Thames Rescue

Billed as the largest water infrastructure project in the northern hemisphere, the London Tideway Tunnels aim to massively reduce sewage discharge into the rivers Thames and Lee. Alexandra Wynne talks to the man in charge of delivery.

It might seem to play second fiddle to Crossrail as the city's most famous planned tunnelling scheme – which at nearly £16bn draws in attention and hogs the headlines – but the London Tideway Tunnels is an ambitious project in its own right. With a £2bn price tag (in 2006 figures) its 39km of tunnels will be able to boast that they are the deepest in the capital.

Click here for diagram of area

Thames Water head of London Tideway Tunnels Delivery Team Phil Stride joined the project in April and he is flying the flag for a project that he says London is in desperate need of. "There is 32M.m3 of storm sewage discharge [into the rivers Thames and Lee] every year. It's just not what you'd expect to see happen in a world class city," he says.

The heritage of the scheme dates back to Victorian times and the birth of Sir Joseph Bazalgette's sewerage system. Stride says although it was and is a fantastic piece of engineering, it was designed for a city with a population of only 4M when the Thames was considered dead. Today there is much more pressure on the capital's combined sewer overflows (CSOs) as a result of its rampant population growth.

Increases in the amount of paved area in the capital have added to this by increasing the amount of water run off after heavy rain. As a result, stormwater sewers frequently become overloaded, forcing them to discharge huge volumes of sewage into the Thames.

On top of these factors, the European Urban Waste Water Treatment Directive has forced the government to act.

So the Department for Environment, Food and Rural Affairs announced in March 2007 that Thames Water would be responsible for bolstering the CSOs that serve about 80% of London.

The plan is to collect storm sewage from CSOs into vast, 7.2m diameter tunnels running close to the route of the Thames between West London and Beckton. These will act firstly as a huge storage tank and then transport stored flows to Beckton sewage treatment works in East London.

Thames Water appointed CH2M Hill as project manager in partnership with Halcrow in March.

The project will comprise two separate tunnels. The first is the shorter and relatively straightforward Lee tunnel, about 7km long, that starts at 60m below ground at Abbey Mills and drops to 75m below ground at its end point at Beckton Sewage Treatment Works.

The second is the mammoth 32.2km long Thames Tunnel from an as yet undecided location in West London running broadly aligned with the river. Stride says this one is complex in places and is very much at consultation phase.

The difficulty is that out of the 57 CSOs that run along the Thames, the Environment Agency has identified 36 that Thames Water will have to connect to the main Tideway Tunnels along the route of the Thames.

This means finding space to create between three and five main shaft locations across central London. These will include site areas around them which will be needed to assemble and install the tunnel boring machines (TBMs). These are  expected to cover an area equivalent to three football pitches.

At the bottom of each CSO shaft will be a connecting tunnel with a diameter of between 1.5m and 2m to take the overflow into the new tunnel.

The location of the shafts is the team's main preoccupation right now. Each will need to be agreed in conjunction with each of the 13 London boroughs affected by the proposals. "You want to make the connection tunnels as short as possible," says Stride. "It is indicative at the moment but we don't want to veer off from the alignment [of the Thames] too much because it would end up costing a fortune to create longer connecting tunnels the further away from the CSOs you go."

Stride has been talking to 10 of the London boroughs about the project and he sees this as the crux of how planning will go forward for the Thames tunnel. The Lee tunnel is simpler because its beginning and end are both under Thames Water owned property.

The talks are designed to help create a methodology for choosing the shaft locations. It is already Thames Water's plan to ignore residential areas and World Heritage Sites but Stride appreciates that the boroughs might be reluctant to allow the  disruption of major construction work in their own backyard. "Some have got to accept that if their borough has more CSOs – they're going to need a lot more work going on there," he says before adding a reminder that the project will ultimately benefit all London residents.

There is another side benefit. "If someone says to us: 'We've got a bad contaminated land area so if you can use that site and can clean it  up at the same time, it would help us,' then the project becomes mutually beneficial," says Stride.

At present, the project's geotechnical management unit – comprising Thames Water staff and experts from a range of consultants – is creating 150 boreholes along the Thames Tunnel route to gather information about the geology.

But it is already known that tunnellers will have to negotiate the entire London geological sequence from the London Clay Formation through to the Chalk. Existing London tunnels – from utilities to the Underground – have already made good use of the London Clay, which is a tunneller's favoured geology. As a result the Tideway tunnels will need to go deeper and into the less well charted territory of the more challenging deposits such as the Lambeth Group and Thanet Sand Formation.

The geology will also pose a challenge for creating the up to 25m diameter TBM shafts and the approximately 15m diameter CSO shafts. The latter will go through the alluvium and river terrace deposits and will need to be isolated against groundwater ingress during construction.

The cost of building the tunnels, as with all other water and wastewater infrastructure projects, will be paid for by customers, through water bills. In addition £400M is already included in Thames Water's Asset Management Plan for the company's five main sewage treatment works improvements - including the expansion of Beckton - to deal with the increased sewage flow and improve the quality of effluent.

The schedule of works is now set to continue apace. For the Lee tunnel, Thames Water hopes to award contracts by summer 2009. Four bidders – Murphy/Hochtief, Morgan Est/Vinci, Bachy/FCC and Laing/Impregilo are in the running. The plans are to advance the Lee much more quickly than the Thames with an expected completion date sometime in 2014.

Because of the longer list of complications on the Thames tunnel, planning applications are expected for 2011, with the build proposed to happen between 2012 and 2020.