The hydro-mechanical coupling, reduction of effective strength, and critical state shearing of faults: Evidence from laboratory testing on natural fault rock

Fault zones in clay-rich formations play a critical role in controlling deformation, fluid flow, and stability in both natural and engineered subsurface systems, including earthquake rupture, underground storage, and radioactive waste disposal. However, the coupled hydro-mechanical behavior of natural fault rocks remains poorly constrained due to the difficulty of obtaining representative samples and performing fully coupled laboratory experiments. Here, we present results from petrophysical and fully hydro-mechanically coupled triaxial compression tests on preserved natural fault material from scaly clay sections of the Main Fault intersecting the Opalinus Clay formation at the Mont Terri Underground Research Laboratory, Switzerland.The hydraulic properties of scaly clay Opalinus Clay were measured using flow-through experiments on back-saturated specimens. Permeability coefficients determined sub-parallel to the orientation of bedding and tectonic shears are up to three orders of magnitude larger than those of the intact rock. The shear experiments were conducted under undrained conditions at different effective confining stresses, allowing direct observation of stress–strain behavior, pore pressure evolution, and effective stress paths up to large axial strains. In contrast to intact Opalinus Clay, the faulted scaly clay exhibits continuous strain hardening without a distinct peak stress or post-peak weakening. Deformation is distributed and accommodated by the reactivation of multiple pre-existing tectonic micro-shear. The shear strength analysis within a Mohr–Coulomb framework reveals that the scaly clay fabric has effectively zero cohesion and a shear strength that is lower than even the residual strength of intact Opalinus Clay. Microstructural observations confirm that deformation proceeds through distributed sliding along an anastomosing network of polished micro-shears surrounding undeformed microlithons.These results demonstrate that inherited fault-zone fabric exerts a first-order control on both mechanical strength and hydro-mechanical coupling in clay-rich faults. Incorporating fabric-and stress-dependent behavior as well as critical-state deformation into constitutive models is therefore essential for realistic predictions of fault reactivation, pore pressure evolution, and long-term stability of low-permeability clay formations.