Local Strengthening of Poorly Executed Plain Tunnel Linings: Design and Construction Features

Several roadway tunnels built in rock masses are made of plain concrete or weakly reinforced with high-bond bars. The geo-radar investigation carried out at the end of the execution in some tunnels along the pass variant located in Tuscany, aimed at the understanding of the source of a crack pattern appeared few months after the completion of the works, showed a significant reduction up to 90% of the design thickness in some situations of the internal final lining. In order to locally intervene, the use of a corrugated curved sheet metal hang to the vault of the tunnel by means of chemical steel anchors is used as a stretched reinforcement for a positive bending moment. Fibre-reinforced concrete is used to fill the inter-space layer of at least 30 mm between the metal sheet and the original lining. Four prototypes, each one made of 4 corrugated steel metal sheets assembled by means of tightened bolts were tested to verify the sagging and hogging bending moment capacity. Four holes, characterized by a Φ200 mm diameter, were practiced in the original lining, 50 mm thick to reproduce the smallest thickness, to favour the shear transmission between the FRC layer and the plain concrete lining. An AR glass fabric is glued to the original plain concrete lining to improve also the hogging bending moment capacity. The predicted values evaluated by means of a plane section and a Finite Element model are considered for the discussion of the experimental results.