Tilbury – October 11, 2026 -- National Grid's tunnel boring machine (TBM) Caroline completed a 2.2km drive beneath the River Thames 49 days ahead of the original programme, ending the tunnelling phase of the Grain to Tilbury project, part of The Great Grid Upgrade.
The breakthrough came on Sunday 27 September. Caroline excavated a new high-voltage electricity cable tunnel between Tilbury in Essex and Gravesend in Kent.
Caroline installs more than 10,000 segments across nearly 1,700 rings
The tunnel lining consists of more than 10,000 concrete segments assembled into nearly 1,700 tunnel rings. In her most productive month, Caroline advanced 926 metres and installed 712 lining rings. Peak weekly progress reached 229 metres, and the best single day delivered 41 metres of excavation and 31 installed rings in 24 hours.
Flooded reception shaft at Gravesend reduces groundwater risk
The reception shaft at Gravesend was deliberately flooded before breakthrough. Because of the local chalk ground, flooding created balanced hydrostatic pressure between the shaft and the surrounding ground, significantly reducing the risk of groundwater entering as the machine broke through.
Logistics and separation plant sustain continuous tunnelling
National Grid credits collaboration with delivery partner Ferrovial BEMO JV, design partner Mott MacDonald and tunnelling specialist Herrenknecht. Favourable ground conditions, the machine's proven performance and coordinated logistics kept segments flowing to the TBM. A dedicated separation plant processed excavated material without interruption.
Digital design tools and connected TBM data improved visibility, quality assurance and decision-making. An integrated underwater telescopic camera enabled remote inspections, reducing the need for personnel intervention.
Herrenknecht supplied TBM, separation equipment and tunnel logistics
Ulrich Schaffhauser, Member of the Board at Herrenknecht AG, said the company delivered the TBM tailored to operating conditions along with the separation equipment and tunnel logistics needed for continuous performance.
Mark Farmer, Project Director at National Grid, said the project will play a critical role in strengthening the electricity network and supporting Britain's future energy needs as demand for cleaner electricity grows. Borja Trashorras, Project Director at Ferrovial BEMO JV, attributed the early finish to planning, efficient logistics, innovative tunnelling technology and a skilled workforce.
Shaft dewatering continues through October before TBM lift
Caroline will advance beyond the shaft wall to reach her final position, allowing the remaining ring-segment works to be completed. Dewatering of the shaft began in September and will run through October. The TBM will then be lifted and disassembled as the project moves into its next construction phase.