Investigations and groundwork are being carried out at the Havant Thicket reservoir site in Hampshire as Portsmouth Water prepares to apply for planning permission next month.
Teams are excavating areas of the site earmarked for the basin and pipelines to learn more about underground conditions and locate any utility assets such as pipes or cables.
Chief executive Bob Taylor said the information gathered will feed into the planning applications for the reservoir and the pipeline, which the company is submitting in September.
Construction is expected to begin from June 2023 and run until December 2026, after which a visitor centre would be built, and the reservoir filled. All work is scheduled to be completed by the end of December 2029.
Last year deep drilling was carried out to assess soil composition and consultations have been held virtually this year to share plans with the public.
The site, situated between Staunton Country Park and Havant Thicket woodland, will hold around 8,700 million litres of water to supply 21 million litres each day.
The reservoir, which is being developed in partnership with Southern Water, will supply water in both companies’ regions as part of a bulk supply agreement with neighbouring Southern. This will allow Southern to reduce its abstraction from protected chalk streams in it western region.
Havant Thicket will be the first reservoir to be built in the UK since 1989.