Installation of Optical Fibre based Sensors within Sprayed Concrete Lined Tunnels to measure Strain and Temperature

Summary

The purpose of this paper is to summarise the practical applications and benefits of optical fibre based sensors over traditional monitoring systems installed within a sprayed concrete lined tunnel. The installation was undertaken as part of the Crossrail project at Bond Street station East ticket hall. The tunnel chosen for installation of the sensors was a concourse tunnel, which is one of the largest sprayed concrete lined tunnels at the station.
The optical fibre based sensor installed is based on the Fibre Bragg Grating strain sensing technique. These cables are designed to survive the installation in harsh environments with aggressive construction techniques like the use of sprayed concrete; whereas standard optical fibres would not survive this process. The cables provide real time structural monitoring with continuous and accurate measurement of strain and temperature values. This provided valuable insights into the mechanical behaviour of the tunnel lining during its early construction and post construction phases. The cables were fully encapsulated within the sprayed concrete lining of a concourse tunnel within the primary lining and at a thickened primary lining location.
The lessons learnt throughout the process have been recorded to aid future installations, as well as suggestions for improvements and further investigations.