Costain has scooped a contract with Thames Water to support the design of a new reservoir in Oxfordshire.

Known as South East Strategic Reservoir Option (SESRO), the new 150Mm3 reservoir will provide a resilient water supply for Thames Water customers in London and the Thames Valley, as well as customers served by Affinity Water and Southern Water. The new reservoir will also create new habitats, increase biodiversity, and provide new leisure and recreation facilities.

The work will see Costain conduct clay compaction trials to understand the ground conditions at the proposed site. Within the proposed reservoir footprint, on land owned by Thames Water, three test embankments will be built, each approximately 50m long, 20m wide, and up to 3m high, using the same plant and methodology as the proposed reservoir embankments. Geologists will then determine the performance of the Kimmeridge clay under different compaction conditions. The data gathered will have a direct impact on the reservoir embankment design and construction methodology.


Costain has also provided constructability advice to inform Thames Water's design proposals for the new reservoir. Drawing on its experience and capabilities in the wider infrastructure ecosystem, Costain provided design, construction and sequencing recommendations for significant elements of the project, including the embankment, tunnelling and the rail siding from the Great Western Mainline, which runs to the south of the proposed site.

In addition, Costain will also provide safety, health, environment and quality (SHEQ) assurance and act as a logistical hub for all ground and environmental investigations, including archaeological assessments on the site.

The clay compaction trials will begin in the summer and are expected to take between six and 12 months to complete, depending on weather conditions. At the end of the project, all land will be restored to its original state.

Graham Sugrue, framework director at Costain, commented: "The UK needs to upgrade its water infrastructure to meet increasing demand and remain resilient in the face of climate change. Working with Thames Water to help it develop a reservoir that will serve around 15 million people across the South East is a huge honour.

"This is a project that will see our customer benefit from the breadth and depth of Costain’s capabilities. Our construction and infrastructure experts in water, road and rail, in addition to our environmental, geological and archaeological experts, will work together to provide Thames Water with the information they need to secure government approval and a Development Consent Order."

Leonie Dubois, head of engagement, land and consent, at Thames Water, added: "We have an excellent track record of working with Costain and we are looking forward to drawing on the team's expertise for this exciting project. We continue to listen carefully to residents, who naturally want assurances about the design and integrity of the reservoir. This is why we’re starting the process to understand local ground conditions at a really early stage."