The breakthrough of a 2.2km tunnel beneath the River Thames between Kent and Essex has been completed 49 days ahead of schedule.

The work is part of The Great Grid Upgrade project to install new high-voltage electricity cables, running from Tilbury in Essex and Gravesend in Kent.

The tunnel boring machine completed the breakthrough on 27 September. The final stage of the journey was underwater, with the reception shaft intentionally flooded to balance pressure between the shaft and surrounding chalk ground and reduce the risk of groundwater entering the shaft during the final stage of tunnelling. 

With more than 10,000 concrete tunnel segments installed to create the tunnel lining, and nearly 1,700 tunnel rings constructed, the breakthrough marks the completion of the project's tunnelling phase.

The achievement was supported by close collaboration between National Grid, delivery partner Ferrovial BEMO JV, design partner Mott MacDonald and tunnelling specialist Herrenknecht.

At peak performance, the tunnel boring machine advanced 229 metres in a single week, while its best daily output seeing 41 metres excavated and 31 tunnel rings installed in just 24 hours.

Mark Farmer, project director at National Grid, described the breakthrough as a “fantastic achievement” and a “major milestone” for the Grain to Tilbury project.

"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. “Everyone involved has worked together to deliver an exceptional performance safely and efficiently, helping us reach this important milestone sooner than anticipated.

“This project will play a critical role in strengthening the electricity network and supporting Britain's future energy needs as demand for cleaner electricity continues to grow.”

Following the breakthrough, activities will continue to allow the tunnelling machine to advance beyond the shaft wall and reach its final position, enabling the remaining ring-segment construction works to be completed. 

Dewatering of the shaft started in September and will continue through October. Once complete, the TBM will be lifted, before being disassembled as the project progresses into its next phase of construction.