Opalinus Clay is an indurated over-consolidated argillaceous rock. The preferential orientation of clay platelets results in a strong anisotropy of its deformational and strength characteristics. The purpose of this study is two-fold: (i) to illustrate the new developments that have been introduced into the combined finite-discrete element method (FEM/DEM) to model layered materials and (ii) to show the effectiveness of this new modelling approach for simulating the short-term mechanical response of Opalinus Clay. A transversely isotropic elastic constitutive law is implemented to account for different elastic properties in a parallel and perpendicular direction to the layering, while a procedure to incorporate a distribution of preferably oriented defects is devised in order to capture the anisotropy of strength. Laboratory results of indirect tensile tests and uniaxial compression tests are used to quantitatively calibrate the numerical model. Emergent strength and deformation properties, together with the simulated damage mechanisms, are shown to be in strong agreement with experimental observations. Subsequently, the calibrated model is successfully validated by simulating the influence of the loading angle with respect to the specimen anisotropy.


Due to the favorable transport and geochemical properties, the Opalinus Clay, an argillaceous rock, has been chosen in Switzerland as host rock formation for a high-level nuclear waste underground repository. It follows that any numerical model that aims at simulating the failure process and EDZ formation in Opalinus Clay should be able to capture the intrinsic anisotropy of the material. In the continuum method, the presence of layers is smeared to produce a fictitious continuous material that exhibits similar mechanical characteristics to the original discontinuous medium. Main formulations of the equivalent continuum approach are the Ubiquitous Joint model, implemented for example in the commercial code FLAC [4] and the models based on the Cosserat theory [e.g., 5, 6].

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