Damage or unusual behavior detection in in-service tunnels: an overview and possible prospects

This work presents an overview on problems that can be met in in-service tunnels and technologies that are usually employed for damage or unusual behavior detection. Operational systems such as air ventilation, traffic management and fire mitigation are installed to help day to day operations. Furthermore, health monitoring systems are usually installed in problematic or special areas of the tunnels, giving information on conditions and helping manage maintenance. Deformation of the inner lining, stress variations of the shotcrete and groundwater level, among others, can be monitored either in real time or at periodic intervals. In most cases, the deformation data is compared with the prescribed safety criteria so that, when the deformation exceeds a certain level, alarms are automatically sent to the maintenance engineer. However, these methodologies have the drawbacks of forcing the interruption of traffic for SHM system installation and of monitoring only selected portions. Alternative solutions that would make it possible to keep the tunnel in normal operation and to analyze the entire infrastructure development, through successive and continuous scanning stages, would be beneficial. In this paper, the authors will review common structural problems such as material deterioration, also extreme events such as earthquakes along with possible monitoring technologies to address these issues. Furthermore, the work is aimed at identifying possible alternatives and solutions that can be used, limiting or avoiding traffic interruptions; referring to methods already used on roads, bridges and viaducts. In particular, fiber optical methods and cameras, machine learning and damage detection algorithms can be considered.