Campaigners question reservoir ground trial

Thames Water will use the excavated clay to create a series of test embankments

Campaigners against the building of a large reservoir have questioned the importance of a ground trial.

Thames Water started the six-month trial in August, which aims to inform the design proposals for the planned £1.2bn reservoir near Abingdon, Oxfordshire.

Group Against Reservoir Development (GARD) said the company should instead focus on the "shortcomings” of the project's plans.

A Thames Water spokesperson said the outcome of the works would help create "a watertight perimeter around the reservoir".

Thames Water started the six-month ground trial in August

The trial is taking place on land owned by Thames Water, to the south of Hanney Road, at Cow Common.

The first phase of the work requires the excavation of 16,900 cubic metres of clay, which will then be used to create a series of test embankments.

They will then be analysed to understand how the strength and water content of the local clay changes when compacted.

But a spokesperson for GARD said the site was 4.2 sq miles (11 sq km) in area "and the large - 11 km (6.8 mile) long - embankment could not possibly be simulated by this tiny test bank which could fit in my back-garden."

They said GARD had commissioned a report by an expert on Earth Dam reservoirs, which was "highly critical of Thames Water’s (lack of) engineering studies to date".

"Thames Water’s consultants have taken no samples of soil quality and mechanics since the 2006-2007 period - there are huge gaps in their necessary knowledge for constructing this reservoir safely and without delays," they said.

The spokesperson added that the company was trying to give the impression that "their plans have approval".

They said: "The Environment Agency advice to the Secretary of State (as of December 2023) is that the plans should not be approved without further modification and that Thames have not proved that the reservoir represents best value.

"They should be getting on with rectifying these shortcomings.”

Thames Water said the reservoir would help to secure future water supply for local Oxfordshire residents, as well as 16 million people across the Thames Water, Southern Water and Affinity Water catchments.

A company spokesperson said the reservoir proposals "will require development consent and we intend to submit an application to the Planning Inspectorate in 2026".

"The clay compaction trial works we are currently undertaking will help to inform the design of the embankments which would create a watertight perimeter around the reservoir," the statement added.

The company said that it had provided a revised Water Resource Management Plan and was "in regular dialogue" with the Department for the Environment, Farming and Rural Affairs (Defra) on the submission.

The results of a public consultation into the reservoir, which closed on 28 August, are yet to be announced.