Eastern Section Meeting

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Geomechanical Characterization for Long-Term Stability Assessment of a Deep Geological Repository

Abstract

As part of geoscience characterisation activities for a proposed L&ILW Deep Geologic Repository (DGR) beneath the Bruce nuclear site, near Tiverton, Ontario, geomechanical investigation of host and cap rocks were performed to support numerical analyses to demonstrate the rock mass integrity and long-term stability of the proposed DGR over a timeframe of 1 Ma. The investigation program involved field and laboratory testing to study the strength and deformation properties of the rock in an 840m thick sedimentary sequence using 6 deep boreholes. Standards test such as uniaxial compression (UCS) and triaxial tests, as well as, special tests to address the strength degradation issue were conducted on selected rock samples. The proposed DGR host rock, the Cobourg limestone, at depth below 650 mbgs has rock strengths (UCS) ranging from 58 and 175 MPa (out of 67 tests) and with a mean of 113 MPa (Std Dev. of 25MPa). With respect to rock mass conditions, evidence from core logging indicates that the rock is moderate to highly fractured within the Devonian and Upper Silurian dolostones compromising the upper 170 m of the sedimentary sequence. Below these formations, the rock encountered is generally in very good condition and sparsely fractured with very high rock quality designation. Since the implementation of the site characterization program, two boreholes have been drilled at shaft locations with additional testing performed. This presentation will show a comparison between the results of updated strength values and those collected from an early regional study. These values demonstrate that the proposed host rock is competent and the strength is significant higher than those of the regional data. The distinctly higher strength of the Cobourg limestone is more visible when comparing with those of other sedimentary formations studied internationally for long-term radioactive waste management purposes. Using 2D and 3D discrete element and continuum models, micro-cracking, time-dependent behaviour of the limestone and its potential degradation and unravelling around the emplacement caverns resulted from various geological and DGR induced loading scenarios were simulated to predict the repository performance in support of the Safety Case. The presentation also shows NWMO's approach on the selection of short- and long-term rock strengths to address upscaling and long-term rock strength degradation in the numerical analysis.