How Fault Zone Fabric Controls the Hydro‐Mechanical Behavior and Evolution of Critical State Shearing in Clay Shale

Abstract Understanding the hydro‐mechanical behavior of faults in clay‐rich formations is essential for earthquake mechanics and underground storage applications. We conducted fully hydro‐mechanically coupled triaxial tests on preserved fault material from scaly clay sections of the Opalinus Clay formation. The faulted rock exhibits no post‐peak weakening and maintains distributed strain accommodation through the reactivation of multiple, sub‐parallel tectonic micro‐shears. The hydro‐mechanical response is stress‐dependent, transitioning from dilative shearing at lower effective consolidation stresses to constant volumetric shearing at higher effective consolidation stresses, eventually resulting in critical state conditions. Shear strength analysis revealed the absence of cohesion in the scaly clay fabric, which reduces the rock's shear strength to below the residual strength of the intact rock. These findings provide direct experimental evidence for distributed, steady‐state shearing in naturally faulted clay shale and highlight the need to incorporate fabric‐dependent behavior into models of fault mechanics.