Monitoring of Sprayed Concrete Lined Tunnels Using Fibre Bragg Grating Sensors

This paper outlines the use of Monitor Optics Systems’ (MOS) Fibre Bragg Grating (FBG) sensor cables for convergence monitoring in tunnels made from sprayed concrete lining (SCL). The Crossrail project is currently installing the new Elizabeth line into the London rail network, and at Farringdon Station, the line will connect to both the London Underground and the Thameslink lines. Due to several of the new tunnels being in close proximity, as well as existing infrastructure and challenging geotechnical conditions, the decision was made to monitor the critical “RTE2” tunnel to ensure it was performing as designed. Pressure sensors and survey prisms were initially selected to monitor the tunnel’s critical locations. However, as pressure sensors had a reputation for unreliable results due to difficulties with their installation, an additional method was sought out to validate the pressure and surveyed results. Following their successful use in a tunnel at Bond Street Station, MOS sensor cables were selected to monitor the tunnel in conjunction with the pressure sensors and survey prisms.

MOS sensor cables that incorporate a nylon coating are designed to survive direct embedment into the SCL of a tunnel and can provide real time monitoring of the lining immediately after embedment. Sensor cables with five FBGs were installed at two locations in the RTE2 tunnel, along with corresponding temperature sensor cables. The FBGs were monitored using an optical interrogator located outside of the installation area through the use of fibre optic signal cables. Data was available for visualisation and manipulation through the MOS web hosting site DaMiNs.

FBGs, survey prisms, and pressure sensors were located at the same locations, and the FBG results generally showed good agreement with the surveyed results. The majority of the pressure sensors were unable to capture reliable results but were also in good agreement with the FBG results where reliable data was captured. The monitoring results validated the tunnel design and allowed additional tunnel construction to continue without additional unnecessary concrete linings.