By applying a conductive coating material on tunnel lining, a circuit was formed and cracks of 0.6 mm or more in width can be detected based on the conduction and insulating states on at circuit terminals (Fig. 1). Crack positions can be located by measuring the conduction and insulating states with an electric resistance meter between the terminals of the circuit drawn in a cascade form with a conductive material on the lining (see Fig. 2).

This system also enables monitoring of the progression of cracking through wireless sensors without using large-scale circuits. To achieve this, wireless sensors are fixed to cracking displacement meters and measuring instruments on the conductive coating material to send measurement data to the tunnel damage monitoring system through a receiver installed on the roadbed. Although the coverage distance of wireless sensors is only about 10 m, this system can cut installation costs as no wiring is required on the lining surface.

The combination of these technologies enables the monitoring of tunnel lining damage where cracking is found to occur and progress.