Observations of fault zones in fine-grained sedimentary rocks such as claystones have limited value when conducted on outcrops. Surface weathering and hydrological alteration commonly overprint primary structures, modifying mineralogical, structural, and mechanical properties relative to undisturbed subsurface conditions. As a result, direct characterization of clay-rich fault zones in their natural state remains difficult. The Mont Terri Underground Research Laboratory (URL) in Switzerland overcomes these limitations. Situated at depths of 280–300 m, it provides access to faulted Opalinus Clay preserved under near in-situ stress, hydraulic and geochemical conditions enabling high-resolution investigation of micro- and macro-scale fault structures and anisotropic hydro-mechanical properties critical to subsurface deformation and fluid transport. These insights are highly relevant for research related to subsurface applications that rely on claystones for their barrier capabilities, such as the safety of deep geological repositories for radioactive waste or the feasibility of geologic CO₂ sequestration. Over the past decade, the Mont Terri Rock Laboratory has been a focal point for experimental studies on clay-rich fault zones, particularly within the Opalinus Clay. In-situ fault reactivation experiments have significantly advanced understanding of hydromechanical coupling in low-permeability faults, provided field constraints for constitutive model validations, and enabled the development of innovative monitoring technologies. Here, we synthesize key findings from these experimental campaigns and outline emerging perspectives and future research directions from clay fault studies at Mont Terri.
Guglielmi et al. (Thu,) studied this question.