Optical fibre strain measurement for tunnel lining monitoring

A trial strain-monitoring system using Brillouin optical time-domain reflectometry (BOTDR) technology was set up to monitor joint movements in the concrete tunnel lining in an existing London Underground tunnel. The BOTDR strain sensor system allows the measurement of strain distribution along an optical fibre using the reflective technique, requiring access to only one end of the fibre. Measurements were obtained by a strain-sensing optical fibre installed along the tunnel lining. The joint movements were captured by measuring the strain along the fibre across the segment joints. The results show that there is good agreement between the joint movements evaluated by the BOTDR strain sensor system and those by conventional vibrating-wire strain gauges. Whereas conventional strain measurement gauges monitor the strain variations at discrete locations, a BOTDR strain sensor can provide a continuous strain distribution of the tunnel lining. The results demonstrate the practicality of using the BOTDR strain-sensing system to monitor the movement of tunnel linings.