National Grid has reached the halfway point in construction of a 2.2-km high-voltage electricity cable tunnel beneath the River Thames between Tilbury in Essex and Gravesend in Kent.

Tunnel boring machine (TBM) Caroline has excavated more than 1.1 km of the tunnel since tunnelling began in May. The project will replace the existing Thames Cable Tunnel, which has been in service since the late 1960s and is approaching the end of its operational life.

The new tunnel is part of National Grid’s Great Grid Upgrade and is intended to strengthen the electricity network and support connections for energy sources serving homes and businesses across the UK.

More than 40,000 tons of material have been excavated during construction, while more than 5,100 precast concrete segments have been installed to form the tunnel lining. The project team has installed more than 840 tunnel rings, with a best daily performance of 29 rings installed during a 24-hour period.

Weighing 464 tons with its trailing gear, TBM Caroline uses a specialized Mixshield system to excavate through ground conditions beneath the Thames. The machine was named after electrical engineer Caroline Haslett.

The project previously reached a milestone with the launch of Verena, described by National Grid as Europe’s largest-diameter vertical shaft sinking machine. Verena was used to construct the tunnel access shafts at Tilbury and Gravesend.

National Grid is delivering the Grain to Tilbury project in partnership with Ferrovial BEMO JV and tunnelling specialist Herrenknecht. Tunnelling is expected to continue toward a breakthrough later this year.

“Reaching the halfway point of tunnelling beneath the River Thames is an important moment for the Grain to Tilbury project,” said Mark Farmer, project director at National Grid. “The progress made so far reflects the dedication of everyone involved and moves us a step closer to delivering infrastructure that will help strengthen the electricity network and support Britain’s future energy needs.”

Borja Trashorras, project director at Ferrovial BEMO JV, said the project team has focused on safely advancing the tunnel through complex ground conditions while maintaining productivity and environmental performance.

Ulrich Schaffhauser, a member of the board at Herrenknecht AG, called the Grain to Tilbury project a benchmark for the company and said the combination of specialized tunnelling technology and project execution had contributed to the progress.