Tunnelling in argillaceous rock is very common because of the widespread occurrence of those materials in the earth crust. Research on argillaceous rocks have been further stimulated in recent times because of their suitability as host rocks in deep geological repositories for radioactive waste. The paper presents a numerical analysis, using an especially developed constitutive model for argillaceous rocks, that reproduces successfully the observations obtained in the excavation of tunnels in Callovo-Oxfordian claystone at the Meuse/Haute-Marne underground laboratory. Particular attention is given to the configuration of the fractured zone and its dependence on the orientation of the tunnel. Using this analysis as reference, the effects of stiffness and strength anisotropy as well as of the in-situ stress anisotropy are examined. The analyses reveal that either strength anisotropy or the initial in-situ stress state have a dominant influence depending on the alignment of the tunnel.
Argillaceous rocks are very prevalent in nature; it has been estimated that they may constitute as much as 50% of the global sedimentary rock mass and that they outcrop in about one third of the emerged earth surface (Gens 2013). Therefore, tunnelling and underground excavations in this type of materials will occur frequently. Interest on argillaceous rocks has been further enhanced by the fact that they are considered suitable host materials for the safe geological disposal of high-level and long-lived radioactive waste, because of their low permeability, significant retention properties and limited economic value. This has prompted the construction of a number of underground research laboratories that allow the observation of their behaviour under controlled and realistic conditions. The amount of data and information available in these facilities usually surpasses what is generally available in more conventional civil engineering projects.
In the context of radioactive waste disposal, the existence of a fractured zone around the tunnels is a key issue, as they affect the hydromechanical response of the rock. Notably, the fractures that develop lead to an increase of permeability that may favour the eventual migration of radionuclides although argillaceous rocks do exhibit a capacity for self-sealing with time (Tsang et al. 2005). In addition, the development of a fractured zone weakens the rock adjacent to the tunnel and affects the requirements for temporary support and permanent lining.
