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Damage initiation and propagation assessed from stress-induced microseismic events during a mine-by test in the Opalinus Clay

Abstract : We report in this paper an original analysis of microseismic events (MSEs) induced by an excavation operation in the clay environment of the Mont Terri underground rock laboratory. In order to identify the MSEs with confidence, we develop a restrictive but efficient multistep method for filtering the recorded events. We deduce the spatial distribution and processes associated with the excavation-induced damage from the spatial location and focal mechanisms of the MSEs. We observe an asymmetric geometry of the excavation damaged zone around the excavated gallery, without notable microseismic activity in the sandy facies sidewall, in contrast with the shaly facies sidewall where a first burst of events is recorded, followed by two smaller bursts: one locates ahead of the excavation front and is associated with a dominant double-couple component, suggesting bedding plane reworking, that is, shear fracture mode, and the MSEs of the other cluster inside the shaly sidewall of the gallery, with a dominant compensated linear vector dipole component, suggesting extensive cracking. We identify and discuss four major factors that seem to control the MSEs source mechanisms: lithology, geometry of the geological features, gallery orientation and direction of the main compressive stress.
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Yves Le Gonidec, J. Sarout, Jérôme Wassermann, Christophe Nussbaum. Damage initiation and propagation assessed from stress-induced microseismic events during a mine-by test in the Opalinus Clay. Geophysical Journal International, Oxford University Press (OUP), 2014, 198 (1), pp.126-139. ⟨10.1093/gji/ggu122⟩. ⟨insu-01131800⟩

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