Analysis of monitoring data from a deep tunnel in a tectonized clay-shale (Raticosa tunnel, Italy).

The Raticosa Tunnel is one of the several tunnels currently under construction for the new Bologna-Florence high-speed railway line (Italy). The tunnel, having an overburden of up to 500 m, is located in the Apennine chain and half of its 10 km length crosses a tectonized clay-shale formation called Chaotic Complex. Full-face excavation was adopted with reinforcement of the tunnel face by means of fiber-glass dowels. The primary lining consists of a closed ring of shotcrete and steel sets. Due to the heavy squeezing ground conditions, predicted in the preliminary investigation phase, the excavation of the tunnel was performed under the strict control of an extensive monitoring system. In this paper, the in situ measurements of face “extrusion” and tunnel wall convergence have been analyzed and correlated with the single construction stages and specific ground conditions. The deformation pattern near the tunnel face generally calls for core reinforcement and early invert closure so as to assure tunnel stability. Large plastic time-dependent deformations of the ground have generally occurred, which have deter-mined local failure phenomena at some locations along the tunnel route. A tentative explanation of the ob-served behavior has been proposed on the basis of simplified analytical models. A more thorough back-analysis based on visco-plastic models is currently under way.