Anisotropic features of the mechanical behaviour of Opalinus clay

The Opalinus Clay is a potential host rock for radioactive waste repositories. Triaxial tests at different load levels and directions carried out on Opalinus clay showed that the mechanical behaviour of this soft rock is anisotropic. The stiffer samples are those where the loading direction is parallel to the bedding plane so that the preconsolidation stress depends on the orientation of the loading with respect to material structure. It is proposed to interpret those features with an elasto-plastic model based on four mechanisms of plasticity which are alternately mobilized depending on the direction of loading. The predicted stress-strain response is of different nature for different directions of the space, because the hardening process depends on the number of plastic mechanisms that have been mobilized. The numerical predictions showed overall good agreement with the experimental data in terms of deviatoric stress versus axial strain