Euro Gunzel

Retrofitting junctions onto existing underground railways – yes, I’m back on the same topic again! This month we stay in London, as we look at the Northern Line Extension.

Photo by sludgegulper, via Wikimedia Commons

The Northern Line Extension was first floated in 2010, as a privately funded extension of the Northern line to serve a urban renewal of the Vauxhall, Nine Elms and Battersea Power Station areas.

The £500 million project was given the go ahead in August 2014, with the 3.22 km long underground line branching from the Northern line at Kennington.

Image via Wikipedia

Construction started in 2015 using a pair of conventional tunnel boring machines, each digging a single track tunnel.

Photo by L.GEORGE ARDELEAN, via Wikimedia Commons

But there was one problem to be solved – joining into the existing London Underground tunnels at Kennington. The solution – a ‘step plate junction’.

OTB Engineering diagram

Where a new larger tunnel was constructed around the existing.

Photo via newcivilengineer.com

But with a modern twist – sprayed concrete was used for the initial tunnel lining rather than the more conventional timber strutting, and the larger tunnel diameter allowed the use of mechanised equipment.

Contracted by Flo J/V (Ferrovial Laing O’Rourke Joint Venture), OTB Engineering was responsible for developing the award winning step-plate junction design – an innovative solution combining century old technology with twenty first century knowhow which will future proof further tunnel expansions for TfL. OTB’s answer was to design a hybrid solution, combining both temporary and permanent works. In this way the team could ensure its solution was the most cost and risk efficient.

To give this challenge some context, the last time step-plate junction technology was employed by TfL was for the Jubilee Line in the mid 1990s. At that point the technology used was over a century old. OTB Engineering’s method developed for the Northern Line Extension is ground-breaking; it provides TfL with a proven means of enlarging any of its tunnels in the future (e.g. to provide a station platform or turnout).

Speeding up the tunnelling process.

Test trains were using the new line from July 2021, with the extension to Battersea Power Station opening to passengers on 20 September 2021.

Photo by Sunil060902, via Wikimedia Commons

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.

Photo by TheFrog001, via Wikimedia Commons

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).

Photo by TheFrog001, via Wikimedia Commons

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.

Photo by N Chadwick , via Wikimedia Commons

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.

Photo by srfurley, via Wikimedia Commons

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.

Photo by TheFrog001, via Wikimedia Commons

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.