The contractor is the Ferrovial Agroman and Laing O’Rourke JV with OTB Engineering as the designer.

Two 25m-deep permanent shafts have already been sunk, allowing tunnelling to begin. Once the extension is complete, the shafts will remain in place and provide ventilation, cooling and emergency access to the tunnels. The 18,000t of spoil has been removed via barge to turn landfill sites into arable farmland.

A Transport for London spokesperson added: “The works will culminate with the building of a step-plate junction around the existing Northern line loop tunnel. This is a technical and very complex piece of work involving hand-mining and very little machinery. The step-plate junction will take around seven months to build and will be the first time that those working on the project interface with the existing railway.”

The mining diameter is 6.1m with a finished i.d. of 5.2m. The Kennington Park tunnel and step plate junction will be 160m long in total while the Kennington Green tunnel and step plate junction will be 270m. The works will predominantly be in London Clay and the shotcrete will vary between 150-300mm depending on location.

A stub tunnel will be constructed at the reverse side of the shaft ready to receive the TBMs in late 2017. The 6m-diameter NFM machines were being transported to London as Tunnels and Tunnelling went to press. They will be assembled at a Battersea work site and launch on their 2.5km journey in early 2017. They are due to break through into the Kennington shafts in late 2017.

Martin Gosling, Head of the Northern Line Extension at London Underground, said: “The tunnelling works at Kennington represent a significant milestone in the construction of the Northern Line Extension which will bring Battersea and surrounding areas to within 15 minutes of the West End and City. Construction is now well underway in Kennington and involves a lot of complex, highly skilled construction work. It’s a significant step forward in the project which, once complete, will help us to support jobs, homes and growth in this area and keep pace with the Capital’s rapidly rising population.”