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Deep Excavation: Undercover upgrade

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Careful planning means that the 90,000 people who enter or exit Bank Tube station in London’s morning rush hour are probably unaware of the expansion work being undertaken just metres away.

Bank Tube station is London’s fourth busiest Underground station, but the busiest interms of interchanging passengers – and passenger numbers are rising faster than predicted just five years ago.

Every one of the 52M passengers that interchange there each year knows the navigational challenges of the labyrinthine layout that connects four major Tube lines.

Work is underway to create more space and improve the interchange between the Northern, Central, Waterloo & City and Docklands Light Railway lines but most passengers will not yet be aware of the change.

Contractor Dragados has worked closely with London Underground to undertake most of the tunnelling work away from the public eye, with breakthroughs not scheduled until closer to the finishing date.

“We are building 600m of new tunnel to form a new station box and running tunnel, new ticket hall and connecting passageways to create step free access to all but the Central Line,”says Bank Station capacity upgrade programme manager Andy Swift.

Bank Station

Bank Station

All the tunnels are being formed using sprayed concrete lining techniques

“Once completed, the existing northbound tunnel will become a new concourse and disused sections of tunnel will become essential storage areas.

”The aim of the work is to increase capacity and decrease journey time through the station for passengers that use it as an interchange by providing more space and more direct routes.”

The design and build contract was awarded to Dragados in July 2013 and there was a three year planning, design and consent process to achieve the Transport & Works Act consent for the scheme in 2016.

Going underground

Work on site started in 2016 with sinking of a shaft in Arthur Street to provide the access for all the tunnelling work.The shaft is formed by a steel sheet pile box through the River Terrace Gravels and made ground. Once into the London Clay, the shaft is supported by a sprayed concrete lining (SCL) which allows it to widen into an oval shape in plan as it extends.

“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 inposition.

“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.

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Bank Station

From the bottom of the shaft, the tunnel is being driven through the London Clay using mining techniques and SCL.The running tunnels 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 the 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. Space is at a premium, but with tunnelling underway 24 hours a day, a temporary tunnel has been driven close to the main shaft as a temporary store for the excavated material.

The project team is also making use of part of the Tube’s heritage to overcome space restrictions. It is using the platform tunnels for the former King William Street Underground station, which closed in 1900, for pedestrian access to the tunnel and for storage. The former station is located part way down the shaft and had to be partially backfilled with foamed concrete to allow the shaft to be constructed through it.

Lack of space on site has also driven use of dry pre-mixed shotcrete, which is stored in two 100t silos at the top of the shaft and mixed on site as needed.

Initially it was piped to the working face, but as the tunnels have moved away from the shaft, re-mixers have been used to shuttle material to where it is needed. Work from the cavern started with a 30m back shunt, which is currently being used for trials of the permanent lining, before the drive along the new platform tunnel started.

Further challenges

According to Swift, the main challenge so far was actually getting the shaft down to the construction level at 40m below ground. Tunnelling is now 35% complete and going well, but creating the access shaft is not the project’s only challenge. Swift says there are more to come – mostly from existing piles.

“There is a complex pile intercept with some large diameter underreamed end bearing piles, which support 6-8 Princes Street which was built in the 1980s,” he says. “Ironically the pile design was selected to avoid putting pressure on the Northern Line.”

This is the first time end bearing piles have been cut off in this way and a complex load transfer platform will be constructed from tunnel level to support these piles.

“We have already intercepted a number of skin friction piles, but this is a first for end-bearing piles. We are expecting to intercept four piles but it could be up to eight,” explains Swift.

“We looked at undertaking the work from the basement of the building, but doing it from tunnel level will be less disruptive and the building will remain in occupation throughout. We will sequence the work overnight and at weekends to avoid noise issues.”

This work will start in summer 2018.The other challenge will be to link the new infrastructure with the existing. At the moment the construction work is mostly isolated from the existing station and the project team is trying to keep it that way for as long as possible.

Consultations are underway regarding the closure of the station over a period of several months in summer 2021 to allow the connection work to be undertaken more quickly than would normally be possible using weekend closures.

Nonetheless, such as closure still does not remove the challenge of breaking through into Victorian-era cast iron tunnels. But if all goes to plan, commuters will see a real difference when the work is completed in late 2021 or early 2022.

The new ticket hall

Arthur Street is the focal point for the tunnelling work, but just to the north on King William Street, on an area known as the Whole Block Site, work on excavation ofthe box for the new ticket hall is already starting. 

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Bank Station - new ticket hall

Keltbray formed a secant piled wall for the station box

 

This will create a new entrance on Cannon Street and is key to improving passenger flows and access with escalators and lifts down to running tunnel level.

Existing buildings on the site were cleared last year so Keltbray Piling could install a secant pile wall to form the retaining walls for the ticket hall.

The box is formed by1.2m diameter piles that extend mostly to 40m to take the box past the level of the Docklands Light Railway, but one section steps down to 50m to accommodate the lift shafts.The secant wall is formed by 84,1,180mm diameter male piles and 84, 750mm diameter female piles.

According to Keltbray, some of the piles were installed within 2.7m of the live Northern line running tunnels. Four 1,200mm diameter piles were also installed to support a tower crane, and two hollow piles were constructed with a 24m long spliced liner to take services down to tunnel excavation level. The box is being excavated using top down techniques and the pile cap is now under construction and the first slab about to be cast.

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