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 now have to return to site to complete these 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 18 December 2025.
Challenges
Since January 2025, our teams have focused on connecting the newly lined pipe into the existing water network. 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. This will be after our current traffic permits expire, so we will need to return in the new 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.
Other works in the area
Other major works on the A23 at Streatham Hill in July meant we needed to demobilise our works on the A24. This was to reduce disruption and impact on traffic.
Next steps
We'll return to the A24 in January 2026. We'll finish installing the new valve, complete the final connections and commission the upgraded main.
We’ve been working with Wandsworth Council and Transport for London to plan our return and safely commission the upgraded main.
We’ll be working at multiple locations along the A24. Our works will start on Monday 12 January 2026 near the junctions of Trevelyan Road and Ansell Road. From Monday 19 January 2026 we will be working at the junction of Balham Park Road. There will be temporary traffic lights in place to manage the flow of traffic safely around these sites.
The project is expected to be completed in May 2026.
How you'll be affected
- Temporary traffic lights on the A24 near the junctions of Trevelyan Road and Ansell Road from 12 January 2026
- Our next phase will take place at the Balham Park Road junction with the A24 from 19 January 2026 until 15 February 2026. We've previously been in this location in 2024 and 2025 to install the new pipes. We're returning to complete the connections and commission the pipes into the network.
- Access will be maintained for residents and businesses via diversion routes
- No bus routes are being diverted as a result of these works
- You may notice an increase in noise and vehicle movements. We’ll do everything we can to keep this to a minimum by installing sound barriers, where necessary.
The following A24 junctions need a closure as part of the approved traffic management plan. We appreciate that these junction closure extensions are disruptive. We apologise for any extra inconvenience caused by the change of programme.
|
Street name |
Start of closure |
End of closure |
|---|---|---|
| Trevelyan Road | 12 January 2026 | 13 March 2026 |
| Carlwell Street | 12 January 2026 | 13 March 2026 |
| Ansell Road | 12 January 2026 | 23 January 2026 |
| Derington Road | 12 January 2026 | 23 January 2026 |
| Balham Park Road | 19 January 2026 | 15 February 2026 |
| Beechcroft Road | 10 March 2026 | 17 March 2026 |
| Brudenell Road | 10 March 2026 | 17 March 2026 |
| Totterdown Street | 16 March 2026 | 23 March 2026 |
| Ansell Road | 24 March 2026 | 31 March 2026 |
| Derinton Road | 24 March 2026 | 31 March 2026 |
| Balham Park Road | 20 April 2026 | 15 May 2026 |
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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