Evolution of excavation damaged zones in Opalinus Clay shale inferred from seismic investigations

We present the results of new and repeat gallery- and borehole-scale seismic experiments carried out in two excavation damaged zones (EDZs) of 10 (gallery 08) and 20 (gallery 98) years of age at the Mont Terri Underground Rock Laboratory in St. Ursanne, Switzerland. The initial geometry and rock mass properties of the EDZs were studied during and shortly after excavation—allowing for a direct comparison of measurements collected in 2018 to those completed in the past. In gallery 08 parts of the EDZ remain unchanged over ten years, whilst other zones show improvement (i.e., self-sealing) or considerable degradation. In addition, velocity tomograms from 2018 measure extensive areas of p-wave velocity reduction in comparison to the surrounding rock mass at depths shallower than 3.5 m. Isolated extensile fractures are still found in boreholes up to a maximum radial extent of 4.2 m. Data from borehole interval velocity measurements and cross-hole seismic surveys from this gallery suggest the damaged zone still extends up to 1.9 m and 3.1 m radial depth. Refraction seismic results in gallery 98, in contrast, suggest a less pervasive, shallower (