Stress-Induced Microseismic Events during a Gallery Excavation in a Clay Formation : Spatial Distribution and Damage Mechanism

International audienceActive acoustic surveys and passive detection of microseismic events have been performed in the clay environment of the Mont Terri underground rock laboratory during a mine-by test. From the active experiments, we highlight the transversely isotropic P-wave velocity field of the rock mass, used to efficiently locate the acoustic sources and validate the location procedure. From the passive experiments, we identify the stress-induced microseismic events with confidence by developing a restrictive but efficient multi-step filtering method. We deduce both their spatial distribution and processes from the location and focal mechanisms of the microseismic event sources. We observe an asymmetric geometry of the excavation damaged zone around the excavated gallery, without notable microseismic activity in the sandy facies sidewall. A burst of microseismic events locates in the shaly facies sidewall followed by two smaller bursts which occurred when the excavation procedure was stopped. The first one locates ahead of the excavation front and is associated with a dominant double-couple component, suggesting bedding plane reworking, that is, shear along bedding planes and associated wing-cracks. The second one locates inside the shaly sidewall of the gallery and is associated to a dominant compensated linear vector dipole, suggesting extensive cracking, that is, typical spalling damage in the vicinity of the stress concentration zone. We identify and discuss four major factors that seem to control the source mechanisms of the microseismic events: lithology, geometry of the geological features, gallery orientation and direction of the main compressive stress