Geological and hydraulic characterisation of the excavation disturbed zone in the Opalinus Clay of the Mont Terri Rock Laboratory

Abstract

The fracture network in the excavation disturbed zone (EDZ) of an argillaceous formation, the Opalinus Clay in the Mont Terri Rock Laboratory in northwest Switzerland, is characterised using structural and hydrogeological techniques. Sampling of unloading fractures is done by analysing resin-injected overcores taken from the EDZ, resulting in a structural data set of fracture orientations, frequencies and extent of the EDZ. These parameters are derived in tunnel sections, which were excavated by blasting, pneumatic hammering and road header techniques. Pneumatic and hydrogeological tests are subsequently carried out to derive hydraulic parameters such as permeability and transmissivity distributions of the fracture network. Synthesis of the structural and hydrogeological data led to a conceptual model of the EDZ in the Mont Terri Rock Laboratory, which is defined by a 2-m plastically deformed zone around the tunnel. This plastic zone is further divided into an inner 1 m zone consisting of an air-filled fracture network and an outer zone with mostly saturated and isolated fractures. Transmissivities in the inner zone are orders of magnitude higher than those of the undisturbed Opalinus Clay. Finally, a methodology is proposed for the structural and hydrogeological characterisation of the EDZ. This methodology is not site-specific and is thus applicable at any argillaceous underground facility.