Thames Water's Lee Tunnel Project, East London, United Kingdom
The Lee Tunnel project is the first of the two tunnelling projects executed as part of the Thames Water London Tideway Improvements scheme in the UK. The second is the Thames Tunnel. The scheme aims at presenting a cleaner River Thames by 2020.
The four-mile (6.4km) Lee Tunnel runs underwater and ground through East London, from the Abbey Mills Pumping Station in Stratford to the Beckton Sewage Treatment Works (STW) in Newham.
It carries sewage from London's largest combined sewer overflow (CSO) at Abbey Mills station to Becton STW. The CSO at Abbey Mills receives 40% of the sewage discharge in London.
Construction of the Lee Tunnel project began with the diaphragm walls at Beckton Overflow Shaft site in September 2010. Work on the actual tunnel began in February 2012. The Lee Tunnel was opened in January 2016.
The tunnel has won the Greatest Contribution to London and Infrastructure Award.
Purpose of Thames Water's London Tideway Improvements
London manages its wastewater through a network of 50 CSOs, which discharge sewage and rainwater overflows into the River Thames and its tributary River Lee.
The 150-year-old stormwater and sewer network has now become inadequate to handle the excessive flow of water. The discharges also exceed the permissible limits set by the Urban Wastewater Treatment Directive (UWWTD).
Thames Water has embarked on the London Tideway Improvements project to comply with the UWWTD and put in place a better mechanism to discharge wastewater. Under the project, five sewage treatment works (Mogden, Crossness, Beckton, Long Reach and Riverside) were upgraded in addition to constructing the two tunnels.
Beckton Sewage Treatment Works was expanded by 60% to handle the sewage that is carried by the Lee Tunnel. Lee Tunnel reduces the water discharge into the River Lee by 16 million tons a year. The Thames and the Lee tunnels will together capture 39mt of sewage every year from 35 CSOs.
Components of the four-mile long Lee Tunnel
The tunnel is 6.9km-long and has a diameter of 7.2m. The tunnel project includes four shafts: three located at Beckton STW and one at Abbey Mills. The three shafts located at Beckton STW are Beckton Overflow Shaft, Beckton Connection Shaft and Beckton Pumping Station Shaft.
Beckton Overflow Shaft directly connects to a new outfall channel at the River Thames. The Beckton Connection Shaft carries the water to a suction tunnel between the connection shaft and the pumping station shaft.
The pumping station shaft has six three megawatt pumps to transfer the sewage for treatment at Beckton STW.
The Abbey Mills Connection Shaft transfers sewage flows from two pumping stations (A and F) to the main tunnel. The new outfall channel, constructed on the top of the overflow shaft, helps in directing discharge of water during heavy storms.
Other major components of the project include surface infrastructure, a new power supply complex at Beckton STW and new surface culverts and sewerage flow connections at Abbey Mills Pumping Station complex.
Construction of shafts and use of TBM Busy Lizzie
The tunnel and the shafts are double-lined for water tightness. The primary lining is 350mm-thick, with fibre reinforced precast concrete segments. The secondary lining is 300mm-thick fibre-reinforced concrete cast. The primary walls of the shafts are of reinforced concrete.
Works at Beckton began in September 2010, and those at Abbey Mills a year later. TBM Busy Lizzie was custom-built to suit the construction condition. It is 120m in length and 8.88m in diameter. The largest part of the machine is the 7m-diameter cutterhead.
The machine was transported from Germany by barge along the River Rhine to Rotterdam, and then across the North Sea to Tilbury. It was carried to the site in 60 lorry loads.
Busy Lizzie was lowered at an 80m depth in December 2011. The machine mixes 100t of excavated chalk with water for every one metre of tunnel dug. The slurry is transported to a pipe for being treated above ground. The tunnel was dug at 17m a day.
Busy Lizzie commenced drilling of the tunnel in February 2012. It reached the connection shaft in November 2012.
Excavation of the last two shafts, Beckton pumping station and Abbey Mills, was completed in September 2012. The TBM was lowered into the shafts in 2014.
Key players involved with London's Lee Tunnel project
CH2M HILL is the programme manager for the project. The construction contract for the tunnel was awarded to MVB, a consortium of Morgan Sindall, VINCI Construction and Bachy Soletanche, in January 2010.
MVB sub-contracted the shaft civil engineering, hydraulic engineering and mechanical, electrical, instrumentation, controls (MEICA) engineering to Mott MacDonald, tunnels and surface structures engineering to Underground Professional Service (UnPS) and shaft diaphragm walls engineering to Bachy Soletanche.
The TBM was supplied by Germany-based Herrenknecht. KSB supplied four 52t wastewater pumps for the project.
The Thames Tideway scheme - coupling improvements to London's three main wastewater treatment plants (WWTPs) with the construction of a storage-and-transfer tunnel nearly 35km long - forms one of the city's most ambitious wastewater projects since its first sanitation system in the mid-19th century.
The plant is the UK's first water desalination plant and one of the largest of its kind in the world.