Deep base tunnels currently under construction through the Alps make the use of mechanized excavation with Tunnel Boring Machines (TBMs) of great interest, as this may contribute to savings in construction time and costs. Questions are however posed on the adoption of such a method when the rock mass is overstressed. Such overstressing can result in brittle failure of massive rock masses (i.e., spalling and rock bursting) and in shear failure of weak and heavily jointed rock masses (i.e., squeezing). For the design of mechanized tunnelling in these conditions, the interaction between the rock mass, the TBM and the tunnel support is to be analyzed in detail and the use of three-dimensional (3D) models which consider all these components is of great help for design purposes. A novel 3D simulator of mechanized tunnel excavation has therefore been set up. The model has been applied to the spalling behavior in granite along the Brenner Base Tunnel and to squeezing in the Carboniferous Formation along the Lyon-Turin Base Tunnel. The rock mass has been modelled as homogeneous or equivalent homogeneous and appropriate elasto-plastic constitutive laws for representing such classes of behaviour have been implemented.
In Europe, a major interest in infrastructure and tunnelling fields is in the new crossing of the Alps in the form of Base Tunnels, which are important components of the high speed (for passengers)/high-capacity (for freights) Trans-European railway corridors. These long and deep tunnels are being or will be excavated mainly by TBMs, as they contribute significantly to savings in construction time and costs. There are a variety of conditions under which such overstressing may occur. Two typical cases, as also described by Hoek & Marinos (2010), are the failure of massive rock and of heavily jointed rock mass.