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Tunnelling: All seeing eyes

As London Underground’s Bond Street station upgrade moves into the tunnelling phase, the scheme’s monitoring system is coming into its own. Claire Smith reports.

There is no doubt that the work to upgrade London Underground’s (LU’s) Bond Street station is a complex task, with piling, tunnelling and compensation grouting all needed to deliver a larger station by the 2017 deadline.

With much of the work being undertaken within a live station, close to Oxford Street’s shops or near to sensitive buildings - all of which must remain in use - the task is also a high profile one.

Given the scale and location of the project it is hardly a ­surprise that the monitoring work being undertaken at the site is a key part of the project.

The project is using almost every type of monitoring equipment that supplier Soldata offers, with the exception of electronic crack meters, according to Frances McDonald, who is survey and monitoring manager for the Costain Laing O’Rourke joint venture responsible for the upgrade work.

While Soldata manages the monitoring system, some of the equipment was installed by LU ahead of the contractor’s appointment in 2010.

“One of the good things about this project is that London Underground installed around 15% of the monitoring before the contract was procured, so there is a lot of background data,” says McDonald.

“The existing network was mainly installed on LU assets and includes automatic theodolites and prisms and electrolevels on escalators.”

According to Soldata project manager Ed Avery, the mix of client and contractor procurement for monitoring has worked well at Bond Street. “In many situations the contractor becomes involved too late because clients view monitoring as straightforward, but that is not the case,” he says.

“The monitoring system at Bond Street is probably the best I have seen, not in terms of just the equipment, but in the way the data is being used”

Ed Avery, Soldata

“I have worked on a large range of monitoring projects and the monitoring system at Bond Street is probably the best I have seen, not in terms of just the equipment, but in the way the data is being used in terms of collaboration between the contractor and client.”

The system developed includes 17 of the firm’s Cyclops robotic total stations, including 10 underground, to provide data on the construction of the new Crossrail link passage, tunnels, platforms and adjacent buildings. The network also includes the first major UK use of its Centaur system, which uses lasers on horizontal surfaces rather than conventional prisms.

Such a complex network of instrumentation is vital to the project, says McDonald, as Oxford Street is too busy an environment for conventional surveying methods. “The system provides information to the project team 24 hours a day,” she says. “We carry out physical surveys once a week to check the information the system is delivering.”

The data being integrated into a single software program - called Geoscope - isn’t just collected from Soldata’s and LU’s equipment: the system is also gathering monitoring information from water cells installed by G-Tec and laser data from instrumentation installed on escalators by Kone. The Geoscope software is also now collecting data from strain gauges installed in an LU “smart step” on escalators since wi-fi has been built into the system.

“Before, LU had to physically download the smart step data, but now we are providing them with alerts,” says McDonald.

“Escalators are very sensitive to movement, but even when there is no construction work underway, they do move as a result of thermal changes. The background data that LU has from the Bond Street area has helped us to identify what is normal and what is unexpected. The background data adds confidence to the information our instrumentation is collecting now construction is underway.”

Bond Street’s 175,000 daily visitors

Bond Street station opened in 1900 and is now a busy interchange between the Central and Jubilee lines, as well as a busy entry and exit station.

The existing ticket hall was built in 1975 and, although there are five escalators in total, only three serve the Jubilee Line and there is only one exit, which can cause issues for passengers getting off trains at Bond Street platforms as well as exiting the station.

The station now handles around 175,000 passengers a day, with the normal morning and afternoon commuter peaks, but the high level of tourists visiting the area means that the station is busy throughout the day and passenger numbers will grow when the new Crossrail interchange opens in 2018.

London Underground’s upgrade includes constructing 350m of new foot tunnels, which will be excavated mechanically and by hand depending on their location.

A new underground ticket hall on Marylebone Lane with two new escalators and four lifts to provide step-free access is also being built, and the station entrance on the north side of Oxford Street is being modernised.

According to Avery, Soldata’s close working relationship with Costain Laing O’Rourke and its compensation grouting contractor Bachy Soletanche has enabled the use of a number of new systems at Bond Street.

“In addition to the Centaur system, we are also using Gorgone to carry out environmental monitoring for noise, vibration and dust, which is being collated by the Geoscope software,” he says.

The investment in instrumentation at Bond Street has been high, but is already paying off, according to Avery and McDonald.

Monitoring during piling work in 2012 showed more movement than was anticipated, with around 15mm of settlement and tilts of 5mm to 10mm, but these were well within the design limit of 25mm. Halcrow and Atkins, which are jointly working on design for the scheme, verified that the movement was acceptable.

“The monitoring showed that the settlement stopped after the piling stage was completed,” says Avery.

Excavation of the shafts did not result in any surface settlement, but some LU assets were affected, with the Central Line escalators showing movements of up to 5mm.

 

Bond Street isometric

Bond Street Station overview

The focus is on the monitoring enabling the construction work, rather than holding up progress, according to ­McDonald.

She points to the breaking out of the floor slab at the new northern entrance site as an example. “There were concerns about the vibration resulting from the break out of the slab, which was more heavily reinforced than initially thought,” she explains.

“Monitoring showed that the vibration levels were within set limits so work could progress more quickly.”

The site team hopes that the vibration monitoring will help to speed up work on the tunnels too, now that the sprayed concrete lining (SCL) work is approaching the point of breaking into existing tunnels at the station.

“Ahead of this stage we have carried out analysis of the breaker equipment and looked closely at the tolerance of LU’s equipment,” says McDonald.

So far the SCL work has had little impact on the existing network, with only very small movements recorded.

“The biggest changes we have recorded were around 5mm to 6mm on one escalator,” says McDonald, who clearly believes that the system is correctly configured to give the site team the confidence to push ahead with the next phase of tunnelling.

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