National Grid has completed the breakthrough of a 2.2km electricity cable tunnel beneath the River Thames 49 days ahead of schedule.
The tunnel forms part of the Great Grid Upgrade’s Grain to Tilbury project and will carry a new 400kV connection between Tilbury in Essex and Gravesend in Kent. It will replace the existing Thames Cable Tunnel, which dates from the 1960s, and strengthen electricity transmission between the two counties.
Tunnelling completed beneath the Thames
The breakthrough was achieved on Sunday 27 September by the tunnel boring machine (TBM) Caroline. The machine completed its journey through an intentionally flooded reception shaft, a method used to balance pressure between the shaft and the surrounding chalk and reduce the risk of groundwater entering the shaft.
Caroline installed more than 10,000 concrete tunnel segments, forming almost 1,700 tunnel rings. National Grid said the TBM advanced 926m and installed 712 rings during its most productive month. Its best weekly output was 229m, while its highest daily output reached 41m and 31 rings.
The project used digital design tools and connected TBM data to support monitoring, quality assurance and decision-making. National Grid also said an integrated underwater telescopic camera enabled remote inspections, reducing the need for personnel to enter the shaft and supporting safer working practices.
Project team credited with early completion
National Grid attributed the early breakthrough to careful planning, favourable ground conditions, the performance of the TBM and coordinated logistics. A dedicated separation plant processed excavated material, while a continuous supply of tunnel segments supported the machine’s progress.
Ferrovial Bemo JV is delivering the tunnel, with Mott MacDonald responsible for its design and Herrenknecht providing specialist tunnelling services.
Ferrovial Bemo JV project director Borja Trashorras said the result reflected “excellent planning, strong engineering expertise and outstanding collaboration across the project team”.
Herrenknecht board member Ulrich Schaffhauser said the breakthrough demonstrated the value of coordinating “all project components, technologies, and services”, adding that the company had supplied the TBM, separation equipment and tunnel logistics.
National Grid project director Mark Farmer said: “Completing this tunnel drive ahead of schedule is a testament to the expertise, dedication and collaboration of our teams and supply chain partners.”
He added that the project would help strengthen the electricity network as demand for cleaner electricity grows.