Picture caption: Ashford Common Water Treatment Works, Surrey

The treatment works is fed by the reservoirs which form part of the West London Abstraction area (WLA).

Mandatory five-year inspections carried out by Barhale’s Tunnels & Aqueducts team identified inconsistencies in the Southern Inlet tunnel’s existing GRP liner. They presented a risk of damage to the underlying wedgeblock tunnel. Under the new contract, Barhale will reline the affected section, safeguarding London’s drinking water supply.

The award follows completion of an Early Contractor Involvement (ECI) phase. Barhale worked closely with Thames Water to finalise designs and address the access constraints presented by the congested underground infrastructure of layered tunnels.

Work will begin with excavation to a depth of 3m to expose the chimney and riser for initial access.

Segments of the riser will then be demolished to allow safe descent to the 19.5m tunnel invert level. From there, scaffolding will be installed to enable access along the tunnel, allowing the team to remove the GRP liner.

Once this is complete, they will break out the grout and concrete and construct a new reinforced concrete liner to restore long-term structural integrity.

Florin Edu, Regional Manager at Barhale, explained the importance of the project to secure Ashford Common WTW’s water inflow and protect a significant proportion of London’s clean water supply.

“The works will ensure the resilience and reliability of this strategic asset for the long term,” he said.

“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.”

The WLA consists of ten large, raised storage reservoirs filled by abstraction from the River Thames and linked by a network of tunnels and large mains to four WTWs supplying the London area via the Thames Water Ring Main (TWRM).

Barhale has previously completed a local repair in the Northern tunnel at Ashford Common WTW, which was identified during routine proactive inspections. Full relining of the section is earmarked for a later project.

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