Structural and hydrogeological characterisation of the excavation-disturbed zone in the Opalinus Clay (Mont Terri Project, Switzerland)

Abstract

The fracture network in the excavation-disturbed zone (EDZ) of an argillaceous formation, the Opalinus Clay in the Mont Terri Rock Laboratory in Switzerland, is characterised by applying structural and hydrogeological techniques. Unloading fractures are studied by mapping side walls of newly excavated niches and by analysing resin-injected overcores taken from the EDZ. The result is a structural data set of fracture orientations, frequencies, and extent of the EDZ. Pneumatic and hydrogeological tests are carried out in short boreholes crossing the EDZ to derive hydraulic parameters such as permeability and transmissivity distributions of the fracture network. Hydraulic transmissivities are orders of magnitude higher when compared to those of undisturbed rock, and are in the order between 1E−8 and 1E−7 m 2/s. Regular repetition of these hydrotests resulted in decreasing transmissivities in the range of two orders of magnitude over a time period of about 2 years. These observations indicate a hydraulic self-sealing of the initially highly transmissive fracture network with ongoing saturation of the EDZ. The dynamic evolution of EDZ fractures can be derived by interpreting pore water pressure measurements during gallery excavation. Finally, all structural and hydrogeological information is synthesised in a conceptual model of the EDZ in the Mont Terri Rock Laboratory.