The Lee Tunnel is the first of two tunnels, which will collectively capture an average of 39 million tonnes of sewage a year from the 35 most polluting combined sewer overflows (CSOs), built by the Victorians as part of a sewerage network that still serves London 150 years on.
The £635m tunnel will tackle discharges from London’s largest CSO at Abbey Mills Pumping Station in Stratford, which accounts for 40 per cent of the total discharge.
The four-mile tunnel will run beneath the London Borough of Newham from Abbey Mills to Beckton.
It will help prevent more than 16 million tonnes of sewage mixed with rainwater overflowing into the River Lee each year, by capturing it and transferring it to Beckton Sewage Treatment Works, which is being expanded by 60 per cent to enable it to deal with the increased volumes.
Constructing the Lee Tunnel
We faced the challenge of boring London’s deepest-ever tunnel. This involved tunnelling through high groundwater pressures and passing through four miles of the most abrasive ground, without any other shafts along the way.
Construction work started in September 2010 to build the 80-metre-deep shaft at Beckton sewage works.
In 2011 we lowered our 120-metre-long tunnel boring machine, named Busy Lizzie by a local primary school for good luck, into position 80 metres below the capital.
The machine had been custom-built to suit the ground conditions, which were predominantly chalk with highly abrasive flint.
Busy Lizzie was a ‘slurry closed faced’ tunnel boring machine. It blended over 100 tonnes of excavated chalk with water for every one metre of tunnel advance, forming a white slurry – a similar consistency to single cream, before transporting it through a pipe the length of the tunnel, so it can be processed above ground.
In 2012, Busy Lizzie began constructing the seven-metre diameter tunnel, the width of three London buses. She finished tunnelling in January 2014.
The last stage of construction will finish at the end of next year when the final shaft has been excavated.