Anisotropy of seismic and mechanical properties of Opalinus clay during triaxial deformation in a multi-anvil apparatus

It is generally accepted that the dilatancy concept offers a reliable basis for the assessment of underground buildings in various host rock formations, e.g., a repository for radioactive waste. Stress conditions are predictable where creep failure and increasing permeability will inevitably develop. However, in argillaceous rocks an experimental detection of microcrack opening by volumetric strain measurements fails because the damage induced volume increase is overlapped by the dominating bedding plane compaction, also during deviatoric loading. To solve remaining uncertainties regarding the stress dependent onset of dilatant deformation in clay rocks it is inalienable to investigate the stress and deformation induced variations of various independently measured physical parameters, e.g., volumetric strain resp. porosity, p- and s-wave velocities, permeability. The aim of our spatial velocity measurements in a multi-anvil apparatus is not only to quantify the seismic anisotropy of the Opalinus clay but also to identify the onset of dilatancy and to monitor its evolution at various states of stresses. We found that the crack sensitive p- and s-velocities is a powerful tool for the determination of the so-called dilatancy boundary. Differences in the sensitivity of V p and V s, resp. anisotropy effects are closely related to the addressed physical process of crack opening depending on stress field orientation and bedding plane orientation. In summary, our results lead to a new and comprehensive synoptic view of the stress and deformation induced changes of rock properties in argillaceous rocks, which impressively confirms the general concept of dilatancy.