A major milestone has been reached on the Costain project to build an underground electricity superhighway deep under London.

Giant tunnel boring machine (TBM) 'Cleopatra' has completed an 8km drive from Eade Road in Haringey, north London, to St John’s Wood Substation, in the northwest of the metropolis. Together with earlier tunnelling by her 'sister' TBM, Evelyn, almost half of the planned 32km of the London Power Tunnels (LPT) project has been completed.

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The project is creating new routes for 400kV high-voltage electricity cables beneath the capital, playing a vital role in ensuring Londoners have reliable power supplies for decades into the future.

Cleopatra, a 400-tonne, 130-metre-long TBM has carved a 4-metre diameter tunnel from Haringey, via Finsbury Park, Camden and Regents Park to St John's Wood.

In the process she has removed 250,000m³ of earth, while some 36,000 concrete segments have been bolted together to form the new tunnel.

A team of specialist tunnelling engineers sits within the length of the TBM, guiding it with literally inch-perfect precision to its destination.

"This breakthrough is the culmination of nearly 18 months' work for the entire team and demonstrates Costain's ability to design, engineer and deliver projects that meet national needs," said Andy Firth, Costain Project Manager.

(CD)