The shear strength of Opalinus Clay shale in the remoulded state

The Opalinus Clay shale formation is considered as a potential host geomaterial for the Swiss deep geological repository for radioactive waste. It presents different facies and it is characterised by a multi-scale heterogeneous composition, by a typical fissile structure with well-defined bedding planes and by anisotropic hydro-mechanical behaviour. This peculiar complexity makes it difficult to assign a unique set of geomechanical parameters to the material. This paper presents an experimental study aimed at characterising the lowest values of the shear strength parameters. In this sense, the shear behaviour was investigated on remoulded samples where the fabric and the diagenetic bonds of the intact material were eliminated. The results of a triaxial test campaign belonging to different facies of Opalinus Clay shale (“shaly” and “sandy” facies) are presented. Furthermore, with the aim to study the mechanical properties of fault zones in Opalinus Clay, the effect of large cumulated shear displacements on shear strength was also investigated. Ring shear tests were performed to determine the residual shear strength. The main geotechnical property which discriminates the different facies of Opalinus Clay is the grain-size distribution; in this sense, well-defined correlations between this intrinsic characteristic and shear strength angles of the remoulded material are presented.