On-site hydraulic and mechanical characterization of a claystone around a non-lined test tunnel in Mont Terri, Switzerland

The rock mass around man-made underground structures inevitably experiences major changes in hydraulic and mechanical properties, commonly referred to as excavation damage. In host rock formations for underground nuclear waste storage, such an excavation damaged zone (EDZ) is potentially critical and therefore requires reliable field data sets for safety assessment and the estimation of the long-term behavior. In this study, different on-site measurements were carried out in the EZ-B niche of the Mont Terri Rock Laboratory in order to characterize the EDZ with regard to its changed hydraulic effectiveness and its mechanical and geophysical properties. The discrete fracture network (DFN) around the EZ-B niche was investigated using a transient airflow permeameter as well as combined microscopic imaging with automatic evaluation. The DFN exposed in the non-lined tunnel with measured hydraulic fracture apertures of 84 ± 23 μm basically represents a network for advective fluid transport. Due to their formation mechanism, we encountered noticeable differences between the newly-formed unloading fractures and the reactivated tectonic discontinuities. Geomechanical and geophysical characterization of the Opalinus Clay was conducted based on needle penetrometer testing at the rock surface. This proved to be a valuable tool for accurately determining the anisotropic uniaxial compressive strength of the claystone, but also led to a general underestimation of other physico-mechanical parameters in bedding-parallel direction, presumably due to unperceived microcracks.