Indurated clay formations are under consideration as potential host rock formations for the disposal of radioactive waste in several countries. The favourable features of such formations for deep geological repositories include an excellent hydraulic barrier function, high capacity for dissolved radionuclide retention and considerable self-sealing potential. The development of a brittle fracture network (excavation-induced damage zone, EDZ) around underground structures during construction and operation cannot however be avoided. To assess the impact of the EDZ on long-term safety of the repository, a detailed structural inventory of the fracture network and the relevant hydro-mechanical phenomena and processes during excavation and the subsequent operation phase is required. This then forms the geomechanical basis for the development of conceptual and numerical models of the EDZ. Here we present some empirical and experimental evidence from the Mont Terri Underground Research Laboratory (URL), summarizing the key phenomena and processes associated with the creation and evolution of an EDZ in the Opalinus Clay. In particular, the significance of rock mass anisotropy is highlighted, both as a likely origin for far-reaching pore fluid pressure changes and as a controlling factor in EDZ development during the excavation process.