Northern line extension tunnelling to commence in March

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The Mayor of London has announced that tunnelling to create an extension of the Northern line between Kennington and Battersea will begin in March.

Sadiq Khan confirmed news of tunnelling for the first major Tube line extension since the 1990s as two gigantic tunnel boring machines were unveiled in Battersea. The two 650-tonne machines, each the length of a football pitch, will create two 3.2km underground tunnels to extend the Charing Cross branch of the Northern line from Kennington to Battersea, via Nine Elms.

According to tunnelling tradition, the machines cannot start work until given a name and, following a vote by local school children, the machines are being named Helen and Amy in honour of the first British astronaut, Helen Sharman, and British aviation pioneer Amy Johnson, who was the first female pilot to fly solo from Britain to Australia.

The two tunnel boring machines were built by NFM Technologies in Le Creusot in central France. They were shipped to London earlier this year and reassembled in Battersea. Each tunnel boring machine has a rotating cutterhead at the front which is pressed against the tunnel face by hydraulic cylinders. A series of trailers behind house all the mechanical and electrical equipment and a conveyor belt removes the earth.

The machines will undertake two individual tunnel drives at depths of up to 26 metres to construct the 5.2 metre diameter east and westbound tunnels. Each machine is capable of tunnelling up to 30 metres per day with teams of around 50 people needed to operate them.  The work is expected to take around six months to complete. As they advance forward, nearly 20,000 pre-cast concrete segments will be built in rings behind them.

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