The tunnel boring machines drilling two new tube tunnels under South London have broken through at Kennington, having started their work at Battersea Power Station.
The machines undertook tunnel drives at depths of up to 26 metres to construct the 5.2 metre diameter tunnels. Each machine was capable of tunnelling up to 30 metres a day with teams of around 50 people operating them.
As part of the tunnelling drive, nearly 20,000 precast concrete segments were put in place to form rings to line the tunnels. Two junctions lined with iron rings will link the extension to the existing Northern line.
A conveyor system was used to take more than 300,000 tonnes of excavated earth to barges on the River Thames where it was transported to Goshems Farm in East Tilbury, Essex.
The cutter head of both machines will now be lifted by crane out of the shafts at Kennington while the rest of the machines will travel back to Battersea and be lifted, in parts, out of the ground there.
Now that the tunnels are completed, they have to spend the next couple of years fitting them out with the track bed, signalling and power supplies.
The Northern line extension is targeted for completion in 2020.