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Future Technology: Movement control

A project to monitor and mitigate the effects of Crossrail tunnel boring machines under a viaduct in Canning Town, east London could mark a new departure in the equipment hire business model.

Considering the size of the Crossrail project, it was always inevitable that it would act as a catalyst for pioneering construction techniques.

But perhaps its greatest contribution to engineering excellence is the advances that have been made in sensing and monitoring approaches during the excavation of the biggest hole in London.

As the machines move further to the east of London, a new project is again demonstrating how monitoring equipment can give engineers and asset managers greater levels of assurance about their structures. For equipment company Mabey Hire, the project also marks a step-change in the way it does business with its clients.

Mabey projects manager David Holland describes how the company used over a hundred instruments to monitor the Canning Town Flyover - a 330m long multi span viaduct carrying the Docklands Light Railway (DLR).

Crossrail tunnel route

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The structure was temporarily lifted by a jacking system to mitigate the effects of the tunnelling happening underneath.

“Nothing is particularly novel about the sensing equipment we used on the project,” says Holland. “The novel thing is the way we’ve applied it. Together with the novel approach to the design of the monitoring system, what we did was put complete redundancy in the system so if any part of it went down the system would still keep logging the highly accurate data utilising alternative means,” says Holland . “This was critical in the case of the Canning Town project because the viaduct in question is a main thoroughfare for DLR trains “.

“What we did was put complete redundancy in the system so if any part of it went down, the system would still keep going”

David Holland, Mabey

Holland explains that sensors were employed to gather data about specific parts of the structure and to interrogate the other sensors on site to check the veracity of their data.

“This is the really clever part of the software,” he says. The system continuously assessed the data against a number of sanity checks which meant programming over 750 virtual sensors to allow for the required self interrogation intelligence. Every hour, a health assessment was automatically sent to all involved providing a full assessment of the data providing complete assurance to the client. The project team also had to distinguish between the effects of the tunnelling and a mix of external natural factors that affected the behaviour of the structure. This included the fact that the bridge is situated next to the River Lea and, as such, is subject to the tidal effect of the river and variations in temperature which caused vertical, longitudinal and transversal expansion and contraction.

“The graphical medium on which the data was displayed meant the trends associated with natural behaviour were easily identifiable. We had a series of warning lights that indicated the degree of seriousness of any deviation,” says Mabey contracts director Chris Carter.

Throughout both tunnel drives under the viaduct the deck was maintained successfully to accuracies of sub millimetre with relation the vertical differentials, twist and cant.

“We had a series of warning lights that indicated the degree of seriousness of any deviation.”

When the project team assessed the tunnelling works, it calculated what the settlement would be based on a 1% face loss. As it transpired, the settlement was a third of what was expected.

“The monitoring showed the bow wave as it [the tunnel boring machine] went underneath, it was that accurate,” says Holland.

“Because the tunnel was not directly below the bridge you didn’t get a vertical, linear settlement of the pier, you’d get quite an eccentric sort of settlement - a sort of rotation and tilting of the pier, so we had to keep the deck in the same place in space and allow the piers to rotate. We had transverse jacks on every pier which gave that transverse control and then on the fixed pier we also had longitudinal jacks which gave the longitudinal control, so as the sensor warnings went off we had to mitigate it by adjusting the jacks to bring the deck back in line to a virtual reference line so it maintained the same profile as before.”

“What we’re looking to do is link up our understanding of structures and jacking with the ability to measure their behaviour and present it to engineers in real time”

Chris Carter, Mabey

For the Canning Town project, the client had the option of using an automated jacking system that would correct the effects of the tunnelling on the viaduct as they happened. But Carter says the project team preferred to take the time to digest the readings from the sensors before deciding on the best course of action.

“The job was completely sub-contracted to Mabey,” says Carter. “We designed it, we serviced it, we provided all the equipment and we were on call 24/7 with the client’s engineering team so that we could react to anything that happened.”

Off the back of the project, the company has launched a new division - Mabey Hire Instrumentation - to seek out additional opportunities in providing real-time monitoring of structures.

“What we’re looking to do is link up our understanding of structures and jacking with the ability to measure their behaviour and present it to engineers in real time,” says Carter. “Canning Town was the first major project where we used the technology in this way, but the number of potential applications is almost limitless.

“We’re looking at providing it as a hire-only solution or a bespoke turn-key system,” he says. “The data management software generates executable written files and you can opt for a high or low-frequency system depending on the application. Then you could have a consultancy service or a full sub-contractor service like the Canning Town project.”

Aside from allowing engineers to take a more informed approach to structural interventions, Holland thinks monitoring technology could also provide opportunities for value engineering. “Some famous modern buildings have been built with greater accuracy because of the assurances of the geophysical surveys,” he says. “If we have higher accuracy monitoring equipment we can start to take a less conservative approach to our buildings and take some of the redundancy out that the Victorians put in.”

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