London’s deepest tunnel, Thames Water’s Lee Tunnel , has been completed on time and on budget.

The tunnel boring machine ‘Busy Lizzie’, which has been digging the four-mile long sewer has reached its final destination.

It reached the end of tunnelling, on time and on budget, after cutting through layers of chalk and flint at extreme groundwater pressures. At its peak the 120 metre long machine tunnelled through an astonishing  54 metres of London earth a day, creating the capital’s deepest tunnel.

Programme management firm CH2MHill, along with tunnelling contractors MVB, (a joint venture of three leading civil engineering contractors, Morgan Sindall, VINCI Construction Grands Projets and Bachy Soletanche), completed the tunnelling work on Sunday 26 January.

When in full working action in 2015 the £635 million Lee Tunnel will help prevent millions of tonnes of raw sewage entering the River Lee every year when heavy rainfall overloads the Victorian sewers that London has outgrown.

The tunnel will act as a storage tank before transferring the flows to Beckton sewage works, Europe’s biggest, which is being expanded by a further 60 per cent to deal with the increased volumes.

Martin Baggs, chief executive of Thames Water, said:

“It is unacceptable in modern-day London to have raw sewage entering the River Lee. The completion of this tunnelling work is an important milestone within our long-term plans to improve London’s sewer system.

“We’re immensely proud of this project, delivered on time and on budget, to clean up the River Lee. As well as the clear environmental benefits, the project has provided jobs for both local people and engineering experts from across the world, allowing London to show it is a leader in engineering technology.”

The Lee Tunnel is the first of two tunnels, along with the Thames Tideway Tunnel which is currently being examined by the Planning Inspectorate, designed to combat tens of millions of tonnes of sewage which spill into the River Thames every year.