A versatile, modular borehole pressure test system was developed and used in a series of constant head injection and pressure drop tests in a granite containing three orthogonal sets of widely spaced discontinuities. Small scale tests on individual or very small numbers of discontinuities gave similar results to those obtained in laboratory tests. The results of large scale tests in test sections up to 28 m long showed that, at this scale, continuum concepts may not be used to study flows through the rock mass.
Un système polyvalent de pressurisation des sections d'un trou de sande a ete mis au point et utilise lors d'une serie de tests dans un massif rocheux granitique contenant trois familles de discontinuites orthogonales largement espacees. Lors des tests, la pression d'injection ainsi que la diminution de pression subsequente etaient mesurees. Les tests à petite echelle sur une ou un petit nombre de discontinuites donnent des resultats simulaires à ceux obtenus au laboratoire. Les resultats des tests à grande echelle sur des sections mesurant jusqu'a 28 m de long montrent que le concept de milieu continu ne peut par être utilise pour etudier les ecoulements dans le massif rocheux.
Ein vielseitig verwendbares modular aufgebautes Druckmessystem fuer Bohrloecher wurde entwickelt, sowie in einer Reihe von Druckverlustmessungen und von Injektionen bei konstantem Druck erprobt. Die Tests erfolgten an Granit mit 3 weitraeumigen, aufeinander Senkrecht Stehenden Kluftsystemen. Kleinmasstaebliche Versuche an einzelnen oder einer geringen Zahl von Klueften ergaben den Laborversuchen vergleichbare Ergebnisse, Die Resultate von grossmassstaeblichen Versuchen mit Messstrecken von bis zu 28 m zeigten, dass in dieser Groessenordnung Kontinuumbedingungen nicht angewendet werden koennen, um die Durchstroemung des Gesteinskoerpers zu untersuchen.
Interest in the transport properties of discontinuous, crystalline rock masses has developed considerably in recent years, largely because of the importance of these properties in studies of the geological isolation of radio-active waste. This concern has generated laboratory studies of the thermo-hydromechanical behaviour of single discontinuities, the development of numerical models for simulating flow in single discontinuities and in networks of fractures, and field studies of the in situ hydraulic characteristics of individual discontinuities and of discontinuous rock masses. Studies of the latter type are expensive and fraught with practical difficulty. As a result, few sets of reliable data are available in the open literature; this paper seeks to add to their number. The work described was carried out as part of a research programme on site assessment methodology for radioactive waste repository design, coordinated by the Building Research Establishment (BRE) for the United Kingdom Department of the Environment (Hudson, 1983). It involved pressure testing of a hard, jointed, crystalline rock mass in which fluid flow was dominated by natural fractures. A fundamental question exists as to the extent to which continuum theories may be applied to such discontinuum. Can flow through these rock masses be described adequately by an equivalent continuum permeability tensor? If so, what representative elementary volume (Bear, 1972) is required for continuum theory to apply?
The tests were carried out at an experimental site-in the Carnmenellis granite near Troon, Cornwall, in South-west England. This underground facility which has been used for a range of other purposes as part of the BRE research programme (e.g. Cooling, Tunbridge and Hudson, 1984), consists of a series of tunnels roughly at right angles to each other (Fig.l) entered from the base of a disused quarry. The tests were carried out in three orthogonal boreholes drilled from a small chamber 7 m long, 6 m wide and 3 m high at the location shown in Fig. 1. Cover to ground surface was 34 m. Monitoring of water levels in boreholes showed the initial groundwater level to be approximately 10 m below ground surface. Two major sets of sub-vertical discontinuities with strike concentrations at 1030 and 1940 are apparent in the quarry and underground excavations. They have spacings in the order of one metre and may have clean, closely matching walls or may be lined with variable thicknesses of secondary minerals. A set of sub-horizontal fractures have spacings of less than one metre immediately below ground surface. This spacing increases to in the order of 5 – 10 m at depths of greater than 100 m. Rare mineralised veins containing sulphides and cassiterite, and a number of porphyry dykes, strike at 0500 parallel with a regional trend. Discontinuities with dips of 45 – 70° and variable dip directions are much less frequent than the steeply dipping discontinuities (Bourke et al, 1981).
