The Importance of In Situ Rock Stress In Repository Design

Pour le design des dépots de déchets radioactifs, les contraintes en place constituent l''un des pôles du modèle d''étude qui couple la structure du massif rocheux, les contraintes en place, les mouvements d''eau et la construction, et dont le but principal est l''étude de la migration des particules radioactives. Ce papier presente les resultats d''une étude qui compare les differentes techniques de mesure des Contraintes en place et qui se situe dans Ie cadre du dévelopement des moyens nécéssaires à l''évaluation des différents sites candidats.

In der Planung einer Felskavernenanlage für die Endlagerung radioaktiver Abfallstoffe ist die in situ Spannung ein wesentlicher Bestand- teil des Modells; hier werden Folgende zusammengebracht - Gesteinmasse, ln situ Spannung, Wasserlauf, Konstruktion. Das Modell ist grundsätzlich für die Kalkulation der Kernteilchenwanderung wichtig. In diesem Bericht werden Resultate einer vergleichenden Untersuchung verschiedener Messungsmethoden für in situ Spannung beschrieben. Diese Arbeit wurde im Kontext der Entwicklung nötiger Einschätzungsmethoden für Kandidatstellen durchgeführt.

The UK Department of the Environment has completed a series of stress measurement tests within the general context of its research programme on the geotechnical aspects of radioactive waste disposal. The background to the work was described in Hudson 1983) and has involved a coordinated study of rock mass properties, permeability, in situ stress, excavation, support and backfilling. The effect of heat flow was not emphasized because the work was mainly directed towards the disposal of intermediate level waste. In this paper, we describe a hydraulic fracturing system and the results of several sets of measurements, using both hydraulic fracturing and overcoring techniques, made in the Carnmenellis granite in Cornwall, south west England. The basic design criterion for a radioactive waste repository is that unacceptable quantities of radionuclides should not be carried back to the biosphere by groundwater flow. The in situ stress is normally regarded as an input parameter for considerations of rock failure around underground excavations. In the case of radioactive waste disposal, however, (where engineering stability may have to be replaced by geological stability), the crucial importance of in situ stress is related to the radionuclide migration modelling through the effect of the stress on rock mass permeability. In other words, the interaction between the stress tensor and the permeability tensor must be known - and each tensor depends on rock structure. Both the stress tensor and the permeability tensor are complex entities, especially when inhomogeneous, discontinuous rock is involved. Some of the factors relating to the stress tensor are described in an associated paper (Hyett et al., 1986); many of the factors relating to the permeability tensor are very well described and analyzed in Long, 1983. The next and most difficult research step is the coupling of the two tensors, because principal permeability directions in a rock mass are a function of the stress state. We concentrate here on reporting the results of a suite of rock stress measurements. These measurements were made to clarify the nature of in situ stress and the measurement techniques, but the interpretation and significance of the measurements must eventually be considered within the general background of rock mass permeability, radionuclide migration modelling and site assessment methodologies.

In order to assess the importance of these parameters and to establish, if possible, the interaction and relation between them in practice, the Building Research Establishment under the aegis of the DOE''s radioactive waste management programme established a collaborative research programme on a test bed site in the Carnmenellis granite in Cornwall (Fig. 1). The site comprised a shallow drift mine, with tunnels at right angles and a series of boreholes, up to 700 m deep, in an adjacent quarry. The in situ stress measurements were, therefore, carried out in conjunction with a detailed exploration of a variety of rock mass assessment methods including scanline measurements, statistical techniques (including geostatistics), geophysics and consideration of the regional and local structural geology. The Carnmenellis granite is part of the Cornubian granite batholith which underlies most of Cornwall. it is thought to be late Carboniferous (310–300 Ma) in age and intruded during the Hercynian orogeny. The discontinuity survey within the mine complex and the nearby borehole cores demonstrated one major sub-vertical joint set striking approximately 025° - 20~ with additional sub-vertical sets striking at 065°- 245°and 0950 -275°.