Transport for London’s £655M project to upgrade Bank station into a spacious, modern interchange is almost complete.

Bank and Monument stations combine to make the third busiest interchange on the London Underground system, serving the Docklands Light Railway, Central, Northern, District and Circle and Waterloo & City lines.

In the heart of the City of London, Bank station is used by around 100,000 people at peak rush hour.

Traffic at the interlinked Bank and Monument stations has increased by 38% since 2008, with 120M passengers a year now using them.

Because of this, Bank – which opened in 1900 – has historically suffered from severe congestion and conflicting passenger flows underground, exacerbated by the small diameter of tunnels between lines and platforms.

Transport for London (TfL) has sought to address these issues with its £655M Bank Station Capacity Upgrade project, which will increase the capacity of Bank station by 40%.

The ambitious project involves creating a new southbound Northern line tunnel and a much wider southbound platform – like the generous Northern line platforms at Angel station. It also involves transforming the existing southbound platform into a new central concourse. As well as this the station will get 12 new escalators, two new moving walkways and three new lifts, plus a new station entrance on Cannon Street.

The improvements are expected to reduce connection times between the Central, Northern and Waterloo & City lines, and the DLR. It will also create step free access between the new lobby area, the Northern line and the DLR.

Dragados was awarded the main works contract by TfL in 2013 and started tunnelling in 2017. Dr Sauer & Partners has also been involved in designing the tunnels and shafts for the project.

The Dragados-led team finished the first phase of the project – the excavation, waterproofing and concrete lining of the new 1.5km long southbound running tunnel – in October 2020. During this phase, three new escalator shafts were excavated to link the Northern line to the DLR and to the new entrance. A new link tunnel to connect the Northern and Central lines was also excavated.

The second and more complicated phase got underway in January this year with the start of a 17 week “blockade”, during which Northern line services on the Bank branch between Moorgate and Kennington were suspended. Work carried out during this period, which ended in mid-May, involved connecting the new southbound Northern line tunnel to the existing southbound tunnel.

The north and south ends of the new running tunnel stopped about 1m away from the existing southbound Northern line running tunnel. As a result, tunnelling was needed to connect the ends of the new tunnel with the existing line. This involved creating a 55m long tunnel with sprayed concrete lining (SCL) for the south tie in and a 44m SCL tunnel for the northern one.

As part of this phase, the team converted the former southbound Northern line platform into a spacious central concourse connected to a wider platform for northbound trains via four new Northern line cross passages.

Once the tunnel connections were completed, Dragados constructed the trackbed and then installed the remaining 185m of track in the tunnels, in addition to the 490m that had already been laid before the closure. It then handed over to TfL so it could start testing and commissioning the signalling systems needed to bring trains into the new southbound platform.

Dragados is now delivering the fit out for the final phase of sectional completion works. This includes installing an escalator linking the Northern line to the DLR and installing an escalator connection to the Central line platform. The tunnel with the moving walkway, also part of this phase, opened in September while the station entrance is due to open later this autumn.

Logistics have been the biggest challenge for the project team during the blockade and sectional completion works. To create the new running tunnel the team constructed a 33m shaft on nearby Arthur Street at the south end of the station. Remarkably, more than 90,000m³ of tunnelling spoil and more than 3,300m³ of spoil from the blockade works was removed through a bell shaped shaft with an opening of 13m by 8m at the bottom and just 4.5m by 8m at the top.

“We had a gantry crane servicing the Arthur Street shaft,” explains Dragados tunnels manager Donal Kelly.

“We had it sized so that one skip, which was 11m³ fully loaded, could be lifted out of the shaft onto a tipping frame and discharged directly into a tipper truck on the surface, which we developed at the beginning of the tunnelling project.”

At the congested central London site, a one-way entry and exit system was devised to manage deliveries and the off-site transport of excavated material. The tight programme meant that tunnelling was carried out concurrently to the north and south.

But this posed another problem, explains Kelly. “Normally all the materials come to the shaft from one direction. But in fact, we were tunnelling in both directions, so there were materials coming from both sides.

“This meant if there was a lift taking place, plant and equipment couldn’t cross the shaft pit bottom. So, it was all about timing it so that we could lift the materials out without disrupting the tunnelling works.”

As the team was working in such a confined area for the tunnelling, space was “always at a premium”, notes Kelly. This was especially the case during the blockade where the team had to transport spoil and shotcrete through the new tunnel which already had a trackbed installed and therefore had a small envelope. “This meant the space that we had to store materials and to transport materials in and out was limited,” Kelly adds.

Because the Arthur Street shaft was used primarily for mucking out, Dragados brought all other materials, including those for the fit-out team, into the underground construction site via a weekly engineering train during the blockade works.

A key part of the blockade was converting the existing southbound platform into a new passenger concourse. To do this, Dragados backfilled the old southbound tunnel in three layers by stage pumping foam concrete from the surface down a hollow pile, and then along about 500m to the face of the foam concrete plug. Once the existing tunnels were backfilled, Dragados could start constructing the tie in tunnels.

Getting large plant into the tunnel was difficult. So, Dragados ended up doing “a lot of investigation to find the right plant”, Kelly says. The team conducted trials on site before the blockade to make sure that all the plant and equipment would fit. To free up space, it converted the new Northern line platform into a construction area to park the tunnelling equipment and store the materials for the blockade.

As well as this there were ground engineering considerations, Dr Sauer & Partners senior tunnel engineer Sebastian Kumpfmueller notes. The new tunnel has been excavated under 31 listed buildings which border the project, so there was extensive monitoring of surface assets during the tunnelling works and blockade works to ensure that ground movements were kept under control.

Monitoring was also needed during excavation of the shaft for the new escalators which was mined uphill because the Central line is on either side of the tunnel shaft at the top. In fact, there is only 500mm of lining between the Central line and the top of the escalator shaft.

Because of the proximity of these other assets, “a lot of effort went in to making sure that design did not cause excessive movements, deformations or damage to anything existing around us, above us or next to us”, explains Kumpfmueller.

The blockade’s tight time schedule was an overarching challenge. Dragados had just 44 days to remove the rails, install the foam concrete in the existing southbound tunnel, complete the tunnelling at both ends, including at first stage concrete, and then demobilise all the tunnelling equipment. Indeed, the team could only start the foam concrete backfill of the existing tunnel on day one of the blockade as that is when the trains were no longer running. It then had 19 days to complete the trackbed at both ends and install 712m of track the whole way through the running tunnel.

Covid also delayed the blockade. It was originally scheduled for summer 2021 but was pushed back because of the pandemic.

Plant parked in the new Northern line tunnel during the blockade

To deliver the project on budget and on time, Dragados had to innovate. Notably, it used only a primary single pass spayed concrete lining (SCL) on both tie-ins instead of applying a further waterproof layer, which was “quite unusual”, Kelly notes.

“We changed the design before the blockade because we knew the programme was so tight – so we optimised it. And we realised that the primary lining is actually quite good as a waterproof structure.”

The use of a single pass lining sped up work during the blockade, but Kumpfmueller observes that the SCL provided other benefits.

“We have 30 different geometries on this job. So SCL gives you that flexibility to build whatever geometry you need. The cross passages, running tunnels, moving walkway tunnel, the escalator shafts – everything on this project is a different geometry. So that’s why SCL was the method of choice at the beginning.”

Indeed, SCL was chosen instead of a step plate junction design which would have involved building a large diameter tunnel around an existing tunnel. These tunnels are usually constructed using hand mining techniques. It is a traditional method that was used throughout the London Underground and was more recently used on the Northern line extension to Battersea.

Kelly explains that a step plate junction was ruled out for the Bank Station upgrade because of the proximity of the Bank of England foundations at the north end, and the London Bridge foundations to the south of the station.

“We were extremely close to both of these structures, so we just didn’t have the space to build a large diameter step plate,” Kelly says.

“SCL also keeps settlements, movements and deformations to a minimum which is important when working in this highly dense area in the middle of London,” adds Kumpfmueller. He notes that the SCL method is also much safer for workers, because it avoids manual handling and the hand arm vibration injuries from mining.

As a result, Dragados opted for the “plug and drive” method of tunnelling. To create the tie ins during the blockade, the team backfilled the existing but soon to be redundant southbound running tunnel with foam concrete. Then the team used a continuous mining tunnel excavator to mine through the London Clay and demolish the existing cast iron lined running tunnels and tie in the new section of tunnel with the existing one at each end of the station.

This complex six year construction project has already improved the passenger experience at Bank. The Northern line’s new southbound platform and wider northbound platforms are a far cry from the previously narrow and unsafe ones. The new entrance on Cannon Street which is due to open by the end of this year will be equally transformative.

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I would like to add that tunnel and shaft design duties, both permanent and temporary works, were shared with OTB Engineering. OTB Engineering has been working on the project as a designer and independent checker since early 2017. Darren Page Director OTB Engineering

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