ABSTRACT: Continuum methods are used widely in rock mechanics for predicting the fluid flow or mechanical response of fractured rock masses. A recent study carried out using data from the Äspö, Sweden Hard Rock Laboratory study site illustrates that continuum models may not adequately represent the connectivity structure of the fracture networks, and that models based upon continuum representations of the fracture rock mass may be seriously in error.
RESUME: Les modèles continuum sont largement utilisees par l'etudes de la mecanique des roches pour predire I'ecoulement des fluides ou la reponse mecanique des massifs rocheux fractures. Une etude recente, realisee en se servant des donnees du site de recherches du Laboratoire des Roches Dures d'Äspö, Suède, montre qu'il est possible que les modèles continuum ne representent pas suffisamment bien la structure de connexite des reseaux des fracture et que les modèles bases sur les representation continuum des massifs rocheaux fracture pourraient être serieusernent erronnes.
KURZFASSUNG: Der Kontinuum-Ansatz ist weit verbreitet bei der Vorhersage der Strömung sowie des mechanischen Verhaltens gekluefteter Formationen. Eine vor kurzem durchgefuehrte Studie mit Daten aus dem Gesteinslabor in Aspö, Schweden, zeigt jedoch, daß Kontinuum-Modelle unter Umstanden nicht in der Lage sind, die Durchlassigkeitsstruktur des Kluftnetzwerkes zu beschreiben. Modelle, die auf dem Kontinuumsansatz beruhen, können daher zu signifikant falschen Aussagen fuehren.
1. INTRODUCTION Many engineering calculations rely upon numerical solutions to equations that describe the mechanical or fluid flow behavior of rock masses, using methods such as finite elements, finite differences or boundary elements. Inherent in this type of numerical solution is the assumption that smaller portions of the rock mass, discretized as elements, behave as individual continua whose mechanical or fluid flow properties are well-modeled by a symmetrical tensor. These elements are usually at the scale of 10 to 100 m. Most rock masses are fractured. These fractures often have two or three preferred orientations, and may range in size from microcracks of millimeter size to fault zones at the scale of kilometers. Commonly a rock mass contains fractures of many different orders of magnitude. If a rock mass is filled with discontinuous geological features over a broad range of scales, can methods that assume that a rock block on the scale of 10 to 100 m behaves as a continuum provide useful answers? A recently completed study in support of methodology development for high-level nuclear waste isolation provides some surprising answers. The Äspö Hard Rock Laboratory (HRL) was established to develop and test methods to safely isolate high-level nuclear waste. An important goal of these studies is to develop reliable methods to predict flow and mass transport at the scale of kilometers or even tens of kilometers with a high degree of accuracy. In Sweden as in many of the countries that are currently studying the safe disposal of nuclear waste, rocks in which fractures dominate permeability are among the leading candidate sites for eventual repository construction. The large scale of the hydrological models has made continuum models attractive for modeling flow, since it is numerically tractable to solve problems ofthis magnitude by discretizing the rock into blocks of tens of meters in size.