Thames Water started a trial to test the clay conditions on land south of Hanney Road, at Cow Common, where a new reservoir is proposed to be built.

The reservoir, known as SESRO (South East Strategic Reservoir Option), is a nationally significant infrastructure project that would guarantee the water supply of 15 million people across the South East, serving Thames Water, Southern Water and Affinity Water customers.

The trial will take place on land owned by Thames Water and is expected to last for up to six months, with Costain undertaking the work. The first phase of the work requires the excavation of 16,900m3 of clay. The material will then be used to create a series of test embankments, up to three metres in height, which will be analysed to understand how the strength and water content of the local clay changes when compacted.


The trial will play a crucial role in informing the design of the 10km of new embankments which would create a watertight perimeter around the reservoir. Current designs propose that the gradual outer slopes would be covered by grassland and pasture, new woodlands, paths and hedgerows.

The main structure of the embankments would be formed from bedrock clays that exist at the proposed reservoir site – specifically Kimmeridge Clay and Gault Clay. The trial will assist in determining how these materials perform when compacted.

Simon Adams, Project Director for Thames Water’s Strategic Resource Options team said: "This is a vital step as we continue to develop our plans to build this much-needed new water storage facility. The results from the trial will help to validate our confidence that the site we are proposing for the new reservoir is the right one.

"If we secure planning consent to build and operate the reservoir then further trials, on a much larger scale, will be carried out. We have every confidence in this first trial and look forward to sharing the results with local communities as part of our ongoing design development and public consultation programme."

To facilitate access to the site, a temporary construction road will be built from Hanney Road. Two-way traffic lights will be in place for approximately six weeks, operating on weekdays only between 8am – 5pm. For the duration of the works, a temporary 30mph speed limit will be in place along Hanney Road to ensure a safe entry and exit from the work site.