Peter Kenyon, TunnelTalk

Most of the media headlines surrounding the Crossrail project in London focus on progress of the TBMs, ongoing station construction and recent completion of one of the largest crossover caverns in Europe at Stepney Green. What is less well known, is that the £16 billion project is about to serve as a vital proving ground for new wireless and fibre-optic sensory technology that will 'revolutionise' ground movement and structural integrity detection methodologies and give infrastructure owners real-time life-cycle data on the internal condition of structures as well as providing readings that could actually give clues as to how to improve the construction processes themselves.

Fibre optic sensors installed in London cable tunnels

The claim of revolutionary potential is made all the more valid in that it is made by Professor Robert Mair, the renowned tunnel engineer whose pioneering work on compensation grouting was applied successfully to ensure the iconic London landmark, Big Ben clock at the UK Houses of Parliament, survived excavation of the Jubilee Line Extension of the London Underground.

As Professor of Geotechnical Engineering at Cambridge University since 1998 and a member of the five-man Crossrail expert advisory panel, Mair has long portfolio of achievements and his expertise on soft ground tunnelling is in high demand.

Mair is most excited about his work on developing so-called smart infrastructure as part of a project on which he is Principal Investigator, and which is supported by £17 million in funding from the Engineering and Physical Sciences Research Council, the Technology Strategy Board, and a host of private companies with an interest in industry application of the new technology.

One of those partners, UK tunnelling contractor Costain, is already working with the research team on testing, in a live demonstration project, new fibre-optic sensory equipment that is embedded inside the individual segments of cable tunnels it is currently excavating under London for the National Grid, and which will be able to relay information about even the most minute changes occurring from inside the infrastructure itself.

TBM Sophia will erect segments featuring embedded fibre optics sensors

The new, embedded, fibre-optic sensors represent an advance on the research project's own surface mounted system that was tested effectively on a section of the Singapore Circle Line that, at the time, was raising particular concerns for the project owner. As an expert adviser to the LTA owner organisation, Professor Mair was quick to spot an opportunity to put a prototype version of fibre-optic sensory technology to the test, and two members of the Cambridge University research team were mobilised to carry out the necessary work.

"There were two new tunnels to be constructed alongside each other: the first one was completed but inclinometer readings in the ground close to it revealed a significant amount of movement. There was a lot of concern about what effect construction of the parallel tunnel, which was going to be very close to the first tunnel, with a clear separation of only a few metres, was going to have on the earlier excavation, and so we installed, on the segment surface, optical fibre to monitor the performance of the first tunnel, and this proved very successful," said Professor Mair.

Singapore Circle Line site used surface mounted fibre optic sensory technology

But it is the first tunnel drive for Crossrail under the River Thames by the Hochtief/Murphy construction joint venture under Contract C310 where all eyes from the research team are now focused, as the very latest generation of the technology is put to the test.

"This is a very exciting area in sensor innovation, and one where we have now developed the technology for attaching fibre optics to the reinforcement cages inside diaphragm walls, or in piles, or even within tunnel segments as they are manufactured in the factory. A major impetus behind our project is to be involved in demonstration projects where we are using these technologies on real, live, construction sites and testing them out to develop them into commercially viable technologies," explained Mair.

Fibre optic cable attached to lining segments in Singapore

On the Crossrail project in London, the segments that line the 2.6km running tunnels under the River Thames towards the eastern terminus at Abbey Wood have been manufactured with fibre optic cabling and sensors embedded within them, and these will be installed when the first of two tunnel drives under the river gets under way after recent breakthroughs into the Woolwich junction box earlier this year (2013).

"The fibre-optic cable is attached to the reinforcement cage of a segment which then gets concreted in while in the segment mould, and there will be a little junction box so when the segment ring is erected in the tailskin of the TBM you then just snap across a connector and you have then got a complete fibre-optic both on the extrados and the intrados of the installed ring of segmental lining. It is going to revolutionise our understanding of how tunnel segments actually perform," said Mair.

"The internal fibre-optic connections, those set inside the segments themselves, will be clipped manually via a junction box as each ring is built. All tunnelling crews will have to do is clip something together and the fibre-optic circuit will then be connected to an analyser. This is work in progress, we are about to be doing this for Crossrail and have already done some work with Costain on this in their National Grid tunnels under London. It is too early to say at this stage how the system will perform, but the technology looks very promising."

So what exactly is 'smart infrastructure', and what are its benefits for the tunnel industry?

Segments awaiting use for Thames crossing precast with internal sensors

"It is all about transforming the construction industry by using sensors in a much more innovative way, and for two reasons," explained Mair.

"First, to actually improve the construction process itself by making the right kind of measurements and actually finding ways of building it more efficiently, faster, and with more confidence, and secondly to actually do it from the point of view of the completed product so that once one has a completed tunnel, or whatever kind of infrastructure it might be, if you have the right kind of sensors embedded within it they can tell the owner from there on afterwards how it is actually performing. For example, are there any issues; are there any cracks opening up; is it performing, is it moving?"

Aerial view of the Thames crossing route

"If you ask most tunnel designers 'how is your structure actually performing five or ten years on?', unlike the aeroplane designer or the car manufacturer, who already use wireless technology to give real time data, they are not able to tell you, other than through a visual inspection. Rolls Royce, for example, can monitor wirelessly the internal condition of its aerospace engines at any time, no matter where in the world."

"The impetus behind our project is that there has been this revolution in sensor technology, and there has been a wireless revolution as well. We take smart phone-type wireless technology for granted, but what we are working on and getting very close to, is having a small sensor that might be the size of a matchbox that could be fixed to the side of a tunnel lining or to an installed diaphragm wall, taking readings and beaming a signal wirelessly to a central gateway.

Professor Robert Mair

"One of the limitations in all of this is batteries. One of the things we are working on is 'power harvesting' and we are getting quite close to having a sensor that will be able to harvest the energy it needs from vibration, if it is inside something that is being vibrated, like a tunnel lining with trains going through it. It can take a reading without a battery and that is another aspect to what we are working on."

The research team is currently collaborating with companies in the sensor industry on various aspects of wireless applications, but because there are currently none among them developing applications for fibre-optic technology, Cambridge University, as lead partner in the Cambridge Centre for Smart Infrastructure and Construction, has had to establish its own companies to explore ways of delivering a market-ready solution.

The project is a clear demonstration of academia and industry working together to deliver technological innovation, and Professor Mair is proud of what has been achieved so far. Ultimately the technology may bring some revenue benefit to the University, but this is not the driving force. "The real interest for us is seeing our research transferred into practice. Just how that happens and who gains commercially is up in the air at present but we would like to see British industry taking up these new technologies and I think there is every chance that will be happening."

References

Cambridge researches smart infrastructure - TunnelTalk, December 2010
Robert Mair delivers Helsinki WTC2011 Muir Wood lecture - TunnelCast, May 2011

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