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The Gallery | Northern Line Extension TBMs installed

Construction of the Northern Line extension has taken a major step forward with the lowering of two giant tunnel boring machines into  20m deep shafts in Battersea.

The two TBMs, named Helen and Amy were lifted into the shafts by a 750t crane in the site of the former Battersea Power Station.

The TBMs will create two 5.2m diameter, 3.2km long underground tunnels to extend the Charing Cross branch of the Northern line from Kennington to Battersea Power Station, via a station at Nine Elms.

Tunnelling is due to start in March.

Transport for London (TfL) said that tunnelling will take six months, with the extension targeted for completion in 2020. It said that it was the first major Tube line extension since that of the Jubilee line in the late 1990s.

Both TBMs will be fully assembled within two launch tunnels, before starting their journeys towards Kennington next month. When fully assembled, Helen and Amy will each be 100m long.

A Ferrovial, Agroman and Laing O’Rourke joint venture was selected by London Underground in 2014 to design and build the extension.

TfL said that after both tunnel boring machines and their gantries are constructed, a conveyor system would be built to take spoil from the tunnels to barges on the River Thames.

It is expected that more than 300,000t of earth will be excavated by the TBMs. it will be taken by river to Goshems Farm in East Tilbury, Essex where it will be used to create arable farmland.

The extension has faced delays and cost increases since the project started.

The machines were named after first British astronaut, Helen Sharman, and British aviation pioneer, Amy Johnson, who was the first female pilot to fly solo from Britain to Australia.

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