Civil engineering, infrastructure and tunnelling specialist Barhale has been awarded a contract by Thames Water to reline a section of the abstraction tunnel at Ashford Common Water Treatment Works (WTW) in Surrey, helping safeguard a key part of London’s drinking water infrastructure.
Ashford Common WTW receives raw water from reservoirs forming part of the West London Abstraction (WLA) system, one of the capital’s most important water supply networks.
Inspections identified need for tunnel relining
The contract follows mandatory five-year inspections undertaken by Barhale’s Tunnels & Aqueducts team, which identified inconsistencies within the existing glass reinforced plastic (GRP) liner in the Southern Inlet tunnel.
According to the company, the defects created a potential risk to the underlying wedgeblock tunnel structure. The relining project is designed to restore the tunnel’s long-term integrity and reliability.
The award follows completion of an Early Contractor Involvement (ECI) phase, during which Barhale worked with Thames Water to finalise the engineering design and develop construction methods capable of overcoming the site’s complex underground constraints.
Complex construction programme
Work will begin with excavation to approximately 3 metres below ground level to expose the tunnel chimney and access riser.
Sections of the riser will then be demolished, allowing safe access to the tunnel invert at a depth of 19.5 metres. Scaffolding will be installed inside the tunnel before the existing GRP liner is removed.
Following removal, contractors will break out the existing grout and concrete before constructing a new reinforced concrete liner designed to provide long-term structural protection.
Maintaining water supply resilience
Throughout the works, Barhale said careful planning will ensure minimum flow rates from the remaining West London Abstraction reservoirs continue feeding the treatment works to maintain water treatment operations.
Florin Edu, Regional Manager at Barhale, said:
“The works will ensure the resilience and reliability of this strategic asset for the long term.
“We have collaborated with Thames Water through the ECI phase to shape our approach. It will be carefully managed to ensure at least a minimum flow rate from the other WLA area reservoirs into the works is maintained to sustain the treatment process and prevent the treatment beds from deteriorating.”
Supporting London’s strategic water infrastructure
The West London Abstraction system comprises ten large raised storage reservoirs supplied from the River Thames. These are connected through a network of tunnels and large-diameter mains serving four water treatment works, which ultimately supply drinking water across London via the Thames Water Ring Main (TWRM).
Barhale previously completed a local repair within the Northern tunnel at Ashford Common WTW after proactive inspections identified defects. A full relining of that section is planned as part of a future project.
FAQ
Why is Barhale relining the Ashford Common abstraction tunnel?
Routine inspections identified defects in the existing GRP liner that could damage the underlying tunnel structure, prompting Thames Water to commission a full relining project.
Where is Ashford Common Water Treatment Works?
Ashford Common Water Treatment Works is located in Surrey and forms part of Thames Water’s strategic West London Abstraction network supplying drinking water to London.
What work will Barhale carry out?
The project includes excavating access shafts, removing the existing GRP liner, breaking out grout and concrete, and constructing a new reinforced concrete tunnel lining.
Will the project affect London’s water supply?
Barhale and Thames Water have planned the works to maintain minimum flows from other West London Abstraction reservoirs, allowing the treatment works to continue operating during construction.
