As London Underground’s Bond Street station upgrade moves into the tunnelling phase, the scheme’s monitoring system and the team that operates it are coming into their own
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 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 by Soldata is a key part of the project.
“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”
Frances McDonald, survey and monitoring manager, Costain Laing O’Rourke
“The project is using almost every type of monitoring equipment that Soldata offers with the exception of electronic crack meters,” says Frances McDonald, who is survey and monitoring manager for the main contracting joint venture Costain Laing O’Rourke. “But I hope that those will not be needed.”
While Soldata manages the monitoring system at Bond Street, some of the equipment was installed by LU ahead of the appointment of Costain Laing O’Rourke 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.
“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 and integrated by Soldata includes 17 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 as McDonald says that 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 by Soldata into a single software program – called Geoscope – isn’t just collected from its own 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. Soldata’s Geoscope software is also now collecting data from strain gauges installed in an LU “smart step” on escalators now 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.”
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.”
The investment in instrumentation at Bond Street has been high and it is already paying off, according to Avery and McDonald.
Monitoring during the 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 limit of 25mm. Halcrow and Atkins are jointly working on design for the scheme and 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.
Settlement from the work was always expected to be in the slight category, according to analysis carried out by Halcrow and Atkins using Oasys’ Disp X software.
Data from the Geoscope system is now being used to guide the active compensation grouting via the 100 tubes à manchette installed by Bachy in a shaft next to the new northern ticket hall site to offset any movement.
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 says. “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 too,” 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,” adds 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.