The monitoring of an existing cast iron tunnel with distributed fibre optic sensing (DFOS)

Instrumentation is vital to tunnelling projects for validation of design assumptions, monitoring of trigger levels during construction and also serving as a feedback loop to understand the deformation mechanisms of the particular problem which could be used to improve future designs. It can also be used for long term monitoring of the tunnel for maintenance purposes. Distributed fibre optic sensing (DFOS) systems based on brillouin optical time domain reflectometry are able to provide continuous and distributed strain measurements to be taken along the entire length of, for example, an existing cast iron tunnel where the fibre optic cable length is fully attached. This enables engineers to understand the stresses and strains that develop within the lining caused by external influences; which in this case, the construction of a new tunnel directly underneath it, rather than relying on discrete point measurements of displacement from conventional methods of monitoring. Nonetheless, proper installation of DFOS is of paramount importance to obtain high quality data. This paper aims to provide some practical guidance on the planning and installation of DFOS and presents a brief case study on the monitoring of London’s Royal Mail tunnel during the construction of the large Crossrail platform tunnel, located directly below it at Liverpool Street Station. It demonstrates the potential benefits of using such systems in complex tunnelling scenarios.The authors would like to gratefully note the contributions in the form of logistical and technical support from Crossrail (particularly Mike Black, Stephen Roberts, Alison Norrish and Chris Dulake), Royal Mail (particularly Mitch Harris), ARUP (particularly Mike Devriendt), CH2 M Hill (particularly Peter Wright). This project would not have been possible without the financial support from Laing O’Rourke for the first author’s PhD studentship as well as the continuous support from the UK Engineering and Physical Sciences Research Council (EPSRC) and Innovate UK through their funding of the Centre for Smart Infrastructure and Construction (CSIC) at Cambridge.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s13349-015-0109-