Sprayed Concrete Lining
After successfully testing the direct embedment of the MOS sensor cables in sprayed concrete lining (SCL), MOS have directly embdedded their GFRC sensor cable into the lining of new tunnel walls. The FBGs have provided information on the curing of the concrete which allows an informed decision on whether the concrete is strong enough to proceed with further boring, and also as a structural health monitoring tool when the tunnel is located in proximity to geological features, other tunnels or critical infrastructure. The largest installation MOS undertook was at Farringdon Station where sensor cables were used to monitor the structural integrity of the tunnel when other tunnel operations were undertaken. Monitoring was also conducted with pressure cells and survey prisms, and the contractor carried out an independent review of all three technologies, where the sensor cable option was shown to be very effective.
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Retrofitting Tunnel Walls
MOS embedded their GFRC sensor cable into the wall of the Holme Rail Tunnel in the UK to monitor the effect of soil movements on the tunnel walls. The sensor cables were installed into saw-cuts in the tunnel wall at three locations, with seven FBGs on each sensor cable, and were embdedded using an additional lining of sprayed concrete. All sensors were embedded sucessfully and as they are based on fibre optic technology, there is no risk of interference with the rail's electrical signals.
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