ABSTRACT
The Monte Baldo twin bore tunnel, along the A27 Highway linking Venice to Cadore, was excavated during the 1970s in marls, with partial section advancing system, systematically installing metal ribs as a temporary support. The final lining consists of unreinforced concrete with average thicknesses of 90cm. As part of Autostrade per l’Italia (ASPI) Tunnel Assessment Programme, the southbound tube has recently undergone major temporary safety works, consisting mainly of crown supporting with steel-welded wire mesh and self-drilling bolts. At the same time, an extensive investigation plan was put in place in order to identify the actual thickness of the linings and the extent of back voids due to over-excavation and casting flaws. The investigation, based on video-endoscopy and phorometric techniques, showed an extensive system of interconnected cavities being almost a meter deep. The extent of the volumes involved determined the need to study specific design criteria, based on the seismic response of the shell lining in the presence of cavities on its back, in order to identify priority sectors for filling as well as the suitable material to be used. It was deemed appropriate to proceed with the injection of expansive epoxy resins of high mechanical properties in the hardened state (strength and dynamic stiffness have been investigated, varying the expansion factor). Resin injection proved to be an optimal solution to minimize impacts in terms of site logistics and time incidence of secondary works (perforations diameter, injection equipment). This paper discusses the evidence gathered from the experimentation of this pilot solution in the field of systematic filling of back cavities on existing tunnel linings, specifically in the conditions of implementation during night-time traffic closures.
