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 13 February 2026.
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 returned to the A24 in January 2026 to finish installing the new valve, complete the final connections and commission the upgraded main.
Our next phase of works will be starting on Monday 9 March 2026, with traffic management being put in place at the junction with Blackshaw Road and Longley Road. This phase of work is in addition to our existing works on the A24 near the junctions of Trevelyan Road and Carlwell Street, which started in January 2026, and will be continuing until June 2026.
Due to significant engineering challenges, we’ve had to adapt the programme for these works. The next phase will require temporary traffic lights at Blackshaw Road Junction. This work will be carried out in two phases to complete the installation. The first phase will be completed in the Southbound lanes, followed by the Northbound lanes. This site will be using urban traffic control signals to have the signal timings managed remotely throughout the works.
We’ll be keeping customers up to date about the remaining sections of the scheme.
How you'll be affected
- Works will continue to take place at the junctions of Trevelyan Road and Carlwell Street
- Temporary three‑way traffic signals will be in place at the Blackshaw Road/Longley Road junction from the week beginning 9 March 2026 for around four weeks
- Longley Road is being closed to help traffic move more safely at the junction, and to minimise disruption
- There will be a vehicle gate installed on the Longley Road junction closure to allow access for emergency vehicles only to and from St George’s Hospital - this will be in place for four weeks from 9 March 2026
- A contra flow will remain in operation on the A24 near the junctions of Trevelyan Road and Carwell Street until 8 March 2026
- 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, but we’ll do everything we can to keep this to a minimum by installing sound barriers, where necessary.
Please see the table below for the updated junction closures of works along the A24.
Working hours
Our working hours will be Monday to Friday, 7am to 6pm. Weekends 8am to 5pm.
|
Street name |
Start of closure |
End of closure |
|---|---|---|
| Trevelyan Road | 12 January 2026 | 10 May 2026 |
| Carlwell Street | 12 January 2026 | 10 May 2026 |
|
Longley Road | 9 March 2026 | 5 April 2026 |
| Balham Park Road | 20 April 2026 | 7 May 2026 |
|
Trevelyan Road | 1 June 2026 | 21 June 2026 |
|
Carlwell Street | 1 June 2026 | 21 June 2026 |
|
Longley Road | 1 June 2026 | 21 June 2026 |
|
Beechcroft Road | 22 June 2026 | 29 June 2026 |
|
Brudenell Road | 22 June 2026 | 29 June 2026 |
|
Totterdown Street | 30 June 2026 | 7 July 2026 |
|
Gatton Road | 30 June 2026 | 7 July 2026 |
|
Ansell Road | 8 July 2026 | 24 July 2026 |
|
Derinton Road | 8 July 2026 | 24 July 2026 |
|
Lessingham Avenue | 8 July 2026 | 24 July 2026 |
Throughout this complex scheme, since work commenced in 2024 we’ve been working with Wandsworth Council and Transport for London to plan our works to minimise the disruption we bring to the area.
We appreciate that these junction closure extensions are disruptive. We apologise for any extra inconvenience caused by the change of programme.
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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