Geotechnical investigations for a deep radioactive waste repository: in situ stress measurements

Abstract

AbstractComprehensive geotechnical investigations of potential repository sites for deep disposal of low- and intermediate-level solid radioactive wastes at Sellafield, Cumbria and Dounreay, Caithness included drilling and testing cored boreholes up to 2000 m deep. The investigation at Sellafield included the measurement of in situ stresses using the overcoring technique at various depths in a 250 m deep, vertical borehole drilled from the surface. The measuring instrument and method used for overcoring had, therefore, to perform reliably under high hydrostatic heads.The equipment used to carry out these measurements was the Swedish Borre Probe. Previous work with this equipment has been carried out in slim holes and hard crystalline rocks. In contrast, this project required in situ stress measurements to be taken in a relatively large-diameter borehole, drilled in weak sedimentary rocks by wireline drilling techniques, and consequently, significant adaptations to the installation equipment.The paper describes the adaption of the Borre Probe and its successful application in this project up to the end of 1992. Possible future applications of the technique are discussed.