Insite: Cleaning up the Thames

During times of heavy rainfall London’s Victorian sewer system is cleverly designed to allow water to overflow into the River Thames instead of backing up in the system and spilling into the streets.

However, this sewage spills out at 57 different locations along the river and this is having some undesirable side effects. Organic material within the sewage consumes the oxygen in the water and damages aquatic life and disrupts river use.

To prevent this damage Thames Water has developed a project called the London Tideway Tunnels Scheme. Two new tunnels will divert the overflowing sewage away from the river into an existing treatment works. These are the 6.9km Lee Tunnel and the 32km Thames Tunnel.

The £400M Lee Tunnel is the first phase of the scheme. It will intercept untreated storm sewage from a single overflow point at Abbey Mills Pumping Station near Stratford and transport it to Beckton Sewage Treatment Works for treatment.

“It will enable 50% of stormwater into the tidal Thames to be intercepted, leading to an improvement in water quality,” says engineering consultancy Aecom regional director Brian Dimbylow.

The tunnel will be around 75m below the surface. It must be constructed at depth using a tunnel boring machine so that it avoids the numerous underground obstructions in London. This also means that the tunnel has to withstand more external forces due to increased ground and groundwater pressure.

The tunnel will have two linings − a 350mm-thick primary lining composed of pre-cast concrete segments, and a 400mm-thick concrete secondary lining, which is to be cast on site. 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,” says Dimbylow.

The scheme’s second phase is the 32km Thames Tunnel. It will be 7m in diameter − big enough to drive three London buses down side by side.

It will roughly follow the path of the Thames and discharge at the Beckton Sewage Treatment Works. Although the precise route is not yet finalised, it will need to connect to the 34 most polluting points where sewage currently flows into the Thames from the original sewerage network.

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