National Grid says its tunnel boring machine Caroline broke through beneath the River Thames on 27 September 2026, completing a 2.2km drive between Tilbury in Essex and Gravesend in Kent. The breakthrough was 49 days ahead of the original programme, according to the company. It completes the tunnelling drive—not the whole Grain to Tilbury project.
Where the tunnel is and what it is for
The new high-voltage electricity cable tunnel runs beneath the River Thames between Tilbury, Essex, and Gravesend, Kent. It forms part of National Grid’s Grain to Tilbury project and The Great Grid Upgrade. National Grid says it will replace the existing Thames Cable Tunnel, which has been in service since the late 1960s and is nearing the end of its operational life. The company describes the wider aim as strengthening the electricity network and supporting connections for cleaner, home-grown energy; its announcement does not quantify added capacity or a specific emissions reduction. National Grid’s 7 October 2026 announcement reports the breakthrough and project status, while its 21 August 2026 project update describes the tunnel’s replacement role.
What the 49-days-early milestone means
National Grid compares the 27 September breakthrough with the project’s original programme and says it came 49 days early. That is a schedule claim about the tunnel drive; it is not evidence that the overall project is 49 days ahead at completion, or that the tunnel is ready to carry electricity.
The company reported that more than 10,000 concrete tunnel segments, forming nearly 1,700 rings, had been installed by breakthrough. It also cited 926 metres advanced and 712 rings installed in its most productive month, a peak weekly advance of 229 metres, and a best 24-hour output of 41 metres excavated with 31 rings installed. These are project figures reported by National Grid, not independently audited statistics.
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Why the Gravesend shaft was flooded
Before the TBM broke through into the Gravesend reception shaft, the shaft was intentionally flooded. National Grid says the water balanced hydrostatic pressure between the shaft and the surrounding chalk, significantly reducing the risk of groundwater flowing into the shaft during breakthrough. This was a planned ground- and water-management measure for the connection point, rather than an indication that the tunnel itself was flooded for operation.
How the tunnelling work was delivered
National Grid attributes the early finish to several factors rather than to a single production record:
- Planning and ground conditions: the company credits careful planning and favourable ground conditions.
- TBM and materials flow: National Grid says Caroline performed well, tunnel segments were supplied in a coordinated way, and a separation plant continuously processed excavated material.
- Project coordination: the company credits collaboration among itself, delivery partner Ferrovial BEMO JV, design partner Mott MacDonald, and tunnelling specialist Herrenknecht.
- Digital tools and remote inspection: National Grid says digital design tools and connected TBM data supported visibility, quality assurance, and decisions. An integrated underwater telescopic camera enabled remote inspections, reducing the need for personnel intervention and supporting safe working practices.
These are National Grid’s explanations of the performance, not an independent assessment of which factors caused the schedule result. The announcement also does not establish that the reported outputs demonstrate cost efficiency or superior safety performance.
What remains before the tunnel is finished
At the time of National Grid’s 7 October announcement, Caroline still had to advance beyond the Gravesend shaft wall to its final position so the remaining tunnel ring-segment construction could be completed. Shaft dewatering had started in September and was expected to continue through October; after that, the TBM was to be lifted and disassembled as the project moved into its next phase. National Grid did not give a final commissioning date or say that the whole project was operational.
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Project context
Caroline was launched for the project in March 2026. National Grid’s launch announcement reported a weight of 271.5 tonnes. That is the machine’s launch-announcement figure, not a measure of the finished tunnel or of its capacity. National Grid’s 12 March 2026 launch announcement gives that specification.
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