Retrofitting a junction for the Northern Line Extension

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.

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.

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

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.

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