The existence of a gently dipping fracture zone at the URL created a clear decoupling of the state of in situ stress. In addition, owing to extremely high horizontal principal stresses at the shallow depths of 200 to 500 m, induced hydraulic fractures were invariably subhorizontal, complicating the interpretation of test results. Thus, six separate hydraulic fracturing campaigns were undertaken in order to obtain a reliable estimate of the local stress tensor.
Un decouplage marque du champ de contraintes in situ resulte de l'existence sur le site du Laboratoire Souterrain de Recherche d'une zone fracturee de faible pendage. De plus, à 200 et 500 m de profondeur, les fractures hydrauliques induites sont invariablement subhorizontales à cause de la valeur tres elevee des contraintes principales horizontales, ce qui complique notablement l'interpretation des tests. De ce fait, six campagnes de fracturation hydraulique distinctes ont ete conduites pour obtenir une estimation fiable du tenseur de contraintes in situ.
Die Existenz einer flach einfallenden Rißzone am URL verursacht eindeutig eine Entkopplung des Span-nungszustandes: Außerdem sind bedingt durch die extrem hohen, horizontalen Hauptspannungen in geringenTiefen von 200 bis 500 m hydraulisch induzierte Risse annahernd horizontal, was die Interpretation der Testergebnisse erschwert. Urn eine verlaßliche Abschatzung des lokalen Spannungstensors zu erhalten, wurden deshalb sechs separate Hydraulic Fracturing Versuchsserien durchgefuehrt.
The determination of the in situ stress regime is a major component of site characterization for the design of an underground radioactive waste disposal facility. Knowledge of the crustal stress is required for such endeavors as the rational design of the underground caverns and canister drill holes, and the understanding of the fracture system and its potential for radionuclide migration. With the emergence of hydraulic fracturing (HF) as a means of in situ stress estimation in deep boreholes (Haimson 1978,1992), it has become the method of choice in site investigations.
However, the experience at the Canadian Underground Research Laboratory (URL), located near Pinawa, Manitoba, serves to alert us that the task of determining the stress regime should never be considered routine. The URL is the Canadian underground facility selected for generic studies of the feasibility of radioactive waste disposal in the Canadian Shield granite. There are two main levels that have level developed for the investigation, at depths of 240 m and 420 m. The Lac du Bonnet bathotith in which the URL is located has, been subjected between 1981 and 1992 to six HF stress measurement campaigns (perhaps more than any other site in the world) in and outside of the present underground facility in an attempt to characterize the state of stress. The two main reasons for this unusually large number of measurement sets have been (a) the existence of Fracture Zone 2 (FZ2), interpreted as a low angle thrust fault, at about 275 m depth (Fig. 1), which appears to serve as a discontinuity in the local stress field, and (b) the frustrating feature of all tests in that no vertical hydraulic fractures (or hydrofracs) could be induced below FZ2 (typically these fractures dip 10° to 45°).