In the Clay formation of the Mont Terri, we have performed acoustic experiments to monitor the excavation damaged zone induced in the surrounding of a new gallery Ga08. The experiments took place in the end-face of a previous gallery Ga04 with an excavation operation coming from the opposite direction to joint both galleries. The study dealt with the rock mass segment, 8 m in length, left for two weeks between the Ga04 end-face and the Ga08 excavation front. Acoustic surveys, performed with a controlled acoustic source introduced inside the rock mass segment, allowed defining the transversely isotropic P-wave velocity field of the medium. Passive acoustic measurements, which consisted in detecting induced acoustic events, allowed identifying actual microseismic events induced by stress redistribution in the surrounding of the Ga08 excavation front. We assessed the damage mechanism of these events and clearly highlighted strong correlations between spatial, temporal and mechanism attributes, in agreement with both the geological facies and the gallery orientation with respect to geological preexisting features. Finally, we present preliminary results on additional acoustic events located inside the rock mass segment.
A large scale geological rock mass is subjected to different natural stresses, including tensional, compressive and shear stresses. The rock response to such ambient stress is controlled by the lithology, the pressure condition the rock is under, i.e. the normal to horizontal stress ratio, and the time the rock is under stress. Depending on these in situ conditions, which control the stress field, different damage structures may appear in the geological structure, including faults, fractures, cracks and/or crystal dislocations [1,2].
At short time and space scales, the stress field can be modified for instance when a gallery is excavated in such a stressed environment : it induces a stress redistribution in the vicinity of the excavation, in particular the so-called excavation damaged zone (EDZ) [3,4]. Characterization of the EDZ, where plastic deformation occurs leading to significant hydromechanical changes , is of primary importance in many research topics related to underground galleries, in particular when dealing with deep geological repositories. Indeed, damage structures constitute preferential pathways for radionuclides to reach the free surface or ground water reservoirs. In this context, clay formation are interesting for both its low permeability and its self-sealing capacity [4,6]. As a consequence, monitoring the EDZ in such geological formation is a major but not straightforward research field which motivate numerous in situ experiments in the underground research laboratory (URL) of Mont Terri (Switzerland), situated in the Opalinus Clay formation of the Folded Jura [7,8].