Thames Water has appointed infrastructure and tunnelling specialist Barhale to deliver a £12.5m programme of statutory tunnel inspection and maintenance on its systems.

The scope includes the Thames Water Ring Main (TWRM) and North London Abstraction and New River Zone raw water assets.

The work, awarded under the AMP8 Tunnels and Aqueducts programme, also includes production tunnels in Thames Water’s treatment works, bringing the total length of tunnels under inspection to almost 48km.

On the TWRM, Barhale will use mainly manual entry with some remotely operated vehicles (ROVs) to inspect 12 sections of the tunnel at depths of up to 65m, representing around 20% of the asset’s extent.

At the North London Abstraction and New River Zone, Barhale will carry out 24 inspections concentrated around the Lee Valley reservoir complex.

The tunnels of varying diameter are buried up to 50m deep and range from 30m to 5,450m in length. The overall length to be inspected in the Lee Valley area is 24.9km.

ROV, CCTV and sonar surveys will be conducted where in-person inspection isn’t possible.

Specialist diving teams

The tunnels will be accessed from existing shafts. Works will be carried out under Confined Space working conditions and will include clearing sludge, silt, stones and invasive species – particularly mussel concentrations – which impact flows.

Barhale estimates that 3,120 tonnes of waste will need to be removed.

The contractor will carry out minor repairs, including fixing mortar, sealing leaks, injecting resin and fixing ladders, landing platforms and covers. Major repair requirements will be identified and taken forward for appropriate action.

Specialist diving teams will repair inlet and eel screens and carry out external shaft repairs.

The final inspections will be conducted by Thames Water Reservoir Engineering teams.

Barhale will also replace nine large-diameter valves under the programme.

Bats and newts

The programme must respect environmental considerations. Several of the Lee Valley projects are in Sites of Special Scientific Interest and Ramsar-designated wetlands locations.

Walthamstow Wetlands is a conservation area accessible to the public and work is planned to protect existing bat and newt populations in the tunnels.

“This programme involves complex work in confined spaces and live water environments,” said Steve Best, senior project manager at Barhale.

“We have developed inspection and isolation processes with Thames Water over more than 20 years, and they are designed to keep our teams safe while maintaining reliable operation of the network.

“We are proud of our safety record and continue to improve how we manage risk through the introduction of a new confined space working training regime. It imposes new site entry controls and will see all operatives qualified to the highest level.

“Our double isolation protocol, developed with Thames Water, also ensures that there is an additional layer of protection for teams working near live tunnel sections.”