Remarks on axisymmetric modelling of deep tunnels in argillaceous formations. II: Fissured argillites

In a companion paper, basic differences between the hydro-mechanical behaviour of plastic and stiff clay formations were highlighted and some issues, such as the influence of hydro-mechanical coupling and boundary conditions, were pointed out. This paper focuses on the effect of the scaly structure, frequently caused by tectonic shearing, typical of many relatively old and deep-sited argillaceous formations. Different engineering models for the prediction of short- and long-term response to tunnel excavation are compared. The proposed approaches consider some typical features of the hydro-mechanical behaviour of the plastic annulus around the tunnel, focusing on the influence of fissure opening, which may cause loss of saturation, in the short-term, and a general increase in permeability in the long-term. The case of the Raticosa tunnel (Italy), excavated through tectonised argillites, is chosen as an example case to apply the proposed approaches and investigate the aforementioned issues. Laboratory test results and in situ measurements are also discussed. The interpretation of convergence measurements, with the help of empirical correlations and the results of hydro-mechanical analyses, highlights the relevance of time-dependent creep deformation, which could therefore be tentatively distinguished from deformation due to consolidation. The potential of an ‘‘observational’’ approach in which early tunnel convergence measurements are used to predict the total final increment in convergence due to time-dependent deformation is also discussed.