Investigation and analysis procedures for tunnel refurbishment

Time evolution has two main effects on a tunnel: degradation and changes in operational requirements. In order to cope with these effects after several years of operation, traffic tunnels need to be maintained. The case of an old railway tunnel in the Alps - built in the second half of the 19th century - will be described. In order to satisfy new traffic requirements, the clearance profile of the tunnel had to be enlarged; moreover, the tunnel is located in a zone affected by a deep slope instability and it has been necessary to evaluate the influence of this phenomenon on the behaviour of the tunnel. After the analysis of the deformations along the tunnel, a procedure was adopted, consisting in coupling geophysical investigations and laboratory testing on boreholes samples. This allowed estimating the main parameters describing the tunnel mechanical behaviour, and provided quantitative information for numerical modelling. Numerical simulation results allowed choosing appropriate reinforcement and refurbishment methods before lowering tunnel invert.