Rock joint and rock mass characterization at Sellafield

The NGI methods of characterizing rock joints and rock masses were utilized extensively in geological investigations undertaken by United Kingdom Nirex Limited (Nirex) at Sellafield to determine whether it was suitable as the location for a deep geological repository for radioactive wastes. In addition to the standard rock mechanical laboratory testing of joints, coupled shear flow testing (CSFT) was also performed on natural rock joints for obtaining the magnitude of joint conducting apertures. The objectives of the CSFT tests were to produce site specific data, albeit on small scale samples, so that the effects of normal and shear stress changes, closure and shearing, could be evaluated and compared with the patterns of behaviour predicted by numerical modelling of the disturbed zone which would be caused by excavation of disposal caverns. Preliminary rock reinforcement designs for the conceptual disposal caverns were derived from the Q-system statistics. Numerical modelling using UDEC-BB was carried out for predicting the behaviour. The purpose of numerical modeling was to investigate the potential stability of various sizes of rock caverns and in particular the rock reinforcement (predicting bolt loads and rock deformations), the extent of the disturbed zone (joint shearing and hydraulic aperture) with respect to cavern orientation, the effect of pillar widths, and the effect of cavern excavation sequence.