Tier one contractor Costain is conducting intensive clay investigations at a proposed site for the UK’s first planned reservoir in more than 30 years, with the aim of reducing the risk of cost overruns.
The firm is leading clay compaction trials (CCTs) at the site near Abingdon, in Oxfordshire. If approved, the reservoir could hold 150 million cubic metres of water to safeguard the south of England against drought.
Costain – the 16th biggest contractor in the UK – was appointed as principal contractor on the CCTs at the South East Strategic Reservoir Option (SESRO) site in July 2024. A joint venture of Arup and Binnies is also involved in the Thames Water-run project.
If built, the £2.2bn reservoir will serve three water authorities – Thames Water, Affinity Water and Southern Water.
Water from the Thames will be pumped to the reservoir when there is substantial rainfall so it can be used later in the year when a drought is a possibility.
The contractor’s task is to assess the different layers of clay at the site for compaction, to see which is best-suited to which part of the project. Understanding those compaction rates will allow Costain and its partners to “bake those understandings into the construction programme”, Costain project manager James Barratt told Construction News.
“The biggest part of the project is shifting 45 million cubic metres of mud [to make space for the reservoir and build the reservoir embankments],” he added. “So to be able to understand things like the compaction rates, and to know how easy it is to transport, how easy it is to dig, we can feed those understandings into a relatively detailed construction programme.”
Thames Water expects to submit a planning application for the reservoir in autumn 2026, and hopes to hear from the secretary of state in 2028. The water authority said the reservoir could be completed by 2040.
If it gets approval, the reservoir would be enclosed by 10 kilometres of embankments, between 15 and 25 metres high.
Speaking on behalf of the Arup Binnies JV, Arup integrated major projects leader Mark Job said early testing at the reservoir site will help to “protect our cost envelope as we try and identify and deploy those mitigations now”.
“If we don’t identify those risks until later, the likelihood of cost overrun is much more likely,” he added. “Understanding the ground gives us the surety that we have the right material on site – it gives us greater certainty as to what types of conditioning and processing we might need to do.”
The reservoir is the first planned in the UK since Carsington Water opened in Derbyshire in 1992 and comes at a time when the UK is grappling with climate change and the heightened risk of drought.
Thames Water said the reservoir is needed to combat a forecasted shortfall of one billion litres of water per day by 2050.
The early preparation work has also clarified which materials Thames Water needs to create the reservoir. The location has been chosen because the clay there can be used to build the reservoir, and only soil from estuaries, gravel and rock armour – also known as rip rap – will need to be transported from elsewhere.
Job said the contractors will use the nearby Great Western Railway line to transport as much of the materials to the site by rail as possible, to keep the environmental impact to a minimum and keep lorries off the nearby streets.
Costain’s Barratt said the early testing could also help streamline the development consent order (DCO) process.
“Being able to use this data to form the construction programmes will give the DCO assessment a better footing, to say, ‘Well, actually, we have done the work here, and we have thought about it’,” he said.
Job argued that identifying risks to the project early will also enable them to “deliver much more benefits” to the wider community. He pointed to the northern part of the site, for instance, where they have identified two onsite watercourses that need to be removed before the clay is used for the reservoir.
The water will be pumped into two lagoons nearby, which will hold the water before it gets transferred to the reservoir itself. However, rather than filling in the lagoons, the contractors are suggesting they keep the lagoons for local swimming.
“It means we can get more value out the project, and protect the risk going forward,” he said. “It would also provide an alternative to swimming in the reservoir, which would not be safe.”
In January, Thames Water said that it was looking for two contractors to lead the development of the reservoir.
An infrastructure provider would be responsible for designing, building and financing the majority of assets within SESRO. It would work alongside a main works contractor, responsible for constructing and commissioning the assets under a target-price contract.
Last month, the UK’s spending watchdog the National Audit Office published a report raising concerns that water companies were not equipped to deliver nine new reservoirs and had underestimated the cost by £52bn.
The report said the new projects, which will see construction start over the next 15 years, were “critical” to the nation’s future water supply.
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Joshua Stein