A 2.2km high-voltage electricity cable tunnel being built underneath the Thames has reached the halfway point ahead of its planned opening in 2029.

The £230m project is a new high-voltage electricity link that will replace the existing 1960s Thames Cable Tunnel. It forms part of The Great Grid Upgrade, the largest overhaul of the electricity grid in generations.

A tunnel boring machine (TBM) known as Caroline has been deployed to create the new infrastructure, and it has passed 1.1km beneath the River Thames since it first broke ground in May 2026. So far, more than 40,000 tonnes of material have been excavated and over 5,100 pre-cast concrete tunnel segments installed to form the tunnel lining. Just over 840 rings have been installed, and the tunnelling team has achieved its best daily performance of 29 tunnel rings installed within a 24-hour period.

Weighing 464 tonnes, the TBM, together with the trailing gear, is excavating the tunnel using a specialist Mixshield system. Named after pioneering electrical engineer Caroline Haslett, TBM Caroline has been designed to operate safely in the challenging ground conditions beneath the Thames. 

While the physical digging of the tunnel is moving quickly the project is still not expected to be ready to go until 2029 at the earliest. Excavating the hole is only the first major phase of a much larger, highly complex electrical engineering project.

The tunnel’s sole purpose is to house critical 400-kilovolt (kV) electricity transmission circuits. But hauling, installing, and securing over 2.2km of heavy, specialised high-voltage cables deep underground will be a slow and meticulous process.

The project also requires building permanent “headhouses” on top of the 35-metre-deep access shafts at both the Tilbury and Gravesend ends. These buildings will secure the tunnel and provide long-term access for maintenance crews.

Borja Trashorras, project director at contractors Ferrovial BEMO JV, said: “Delivering more than 1.1km of tunnel safely through complex ground conditions, while maintaining strong productivity and environmental performance, is a testament to the planning, engineering and innovation at the heart of this project.  We’re proud of the progress made so far and remain focused on carrying that momentum forward as Caroline continues her journey towards a breakthrough later this year.”