Surbiton trunk mains project
We're carrying out a £15 million upgrade of our clean water main along the A24. This project is part of our wider work to strengthen and modernise the water network in South London. The next stages of the project have not yet been fully completed due to a number of challenges. This means we'll need to return later in 2025 to complete the works.
We appreciate that this extension to the scheme will be disruptive. We apologise for any extra inconvenience caused by the extension of the programme.
Work began in July 2024. By December 2024, we had installed a new lining inside 2.34 km of the existing 30-inch trunk main. “Slip lining” means inserting a new pipe inside the old one, which strengthens it and helps prevent future leaks or bursts. This section was completed about three months ahead of schedule.
This page was last updated on 28 July 2025.
Challenges
Since January 2025, our teams have focused on connecting the newly lined pipe. One of the most important tasks was to join it to a larger 42-inch pipe at Balham Park Road. During preparation, we found that the pipe was surrounded by a large block of concrete. This meant we couldn’t connect it at the planned spot where we had permission to manage traffic. When we tried digging at another point, we found phone and internet cables encased in concrete too. Diverting these cables would have caused major delays to the scheme.
After discussions with our design experts and operational teams, we found an alternative solution. However, this change required us to redesign our plans, carry out new checks, and get fresh approvals. This all added approximately three months to the overall programme.
The Balham connection was completed in May 2025. However, we still need to fit a special valve and chamber. This will help to control how water flows through the network and direct supply where it’s needed. Because this valve wasn’t in our original plan, it had to be custom-made, which takes time. We expect it to be completed in September 2025. This will be after our current traffic permits expire, so we will need to return later this year to finish the work.
Testing
After the connection activity, we needed to test the new section of pipe. “Pressure testing” means we pump water through the pipe at a much higher pressure than normal. This makes sure there are no weak points or leaks.
In June, our team carried out the pressure tests but they didn't pass at first. This isn't unusual. The test pressure is more than three times what the pipe will experience in normal daily use. It shows us where any tiny leaks are so we can repair them before the pipe goes live.
In this area, leak detection had to be done in the early hours of the morning. This avoids road traffic and the underground trains running beneath the route. Once the leaks were found, we needed emergency permission from TfL and Wandsworth Council to dig up the road and repair them. This added more time to the programme. The final pressure test passed successfully on 1 July 2025.
Next steps
Because of these challenges, we haven't been able to complete before our permits expire. This means we will need to return to the A24 later in the year. We'll finish installing the new valve, complete the final connections and commission the upgraded main.
All current sites related to this project will be cleared and fully reinstated by 26 July 2025.
There are other major works taking place in the local area, including on the A23 at Streatham Hill. We're working closely with TfL and Wandsworth Council to plan our return to site. We need to consider the potential impact of these works on traffic on the A24. We'll coordinate our plans to avoid excessive disruption to the local community. Once we have a confirmed plan for our return, we will provide further details.
Get in touch
If you have any questions, call us on 0800 316 9800 or email customer.feedback@thameswater.co.uk. Quote number: BB70085755.
For business customers, please contact your retailer for information relating to our work.
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