Characterizing excavation-induced perturbations around a short tunnel in an overconsolidated clay shale

The Opalinus Clay is under investigation for deep geological disposal of nuclear waste in Switzerland. Past investigations indicate that the Excavation Damaged/Disturbed Zone is comprised of macroscopic fracturing. Recent excavation of the EZ-B niche at the Rock Laboratory revealed only a few induced macroscopic fractures from drillcore mapping and televiewer imaging. However, borehole seismic profiling showed marked velocity and amplitude decreases within the first 50 cm. Due to the limited induced macroscopic fracturing identified, the zone of excavation-induced perturbation was characterized through the integration of borehole data to reveal a non-uniform zone around the niche. ©2008, ARMA, American Rock Mechanics Association