The £635 million tunnel will take discharges from London’s largest combined sewer overflow at Abbey Mills in Stratford, located in east London, for treatment at Beckton sewage works, which is being expanded so it can deal with the additional flows.

Work has already started on phase one of Thames Water’s London Tideway Improvements program to clean up the Thames – a £675 million scheme to modernise London’s five major sewage works so they can treat more waste to prevent the sites being overloaded, and treat it to a higher standard.

Phase two is the Lee Tunnel, and the final and most challenging piece of the overall plan is the proposed Thames Tunnel, which will run up to 20 miles from west to east London to tackle the 34 most polluting overflows along the River Thames.

Construction has started on one of four shafts, which will be up to 75 m deep.

Article continues below…

Thames Water Chief Executive Officer Martin Baggs said “Abbey Mills is the largest sewer overflow in London so the Lee Tunnel will deal with this first so we can have the biggest impact, most quickly.”

The Lee Tunnel, which will be the width of three double-decker buses, will help prevent 40 per cent of the 39 million tonnes on average of sewage that enters the River Thames and River Lee each year from overflow points.

The 57 overflow points were designed in Victorian times as a solution to relieve sewage from backing up into streets and homes when they reached capacity following exceptionally heavy storms.

“Sewer overflows, used occasionally during the 1800s, are now used almost weekly on average and can be triggered by as little as 2 mm of rainfall. The overflows cause significant environmental damage – killing fish, contaminating the river for those who wish to use and enjoy it and affecting the wellbeing of our capital.

“The sewers are simply not big enough to cope with a population which has trebled in size and continues to grow, and a city which has paved over many green spaces preventing natural drainage,” Mr Baggs said.

The Lee Tunnel will be the deepest tunnel ever constructed in London, and is Thames Water’s biggest engineering project to date.

Three civil engineering contractors – Morgan Sindall, VINCI Construction Grands Projets and Bachy Soletanche (MVB) are working together to deliver the Lee Tunnel.

Having collaborated on some of the UK’s highest profile projects, the joint venture partners have a reputation for excellence, quality and innovation throughout the world, Thames Water said.

Tunnelling work will start in 2012, with the project due for completion in 2014.