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. A microphysics-based understanding of mechanical andhydraulic processes in clay shales is required for developing advancedconstitutive models, which can be extrapolated to long-term deformation.Although many geomechanical tests have been performed to characterise thebulk mechanical, hydro-mechanical, and failure behaviour of Opalinus Clay,important questions remain about micromechanisms: how do microstructuralevolution and deformation mechanisms control the complex rheology? What isthe in situ microstructural shear evolution, and can it be mimicked in thelaboratory? In this contribution, scanning electron microscopy (SEM) wasused to image microstructures in an Opalinus Clay sample deformed in anunconsolidated–undrained triaxial compression test at 4 MPa confining stressfollowed by argon broad ion beam (BIB) polishing. Axial load was applied(sub-)perpendicular to bedding until the sample failed. The test wasterminated at an axial strain of 1.35 %. Volumetric strain measurementsshowed bulk compaction throughout the compression test. Observations on thecentimetre to micrometre scale showed that the samples exhibited shear failure andthat deformation localised by forming a network of micrometre-wide fractures,which are oriented with angles of 50∘ with respect to horizontal.In BIB–SEM at the grain scale, macroscale fractures are shown to beincipient shear bands, which show dilatant intergranular and intragranularmicrofracturing, granular flow, bending of phyllosilicate grains, and porecollapse in fossils. Outside these zones, no deformation microstructureswere observed, indicating only localised permanent deformation. Thus,micromechanisms of deformation appear to be controlled by both brittle andductile processes along preferred deformation bands. Anastomosing networksof fractures develop into the main deformation bands with widths up to tensof micrometres along which the sample fails. Microstructural observations andthe stress–strain behaviour were integrated into a deformation model withthree different stages of damage accumulation representative for thedeformation of the compressed Opalinus Clay sample. Results on themicroscale explain how the sample locally dilates, while bulk measurementshows compaction, with an inferred major effect on permeability by anincrease in hydraulic conductivity within the deformation band. Comparisonwith the microstructure of highly strained Opalinus Clay in fault zonesshows partial similarity and suggests that during long-term deformationadditional solution–precipitation processes operate.

Micromechanisms leading to shear failure of Opalinus Clay in a triaxial test: a high-resolution BIB–SEM study