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Stepping up to tunnelling’s biggest challenge on the Northern Line Extension

.... and let sleeping dogs (and cats) lie.

To much fanfare this week, the Northern Line Extension of the London Underground was opened. OTB Engineering is proud of its role as tunnel designer in the first expansion of the #LondonUnderground this century.


The £1.1 billion major construction project began in 2015 on the two-mile twin railway tunnel between Kennington and Battersea. The two new stations, at Battersea Power Station and Nine Elms, are on a spur that connects with the rest of the Northern line at Kennington. TfL estimated that the new services would support 25,000 new jobs and 20,000 new homes and is widely seen as the catalyst for the Nine Elms area, with Battersea Power Station at its centrepiece. The revamped power station is set to fully open to the public by 2022.


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. The engagement also encompassed cross-passages, the upgrade at Kennington Station, as well as sprayed concrete lined (SCL) shaft and tunnels and critical tunnel temporary works, including those to support earth pressure balance tunnel boring machine (TBM) tunnelling.

Known throughout the industry for its unique approach to solving civil engineering’s most challenging problems, OTB Engineering turned its attention to this project's unique situation - namely the technical solution for the step-plate junction design at Kennington. 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).



Engaged to value engineer the existing concept designs, Flo J/V needed OTB Engineering to deliver an efficient, low-risk, economic and buildable design that was ‘on-point’ and right first time. The project was delivered 4 months ahead of schedule, and saved the client millions of pounds as a result.


In fact the entire project came in under budget. In part this was because two overrun tunnels were deleted. The tunnels would have taken the line under Battersea Dogs and Cats Home - but the design was changed to ensure the rescue animals did not have their sleep disturbed.


The successes didn’t end there. The solution won plaudits at the Ground Engineering Awards, NCE100 Awards and New Civil Engineer Tunnelling Awards. And more importantly, gave London Underground a new method of partitioning its tunnels and adding new branch lines that will future proof this development for years to come.


OTB Engineering developed the following:

  • Hybrid step-plate junctions combining SCL and traditional timbering;

  • Mechanised construction less reliant on manual handling to improve safety and reduce risks;

  • Deletion of compensation grouting above the step-plate junctions at Kennington;

  • Permanent SCL running tunnel linings incorporating lining drainage;

  • Permanent SCL cross-passages incorporating lining drainage or RamArch in place of steel fibre reinforced concrete (SFRC);

  • Improved cross-passage geometries with improve space-proofing;

  • Full-round SGI opening-sets to avoid the need for internal propping (a solution developed during earlier OTB Engineering work on the Crossrail construction);

  • Sequencing of works at Kennington Station to avoid internal propping (based on work done at Paddington for the Bakerloo Line Link and Crossrail);

  • Full-face SCL excavation sequence to improve production and safety (through the omission of radial construction joints).

For more information contact @DarrenPage | Director, OTB Engineering



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