The investigation of damage zones around cylindrical excava-tions such as sealing sections in tunnels or shafts is a key issue in the field of underground waste storage. Emphasis is on the geometry and extent of the excavation-induced damage zones, on their mechanical and hydraulic characteristics and, in a broader perspective, on the stability of the cavities before and after waste emplacement. Formation damage control in pro- duction and injection wells represents a similar stability problem, suggesting that a detailed understanding of coupled hydromechanical processes in the vicinity of cylindrical cavities is just as valuable for the oil and gas industry. As part of a geoscientific research programme at the Mont Terri underground rock laboratory in the Jura Mountains of Switzerland, a horizontal microtunnel was drilled in an over- consolidated claystone formation. Prior to the excavation, the rock formation was instrumented with piezometers, incline- meters, extensometers and stress cells for monitoring the hydromechanical response of the rock formation to the drilling process. After completion of the excavation, a geoscientific characterization campaign was initiated, comprising detailed geological mapping of structural features, laser scanning of the tunnel surface and the instrumentation of the tunnel walls with surface extensometers. Complementary investigations were conducted to estimate rock stress at the site and the geo-mechanical rock properties, such as elastic moduli, rock strength and anisotropy. The observations to date indicate that a distinct damage zone was created by the drilling process, comprising massive breakouts and localized spalling.Break-outs with magnitudes of up to 15 cm were observed in one of the upper quadrants of the tunnel cross section and less marked features were mapped on the opposite lower quadrant.Long-term monitoring of pore pressure and deformation around the tunnel and progressive spalling along the tunnel wall suggest that the damage zone is still developing.The complex geometry of the damage zone is explained by the interaction of a distinct stress anisotropy and rock strength anisotropy. The initiation of excavation-induced fractures can be linked to isolated tectonic features intersecting the micro- tunnel.The geometry and the temporal evolution of the damage zones are similar to observations in boreholes, indicating that the basic hydromechanical mechanisms are comparable.