Modelling a deep tunnel excavation in a low-porosity tectonised clay

The design of excavation in clayey soils is usually based on the hypothesis that the material displays undrained behaviour during construction. In the case of a deep tunnel, the high stress relief induced by excavation is associated to the development of a wide plastic zone where pore pressure may significantly decrease up to reach negative values in a wide area surrounding the tunnel walls. This type of hydro-mechanical behaviour, however, hardly applies to pervasively fissured scaly-clays, which can exhibit a macroscopic opening of fissures during stress release, resulting in a loss of saturation. In the paper, the stress-strain behaviour observed during construction of a large-diameter tunnel in scaly clays at depth as high as 400 m is reviewed. Different modelling approaches both for the short-and long-term conditions are investigated, ranging from a roughly simplified dry-medium model to a two-phase model with a coupling between permeability and plastic deformation.