A new 2.2km cable tunnel beneath the River Thames has been completed 49 days ahead of schedule, providing a major step forward for National Grid’s Great Grid Upgrade.
The 400kV tunnel runs between Tilbury in Essex and Gravesend in Kent. It will replace the 1960s Thames Cable Tunnel and reinforce electricity transmission between the two counties.
High-speed tunnelling beneath the river
The breakthrough was completed on 27 September by the tunnel boring machine Caroline. During the drive, it installed more than 10,000 concrete segments across almost 1,700 tunnel rings.
Caroline’s best month saw 926 metres of progress, while its strongest week covered 229 metres. Digital design tools and live data from the machine helped teams monitor quality and make decisions during construction.
The final stage took place through an intentionally flooded reception shaft, helping balance pressure in the surrounding chalk and reduce the risk of groundwater entering the shaft. An underwater camera also allowed remote inspections, limiting the need for personnel to work inside the shaft.
The tunnel was delivered by Ferrovial Bemo JV, designed by Mott MacDonald, with specialist tunnelling services from Herrenknecht. National Grid credited careful planning, favourable ground conditions and coordinated logistics for the early completion.
Why it matters for Britain
The project will strengthen electricity transmission between Kent and Essex and help support future demand for cleaner electricity. Replacing the older tunnel also provides an upgraded piece of infrastructure for the network serving southeast England.
