Raticosa Tunnel, Italy: Characterization of Tectonized Clay-Shale and Analysis of Monitoring Data and Face Stability

Abstract

The Raticosa Tunnel (Italy) is one of several tunnels currently under construction for the new Bologna-Florence high-speed railway line. The tunnel, having an overburden of up to 500 m, is located in the Apennine range and half of its 10 km length crosses a tectonized clay-shale formation known as "Chaotic Complex". The construction method is based on full face excavation combined with face reinforcement by means of fiber-glass dowels. Primary lining consists of a shotcrete layer and closed-ring steel sets. Due to heavy squeezing ground conditions, as predicted in the preliminary investigation phase, the excavation of the tunnel was performed under the strict control of an extensive monitoring system. The paper presents an overview of the physical and mechanical parameters of the Raticosa shale, summarizing the results of laboratory and in situ tests. Face "extrusion" and tunnel wall displacements recorded during the excavation advance have been analyzed and correlated with construction stages. The displacements measured near the tunnel face generally confirm the validity of the design criteria of face reinforcement and early invert closure, which assures tunnel stability before casting the final lining. Large, time-dependent, deformations of ground have generally been recorded, causing local failure phenomena at some locations along the tunnel route. A tentative explanation of the observed behavior has been proposed on the basis of empirical models. Finally, the effect of fiber-glass dowels on face stability has been evaluated by using limit analysis.