Insights from an extensive triaxial testing campaign on a shale for comparative site characterization of a deep geological repository

Abstract

Several boreholes were drilled for site comparison of a deep geological repository (DGR) in Northern Switzerland. The main target of the exploration program was the >100m thick Opalinus Clay, the designated host rock encountered at approximately 450 to 1000 m depth in three different sites. This contribution focuses on the evaluation of geomechanical properties and the deformation behavior from the triaxial testing campaign, both aspects relevant to construction and the assessment of the long-term safety of a DGR. Some 140 triaxial tests were performed on cores from seven different boreholes to evaluate potential differences in material properties by depth and geographic location. Core sampling, preparation chain, and testing protocols were validated before the campaign, and three laboratories were commissioned to perform the tests. A comparison of basic properties from cores used for triaxial testing with a much larger database of complementary core analyses and geophysical logging demonstrates that the performed tests cover the range of expected material properties. Limited to no differences in strength and stiffness are detected from cores at different depths and sites. Despite a relatively large variation in bulk mineralogy of the formation (e.g. clay-mineral content varying between 35 and 75 wt%), the strength values of Opalinus Clay vary only moderately, with equivalent (calculated) unconfined compressive strengths of 21 ± 5 MPa, for loading directions parallel or perpendicular to bedding. This contrasts with the results of Opalinus Clay from the Rock Laboratory at Mont Terri, where the effect of material composition was more relevant. Assuming a Mohr-Coulomb-type failure law, the transition from peak to post-peak strength comes at the expense of cohesion, and only a small reduction of the shear strength angle. Hence the burial history, tectonic overprint, and current depth mainly control the intact properties by additional cohesion, whereas the post-peak behavior is mainly controlled by bulk mineralogy.