The four mile tunnel runs beneath the London Borough of Newham, from Abbey Mills Pumping Station to Beckton Sewage Treatment Works.
The tunnel will aid in preventing more than 16 million tonnes of sewage mixed with rainwater from overflowing into the River Lee each year, through capturing it and transferring it to Beckton, which has been expanded by 60 per cent to enable it to deal with the increased volumes.
The £635m tunnel collects discharges from Londonís largest combined sewer overflow at Abbey Mills situated in Stratford, which previously accounted for 40 per cent of the total overflows each year.
This project is the largest delivered by the UKís privatised water industry and the tunnel is the deepest ever constructed under London. It forms a key part of the biggest expansion of Londonís sewerage network since the 1860s and will transform the quality of the capitalís major rivers.
Constructing the Lee Tunnel
We faced the challenge of boring Londonís deepest-ever tunnel. This involved tunnelling through high groundwater pressures and passing through four miles of highly abrasive ground, without any other shafts along the way.
Construction of the 80 metre deep shaft at Beckton began in September 2010. The following year, we lowered our 120 metre long tunnel boring machine into position, 80 metres below the capital.
The boring machine, affectionately referred to as Busy Lizzie, having being named by a local primary school pupil for good luck, was custom built with the ground conditions in mind, namely a mixture of chalk and flint.
Busy Lizzie was a Ďslurry closed facedí tunnel boring machine, which blended over 100 tonnes of excavated chalk with water, for every one metre of tunnel advance. This formed a white slurry, which was then transported through a pipe the length of the tunnel, so it could be processed above ground.
In 2012, construction of the seven metre diameter tunnel, the width of three London buses began, with the tunnelling process completed in January 2014.
In addition to the primary lining laid by Busy Lizzie, the tunnel also benefits from a 300mm thick secondary inner-lining, constructed from fibre reinforced concrete. This added lining provides further assurance against leakage.
More recently, the focus has been on the installation of the mechanical and electrical equipment, which includes six giant pumps that operate at 3.05 cubic metres per second.
The Lee Tunnel†will be†commissioned by the Mayor of London, Boris Johnson, on 28 January 2016.