The first phase of the London Tideway Tunnels Scheme will prevent overflows of sewage into the Thames and will need extensive volumes of concrete. Jessica Rowson reports on the pressures of working deep underground.
London’s Victorian combined sewerage system revolutionised public health, but the unrelenting pressure created by the capital’s sustained growth is creating modern day environmental issues.
To prevent sewer flooding during heavy rainfall, sewage is discharged into the River Thames at 57 different combined sewer overflow (CSO) points. The organic material in the CSO discharge can lead to reduced levels of oxygen in the river, causing harm to aquatic life and disrupting river use.
The £2bn London Tideway Tunnels scheme is a major project intended to improve the capacity of London’s sewerage system and prevent these sewage overflows. The first phase of this work, the £400m Lee Tunnel, will intercept untreated storm sewage from a single overflow point at Abbey Mills Pumping Station near Stratford.
“It will enable 50% of stormwater into the tidal Thames to be intercepted, leading to an improvement in water quality.”
Brian Dimbylow, Aecom
The proposed Lee Tunnel will collect this storm sewage and then transport it around 6.9km to Beckton Sewage Treatment Works for treatment. By capturing this single overflow, the Lee Tunnel will halve the 32M.m³ of storm sewage that enters the tidal River Thames each year.
“It’s the first phase of the London Tideway Tunnels,” says Aecom regional director Brian Dimbylow. “It’s intended to prevent sewage discharges into the [Thames tributary] River Lee. It will enable 50% of stormwater into the tidal Thames to be intercepted, leading to an improvement in water quality of the Rivers Lee and Thames.”
Aecom is the reference designer on the project for client Thames Water and CH2M Hill has been appointed as programme manager. An announcement on the Lee Tunnel contractor is due shortly and construction is scheduled to start next year. “We are now in the final stages of tender,” says Dimbylow.
“We have two bidders left at the moment − a joint venture between Hochtief and Murphy and a joint venture between Morgan Est, Vinci and Bachy.”
Major material need
As well as 6.9km of 7.2m internal diameter tunnel, the project includes a connection shaft, an overflow shaft and a pumping station shaft at Beckton. There is also a shaft at Abbey Mills where the flows will enter the Lee Tunnel.
“Concrete is our major material need,” says Thames Water project manager for the Lee Tunnel Nick Butler. “The project will require close to 300,000m³ of concrete with 11,000m³ a month needed at peak construction.”
The tunnel runs around 75m below the surface and all of the Beckton shafts are over 80m deep with internal diameters of 25m to 38m. The project has to be constructed at depth so that the tunnel can avoid the numerous underground obstructions in London, but it means that the structure has to withstand more external forces due to increased ground and groundwater pressure.
“The real challenges come from the depth we will need to work at due to the need of avoiding obstructions like the Olympic Park’s cable tunnels,” says Aecom technical director Richard Sutherden.
“The real challenges come from the depth we will need to work at due to obstructions like the Olympic Park’s cable tunnels.”
Richard Sutherden, Aecom
The tunnel boring machine (TBM) will be mostly working in chalk, but there is a location where the Thanet sands are downthrown into the alignment. There is also a large quantity of flint in the chalk which will cause wear to the head of the TBM.
“The invert is around 75m below ground and so we will be dealing with nearly 8 bar of water pressure,” adds Sutherden.
“This water pressure will need to be managed in the construction phase. Interventions to the tunnel face will be difficult, especially if we have to enter in the Thanet Sands which will be free flowing, and a robust strategy is essential.”
The tunnel will have two linings − a 350mm-thick precast concrete segmental primary lining and a 400mm-thick insitu concrete secondary lining. The reference design has adopted fibre-reinforced segments for the primary lining.
“There is a secondary lining within the precast [concrete] due to the high internal pressures,” says Sutherden. “During a severe storm event, the internal water pressure would exceed the external water pressure. We need to eliminate the risk of the sewage going into the groundwater.”
Strength and durability
The concrete has to have a high compressive strength to withstand the water pressure and it also has to be durable.
Sewage environments can be very corrosive and so tests are being carried out on the chemical composition of sewage to try and predict how it would react with the concrete over time.
“The tunnel has a design life of 120 years − it’s a serious bit of London infrastructure,” says Dimbylow. “Durability is key − basically you want a very good finish, a good sulphate and hydrogen sulphide resistance and a high-strength dense mix.”
“The tunnel has a design life of 120 years − it’s a serious bit of London infrastructure.”
Brian Dimbylow, Aecom
The deep diaphragm walls for the shafts require considerable work in themselves. The walls are up to 100m deep for the pumping station shaft, and between 1.2m and 1.5m thick in the reference design.
Large concrete pours of up to 1,000m³ are required due to the depth and size of the panels. “One of the main issues that we want is consistency of supply,” says Sutherden. “We need a batching plant on site with off-site back-up facilities to prevent cold joints within the panels. The rate of pour also needs to be carefully controlled.”
Lee Tunnel construction is scheduled to start in early 2010 and finish in 2014. Meanwhile, work on the 32km Thames Tunnel is expected to be carried out between 2012 and 2020.
The Lee Tunnel overflow shaft at Beckton, which doubles up as the tunnel drive shaft, is located next to the River Thames in a site which housed the original Bazalgette sewage treatment works from the 1880s.
Historical sites present challenges but this one more than most. In this area there are three levels of masonry arched voids that run to about 14m deep. When the treatment works were operating, they were used for sludge separation. The works have been redundant for decades, but the sludge and the underground voids remain.
It was decided to fill the voids with foamed concrete to stabilise the ground, rather than excavate and demolish the arches. The structures are founded on very permeable river terrace gravels so demolition would have required monitoring of groundwater to ensure no contaminants were released. Also, any excavation could undermine the adjacent Northern Outfall Sewer − the main artery of north London’s sewerage network − and other above ground tanks.
“The site contains abandoned sludge settling channels,” says Aecom regional director Brian Dimbylow. “We have taken out 20,000m³ of contaminated sludge and replaced it with 32,000m³ of foamed concrete to provide stable foundations for the large equipment necessary for the construction of the main Lee Tunnel works. Demolition would have been too risky on the river gravels next to the Thames.”
“We chose foamed concrete as it is very good at flowing and filling voids,” adds Aecom technical director Richard Sutherden.
“Additionally, with a design compressive strength between 2.5N/mm² and 5N/mm² we can get close to the strength of the historic brickwork. The density has been designed above that of water so water is displaced but still lower than normal weight concrete to minimise wet loads on the existing structure.”
The voids were filled in August using a batching plant on site to minimise vehicle movements. Around 800m³ to 900m³ a day of foamed concrete was being placed at peak production.
“The works have made safe a previously hazardous area of the Beckton site and will bring it back into use, aiding the sewage treatment process, as originally intended by Bazalgette”, says Thames Water project manager for the Lee Tunnel Nick Butler.