The paper examines and compares different schemes proposed for the evaluation of the equivalent continuum hydraulic properties of rock masses. With reference to simple 2D arrangements of two orthogonal joint sets, the equivalent permeability and compressibility parameters are assessed for anisotropic jointed rock mass conditions.
Dans ce texte les differents schemas proposes pour l'evaluation des caracteristiques hydrauliques du massif rocheux avec Ie modèle continue equivalent sont analyses et compares. En particulier, les caracteristiques de permeabilite et compressibilite du massif rocheux qui contient deux systèmes de discontinuite orthogonaux, dans l'hypothèse du comportement anisotropic, sont evaluees.
Es wurden einige der allgemein verwendeten Schemata zur Bewertung von hydraulischen Merkmalen in Felsmassen untersucht und mit dern kontinuierlichen Äquivalenzmodell verglichen Insbesondere werden die Merkmale fuer Durchlassigkeit und Komprimierbarkeit von Felsmassen, die zwei Systeme von orthogonaler Diskontinuitat enthalten, unter der Annahme von anisotropem Verhalten bewertet.
Groundwater flow prediction in jointed rock masses requires a suitable appraisal of the rock mass hydrologic properties. The rock mass characteristics, above all the natural joints, play a fundamental role in establishing the global permeability/conductivity and the storativity that controls groundwater flow. There is a tendency, at least for a preliminary assessment of the flow conditions, to use the equivalent continuum model even when a rock mass cannot be immediately classified as being highly or extremely jointed. At least two reasons are responsible for this choice: the first one of interest in design, is to have an immediate evaluation of the average characteristics (inflow/outflow, piezometric head, time to steady response) of the groundwater flow; the second, mostly of interest for the analyst, is to reduce the burden of the rock mass structural hydrologic characterization and the great deal of time necessary for discrete feature modeling, which usually requires a simulation strategy. Furthermore the equivalent continuum characterization of hydraulic properties, as expressed by a permeability tensor, can be used to interpret the results of in situ flow experiments. A great deal of research work has been carried out to building a set of conceptual schemes for the equivalent continuum approach to groundwater flow (e.g. Barenblatt et aI., 1960, Huyakorn et al., 1983). The aim has been to make available some useful procedure to find a direct (Long et al., 1982, Samaniego and Priest, 1984) or indirect (Bear, 1972, Oda, 1985) analog between the continuum and discontinuum approach and also to summarize practical rules on the conditions that make the discontinuum-continuum equivalence meaningful (Oda, 1986, Wei et al., 1995). At the basis of the equivalent continuum approach there is the concept of the Representative Elementary Volume (REV), linked to an averaging process of the hydrological discrete properties of the joint network. This concept assumes that the rock volume is sufficiently large to allow a coherent hydraulic characterization of the rock mass which is conditioned by the prevailing conductance of joints. The implicit assumption that also makes the analysis of infinitesimals suitable for a fractured rock mass would seem to contrast with the discrete nature of jointing. That is, like soils where a very high number of pores are present in small volumes, any small volume of the equivalent continuum contains a high number of rock blocks (Barenblatt et aI., 1960). Following a list of alternative schemes that are useful to describe the groundwater flow in rock masses and a summary of methods for equivalence assessment, this paper applies these methods to find a REV for both the permeability and compressibility parameters. The analysis is applied to simple 2D schemes for anisotropic jointed rock mass conditions.
