Geomechanical characterization of a shale for site selection of a deep geological repository

ABSTRACT: A large exploration campaign was executed for the final phase of the Swiss site selection program for a deep geological repository of radioactive waste. Nine boreholes were drilled, penetrating the 100 to 120 m thick Opalinus Clay shale host rock at depths of 400 to 1000 m. Besides substantial geophysical logging, the borehole testing program involved 27 successful micro-hydraulic fracturing (MHF) tests and 4 pressuremeter tests (PMT) in Opalinus Clay alone, using a wireline formation tester (WFT). Complementary sleeve-reopening tests using the same WFT allowed also the estimation of the maximum horizontal stress magnitude. As part of the laboratory program, oedometric tests were performed to assess the volumetric deformation behavior and transport properties, and more than 140 triaxial tests to assess the shear deformation behavior. The triaxial tests were done following state-of-the-art protocols, which were benchmarked prior to the execution of the large campaign. These tests provide insights which can hardly be gained with smaller testing volumes, such as the impact of basic properties or different stress paths on the strength criterion. Good agreement of laboratory-derived shear moduli with those derived from PMT and logging provides additional confidence that laboratory results can be scaled to the Opalinus Clay rock mass.