination of parameters for description of roughness and aperture. This set of tests allowed the determination of parameters that were subsequently used as input for a global hydromechanical numerical analysis of the Alto Lindoso high pressure circuit (Lamas 1993).

2
HYDROMECHANICAL IDEALISATIONS

One of the fundamental parameters for determination of the flow rate in a joint is its aperture. Using Darcy's law of laminar parallel flow, the flow rate is proportional to the cube of aperture. This, in turn, is very dependent on the normal stress on the joint. In order to study the influence of this parameter, many authors have tried to establish experimental relations between the closure of rock joints under normal stress and their hydraulic properties.

3
TESTING EQUIPMENT

All the hydromechanical tests in joints were conducted on the hydro geothermal rock test system existing at Imperial College. Complete descriptions of the facility were given by Zhao (1987) and Elliot and Brown (1988). The system comprises a triaxial compression testing machine with a stiff load frame, with arrangements for raising the temperature and including pore pressure within the sample.

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