Thames Water has launched a £5.7 billion tender notice to bring a contractor on board to build its proposed White Horse Reservoir in Oxfordshire.
The 150 billion litre reservoir is one of nine new reservoirs that are intended to provide water security for the UK, supporting the government’s agenda for 1.5 million new homes, unlocking economic growth, and providing long-term resilience against climate change.
During 2025, Costain conducted clay compaction trials to inform the design of the new reservoir.
Working closely with technical partner Arup-Binnies Joint Venture on the trials, Costain also provided constructability advice to inform the design proposals for the new reservoir, including design, construction and sequencing recommendations for the embankment, tunnelling and the rail siding from the Great Western main line, which runs to the south of the proposed site in Abingdon.
The clay compaction trials help inform the detailed design and construction of engineering projects for which the behaviour of clay needs to be understood. At the proposed SESRO (South East Strategic Reservoir Option) site, Costain undertook the trials to understand the ground conditions and the behaviour of the local Kimmeridge clay, such as its absorption quality and performance under different compaction conditions.
Around 16,900 cubic metres of clay were excavated from within the site to assemble three test embankments, each approximately 50 metres long and 20 metres wide. Geologists then tested the clay on-site and in laboratories, using specialist equipment, to determine the performance of the Kimmeridge clay under different compaction conditions.
Costain also provided safety, health, environment and quality (SHEQ) assurance and acted as a logistical hub for all ground and environmental investigations, including archaeological assessments on site.
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