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Ofwat questions £2bn Thames Water stormwater tunnel plan

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WATER REGULATOR Ofwat this week expressed doubts about Thames Water's claims that it would deliver a new 32km stormwater sewer tunnel for £2bn.

Environment minister Ian Pearson last Thursday gave Thames the go-ahead to develop the 7.2m internal diameter tunnel under the route of the River Thames. Construction is due to start by 2012 and nish by 2020.

But regulator Philip Fletcher doubted that the £2bn budget would be met: 'Without being pessimistic, this is more complicated than anything undertaken by any water company since privatisation, and for some considerable time before that. It's a project that'll span several regulatory periods.' 'As with any major tunnelling scheme, there are elements of uncertainty. There may be problems with ground conditions that turn out to be worse than expected.' But Thames Water senior consultant for pipelines and tunnels John Greenwood said that a substantial contingency been factored into cost forecasts.

The contingency budget is believed to be bigger than the normal 10% to 15% set aside for major civils projects and extensive geological data has already been gathered from Thames Water's boreholes and site investigation.

The Thames Tideway tunnel will intercept 37 combined sewer over flows between Hammersmith in west London and its Beckton sewage treatment plant in the east.

Part of the project will also involve diverting effluent into Beckton from Abbey Mills pumping station via a new 5.5km long tunnel.

Each year around 20Mm 3 of sewage discharges into the River Lea because Abbey Mills lacks capacity to handle the flows. The Lea is a tributary of the Thames.

The main 32km tunnel would be driven 25% through clay, 25% through Woolwich & Reading Beds and 50% chalk. It will begin at a depth of 40m and have a gradient of 1:800. At its deepest point the tunnel will hit a depth of 80m.

Greenwood said that tunnelling for the Channel Tunnel Rail Link in east London had showed it was possible to drive tunnel boring machines through difficult and mixed ground, and to stay on time and budget.

The sewer is being built to meet European Union water quality legislation. Each year 32Mm 3 of stormwater sewage is discharged into the Thames because the capital's sewerage system is unable to cope.

But Fletcher has also questioned the need for the sewer, and claimed that upgrades to London's sewage treatment works already under way would deliver the water quality improvements required.

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