Constructing tunnels in highly congested urban cities is a challenge, as the construction willinevitably take place in close proximity to existing structures both above and below ground. As more tunnels arebeing constructed, it is inevitable that some tunnels will be constructed in close proximity to existing tunnels andmonitoring of the existing tunnels in this case is paramount for the design and construction works. The discrete natureof conventional monitoring instrumentation requires significant interpolation and judgement in order to understandthe overall behaviour of the tunnel itself. Subsequently, conservative design approaches will have to be adopted tocater for the gaps in knowledge, which could lead to unnecessary delays and high costs. Distributed fibre optic strainsensing systems based on Brillouin Optical Time Domain Reflectometry (BOTDR) could offer an alternative; in thispaper a case study is presented where fibre optic cables were deployed to monitor the response of the cast iron RoyalMail tunnel in the vicinity of London Liverpool Street Station during the construction of Crossrail’s new platformtunnel directly below it. Single mode single core tight-buffered cables were attached directly to the intrados of thecast iron tunnel lining of the Royal Mail tunnel to understand its response during the construction works. This paperfocuses on the challenges and considerations in deploying the fibre optic system in the tunnel and presents some ofthe data, which demonstrates the potential benefit of using such a system in real, complex tunnelling scenarios.
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