Since a decade, observations and experiments conducted in underground environments worldwide (mines, underground research laboratories - URL) allowed to bridge the scale gap between laboratory scale and large scale faults. This is particularly important in the study of faults affecting clay formations which could not be observed at the Earth’s surface because of weathering processes. Here we show how examples from the Mont Terri (Switzerland) and Tournemire (France) URLs may help in (i) defining the concepts of fault architecture in shales, and (ii) estimating fault hydromechanical properties from decameter scale field experiments. Some key results are that critically stressed shale faults can be reactivated with little stress changes in the range of those produced by shallow underground excavations, and that micrometer-to-millimeter scale reactivations may lead to an increased permeability of one order of magnitude without generating any noticeable seismicity. This appears to be related to complex multi-scale geological processes linked to fault history such as differential hardening, partial sealing, pressure solution and gouge development within the fault zones rather than to the regional state of stress. Such results are important in evaluating fault seal integrity in oil and gas exploration, production or CO2 storage or monitoring underground excavations for deep geological repositories.
Little is known about the rock mechanical characteristics of pre-existing tectonic faults and their influence on rock mass properties and behavior in clay rocks. Underground research laboratories provide exceptional conditions to observe unaltered fault zones in shales and to perform relatively well constrained experiments. Studies on induced macroscopic fracturing around tunnels in faulted shale indicate that reactivation of tectonic faults as a consequence of excavation-induced stress redistributions play an essential role in the development of the Excavation Damage Zone (EDZ) fracture network [1-4]. Hence, faults significantly alter the homogeneity of the rock mass in strength, stress and deformability, and thus have a significant effect on induced fracturing.