I’ve written many a blog posts about how metro systems have retrofitted new underground stations into existing operating railways, and this time we’re looking at Bank station on the London Underground.
Bank station forms part of the Bank-Monument station complex, constructed between 1884 and 1991, and served by five lines of the London Underground as well as the Docklands Light Railway (DLR).
Between 2003 and 2014 the number of passengers using the Bank–Monument station complex rose by over 50% to 337,000 per day, leading to congestion in the narrow interchange passageways and staircases between the multiple platforms that make up the station.
As a result, the Bank Station Capacity Upgrade Project was launched to build a new southbound platform for Northern line trains, convert the existing platform into an interchange passage, and provide new escalators to the Central line, DLR platforms, and the surface exits.
The tunnelling contract was awarded to Dragados in 2013, and work started in 2016. The first phase was uneventful.
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.
With pedestrian walkways ready to be connected into the current station.
But a sticking point was the tie in between new and old southbound running tunnels. A ‘step plate junction’ was considered, where trains could keep running while a large diameter tunnel was built around the existing one, but this was not possible due to the constrained site.
Unfortunately, the locations where the new running tunnel connects with the old do not permit the construction of step-plate junctions, which could have been built around the running tunnels with shorter closures.
The southern connection is close to the river and possibly in ground disturbed by the piles of the original London Bridge; this is the same situation encountered when the new southbound Northern Line platform was constructed at London Bridge station.
To the north, the junction will be beneath the junction of Lothbury, Princes Street, and Moorgate. At this point the Northern Line running tunnels are one above the other as they curve north into Moorgate, and there is insufficient room to build a step-plate tunnel.
And so a more disruptive method was chosen, which required a 17 week shutdown of the line in January 2022 to tie in the new to the old.
The second and more complicated phase got underway in January 2022 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.
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.
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.
The new Northern line southbound platform and concourse was opened in May 2022, with the new interchange passageways and escalators opening in the months to follow, with the project completed in February 2023.
The station now has 27 escalators, the most of any station on the London Underground.
Further reading
Behind the scenes at Bank tube station’s huge upgrade project at ianVisits.