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Tunnelling: Heading for halfway

The Bank Station Capacity Upgrade project team have cause for celebration with the halfway point reached and tricky pile intercepts completed. 

pile intercept

pile intercept

Swift: The pile intercepts were one of the most challenging elements of the project

Passengers who voted London Underground’s Bank Station as the worst on the network will be pleased to know that the £607M project to add capacity and improve interchange is now 50% complete. However, Transport for London Bank Station Capacity Upgrade project programme manager Andy Swift says that great effort to plan and phase the work means that few passengers will be aware of the project, let alone its milestone.

Work on the project started in 2016 and is not due for completion until 2022 but this last year has seen some major progress on site with 80% of the sprayed concrete lined tunnels complete and intercepts with piled foundations successfully undertaken.

Swift presented the latest progress on the project, as well as the overall aim and drivers for the scheme at GE’s sister title NCE’s Tunnelling Festival earlier this month.

Bank is the fourth busiest tube station on the London Underground network with interchanges between the Dockland Light Railway, Northern Line, Central Line and Warerloo and City. The completed upgrade will feature a new ticket hall on Cannon Street – currently known as the Whole Block Site, a 600m long new south bound running tunnel and platform on the Northern Line, a 95m long moving walkway and numerous new cross passages and use of the space occupied by the former southbound platform as a concourse.

The aim of the work being undertaken by Dragados is to increase capacity, minimise journey times and provide step free access.

“The project involves construction of 1,214m sprayed concrete lined (SCL) tunnels below 70 listed buildings that have resulted in 21 separate legal agreements with land and building owners,” says Swift.

The work is being undertaken from two sites – the new concourse is being built by top down techniques from the Whole Block Site and the tunnelling from a temporary access shaft within Arthur Street.

“The shaft at Arthur Street is small considering the scale of the tunnelling work and one of the team has described it as being like decorating your front room through your letter box,” says Swift.

The shaft is formed by a steel sheet pile box through the River Terrace Gravels and made ground and, once into the London Clay, the shaft is supported by sprayed concrete lining (SCL) which allows the shaft to widen into an oval shape in plan as it extends.

tunnel progress

tunnel progress

The project has reached the halfway point with 80% of the sprayed concrete lining tunnels completed

“Arthur Street is very narrow so there is small envelope to get underground but once we were below the street level, and ground conditions changed, we were able to bell outward to enlarge the work area,” says Swift.

The excavation extends under 33 King William Street and the team has installed jacks within the basement to keep the building in position.

“It has hardly moved though – we were anticipating up to 6mm of movement – but it has actually only moved by 2mm,” adds Swift.

There are four jacks under the two main columns – one each to deal with horizontal and vertical movement. The jacks are sacrificial and will be locked off and cast in at the end of the work.

From the bottom of the shaft the tunnel is being driven through the London Clay using mining techniques and sprayed concrete lining (SCL).

The running tunnels at 40m below ground level have a 4m diameter, while the platform tunnels have an 8m diameter and the link to the Central Line will have a 10m diameter.

The excavation is mostly full face, except below buildings where it is undertaken in two stages – heading and invert – to ensure additional control and minimise ground movement at surface. The project is operating three eight hour shifts per day and the teams are achieving up to five advances of 1.2m per day.

Despite the access challenges, the tunnelling is now four fifth complete.

whole block progress

whole block progress

On the Whole Block Site, the station box has reached structural completion with the base slab in place

“On the Whole Block Site, the station box has reached structural completion with the base slab in place,” says Swift. “All the slabs are currently hung from a temporary 100t truss which has been jacked at either end and will be removed once the lift shaft construction has been completed.

“It has called for very complex temporary works.”

At the northern end of the tunnelling, the line of the tunnel intercepts a number of large diameter underreamed piles that support a building above the site.

“Work on the pile intercepts was one of the most difficult aspects of the job,” says Swift.

“We looked at undertaking the work from the basement of the building but doing it from tunnel level was less disruptive and the building remained in occupation throughout.”

It is believed that this is the first time that end bearing piles have been intercepted in this way.

The tunnel route was planned carefully to minimise the impact on buildings above but below the building on Princes Street there was no other option. A load transfer structure had to be built to transfer the loads of the piles around the tunnel and enable the piles to the drive through.

“Ironically the original pile design was selected to avoid putting pressure on the Northern Line,” says Swift.

“The tunnel threaded through most of the piles but still intercepted four and exposed the side of piles at other locations too.

load transfer structure

load transfer structure

A load transfer structure was built to support the piled foundations

“The building moved 5mm in total and the work has been successfully completed but the building owner was very nervous ahead of the work.

“We built a substantial concrete structure to transfer the load around the tunnel and into the ground below. This ensures the building is not affected by the tunnel and will allow for future redevelopment of the building too.”

Despite the progress, there are still challenges ahead. The team still needs to tackle the construction of three escalator barrels and a number of cross passages.

Originally the cross passages were due to be constructed during a blockade of the station but the project team has brought the work forward to reduce the risk of delays on the scheme.

Swift says that despite the re-phasing of the work the public is oblivious.

The project team has also looked to use innovation solutions, including top down construction from a temporary shaft for some of the cross passages so that the work can be undertaken without impact on passengers.

“It allows closures to be limited and to undertake the work earlier,” says Swift. “It is very intricate.

“Nonetheless, there are still areas where we have to undertake conventional square works where there is limited space. There is lots of innovation on the project but traditional techniques are still needed.”

So far the work has remained hidden from public view and it looks set to remain that way for some time yet with closure of the station not scheduled until 2021 to allow faster connection of the new tunnels. Nonetheless, the closure does not remove the challenge of breaking through into Victorian-era cast iron tunnels though but if all goes to plan then passengers will see a real difference when the work is completed in three years’ time.

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