Micro-seismicity recorded during a gallery excavation in the Opalinus Clay of the Mont Terri URL: preliminary description of in situ damage mechanism

A gallery Ga08 has been excavated in August 2008 in the Opalinus Clay formation at the Mont Terri Underground Rock Laboratory to join the end-face of the pre-existing gallery Ga04. In situ acoustic experiments were performed to monitor the evolution of the induced Excavation Damaged Zone (EDZ): the end-face of Ga04 was instrumented with acoustic transducers allowing for both active (acoustic surveys) and passive (acoustic emissions, AE) seismic monitoring. The multifrequency acoustic surveys put in evidence the heterogeneous structure of the rock mass, which acts as a frequency filterfor acoustic waves, and its elastic anisotropy properties modeled by using the Thomsen's Weak Transverse Isotropy model. The in situ acoustic emissions, recorded during and following the excavation of the Ga08 gallery, put in evidence the activity of the excavation process mainly located on the excavated face, only few AE sources being localized inside the rock mass. Some are associated to good quality seismic traces and we attempt to analyze the mechanisms of the associated sources to inform about the damage mechanisms related to the gallery excavation.