Image: CSIC's Smart Tunnel
Hundreds of kilometres of London’s tunnel and pipe infrastructure are made of cast iron and many London Underground tunnels are now between 50 and 150 years old. Better assessment of their condition has become an increasingly significant issue for maintenance and calculation of residual life.
Currently new tunnels are being built close to London’s ageing tunnel network. CSIC Industry Partner, Crossrail, is in the process of building a £15 billion subterranean rail link crossing the city over a distance of around 21km. This project has presented new engineering challenges – never before have new tunnels been dug in London so close, parallel and perpendicular to existing tunnels over such a long distance.
Crossrail engineers faced uncertainty about the likely modes and levels of deformation of existing tunnels. In this case, no one technology could provide the required mode of deformation information for Crossrail. CSIC combined a selection of devices to collect the data.
As cities grow, there will be a pressing need to build new structures close to existing pieces of infrastructure, both in the UK and abroad.
Working with Industry Partners Arup, CH2M HILL and iMETRUM, CSIC deployed a team of researchers to install four different pioneering monitoring devices at specific locations inside a 40-metre stretch of the disused, 100-year-old Royal Mail tunnel, at Crossrail’s Liverpool Street Station, where the new tunnel was being constructed parallel beneath it.
The same technologies were also deployed at London Underground’s Bond Street Station Upgrade project, where a new tunnel is currently being excavated by the Costain/Laing O’Rourke joint venture, perpendicular to and actually touching the Royal Mail tunnel above.
The latter conditions presented an opportunity for CSIC to measure the performance of an existing tunnel in an extreme engineering scenario and data is currently being collected and collated.
The monitoring devices deployed include:
• wireless sensor network (WSN) displacement transducers
• fibre optic strain sensors
• wireless linear potentiometric displacement transducers (LPDTs)
• photogrammetric monitoring
CSIC’s bespoke combination of different devices, featuring existing instrumentation currently used by industry and new image correlation techniques, including CSattAR Photogrammetric Monitoring – a new digital image correlation (DIC) technology – captured the various strains and modes of deformation and, in addition, provided valuable comparative data.
This combined instrumentation created a unique ‘Smart Tunnel’ capable of measuring and monitoring the structural performance and stress levels of the older tunnel as the new, large Crossrail tunnels were excavated immediately below.
Impact and benefits
The impact of CSIC’s Smart Tunnel monitoring has delivered measurable benefit to asset owners:
• the digital image correlation technique measured tunnel movements at sub-millimetre scale using digital images taken at different times
• the joint movement and bolt strain measurement techniques provided new insights in terms of joint movements, which have never been measured in such detail before
• the data from the fibre optic distributed strain measurement gave the overall distortion of the tunnels
• the wireless sensor motes measured tunnel movements in three-dimensional directions
• these insights deliver valuable, accurate and validated information to the asset owner enabling better-informed decisions on the effects of new excavation on existing structures
• the combination and deployment of CSIC’s advanced sensing technologies allows us to understand the engineering performance of cast iron tunnels, which exist in many parts of London
• the knowledge gained from this project will be shared with asset owners and industry to assess other cast iron tunnels in London, to build safe and long-lasting tunnels for the future and, ultimately, deliver benefit to clients, contractors and tax payers