A 271.5t tunnel boring machine (TBM) has begun excavating a 2.2km tunnel beneath the River Thames as part of National Grid’s Grain to Tilbury project.
The machine, named Caroline, was launched from a 48.7m-deep shaft at Tilbury in a ceremony attended by energy minister Michael Shanks. The tunnel will replace the existing power tunnel under the Thames, which was opened in 1969, and be fitted out with higher capacity transmission infrastructure.
The tunnel will be 4m in diameter and cut through chalk about 15m below the riverbed. Contractor Ferrovial Bemo JV (FBJV) is delivering the scheme for National Grid. The passage will form part of new infrastructure intended to support the electricity network that connects the Isle of Grain and Tilbury, an upgrade National Grid describes as vital to long‑term security and decarbonisation of supply.
Because the TBM is too long to fit fully inside the 15m diameter Tilbury shaft, engineers have begun an “umbilical” launch. Speaking to NCE about the project in-depth last year, FBJV project director Borja Trashorras explained that a 12m section of the machine will start the initial excavation inside the shaft while temporarily supplied with power. The team will then add further gantries and TBM sections incrementally, a process described as a “flying launch” that uses a steel portal frame and jacking system rather than pushing against temporary rings.
The tunnelling rate is expected to be slow at first, roughly 0.9m per day during the initial phase, rising to about 2m per day after further sections are added. The TBM will reach its full 100m length between March and August 2026. Once at full length, the machine is expected to tunnel at an average of about 23.3m per day, allowing the remaining drive to be completed between August and November 2026.
On the Gravesend side, a second vertical shaft is being created using a vertical shaft sinking machine (VSM). The same VSM was used to excavate the Tilbury shaft over the course of a few months, starting last November. The Gravesend shaft will be 46.3m deep and will receive the TBM when it completes the drive.
Both the TBM and VSM were manufactured by Herrenknecht specially for the project.
Although the tunnel excavation phase is scheduled to finish next year, National Grid said years of fit‑out and testing of electrical infrastructure will follow. Project completion and powering up of the new 400kV circuits is expected in 2028.
In a move to link the scheme with local communities, the TBM was named Caroline after Caroline Haslett, an early 20th‑century electrical engineer and advocate for women in engineering. National Grid and project partners said they are running school outreach and skills programmes alongside the construction works, intending to support apprenticeships, graduate roles and short work‑experience placements. The project currently employs 76 local people and engages 15 local businesses, the company said.
The Grain to Tilbury upgrade has been framed by National Grid as part of wider efforts to reinforce Britain’s electricity network for future low‑carbon generation under its Great Grid Upgrade programme.
Safety and careful site management remain central to the operation as the TBM progresses beneath the Thames; engineers will monitor ground conditions and river impacts as excavation proceeds.
Mark Farmer, project director, National Grid, said: “The arrival of the tunnel boring machine is a significant milestone for the Grain to Tilbury project and a major step forward in strengthening the electricity network in this area for years to come. This important project will help ensure secure, cleaner, home‑grown energy can continue to flow across the network as demand increases.
“It was a great privilege to welcome Energy Minister Michael Shanks, along with representatives from Ferrovial Bemo, Herrenknecht and our National Grid colleagues, to mark the launch of TBM Caroline.”
Borja Trashorras, Project Director, FBJV, said: “The launch of TBM Caroline marks a major achievement for our integrated JV, highlighting the strength of our teamwork, technical expertise and our capability to deliver world-class engineering for National Grid.
“As tunnelling begins, my thanks and congratulations go to everyone whose hard work has achieved this milestone so swiftly with efficiency and excellence.”
Ulrich Schaffhauser, member of the board of management, Herrenknecht AG, said: “River crossings are key in grid construction, and safety comes first in shaft and tunnel works. In addition to the VSM, which has safely sunk the launch shaft, we are proud to deliver a tailor-made Mixshield TBM for the cable tunnel.
“My best wishes to the team of our partners Ferrovial BEMO JV and National Grid on the launch of the TBM.”
Energy minister Michael Shanks said: “Since 1969 the Thames Cable Tunnel has transported electricity under the river between Essex and Kent. But after more than half a century, it now needs replacing.
“This major engineering project is part of the biggest upgrade in Great Britain’s electricity network in a generation. Not only will this boost jobs and growth, but it will help get lower cost homegrown clean power to homes and businesses and get us off the rollercoaster of fossil fuel markets.”
Tim Self, careers leader at St John’s Catholic Comprehensive, said: “We’re really pleased to be working with National Grid and FBJV, and to see such a strong commitment to engaging with our students. The team have provided wonderful support and direction, challenging our students with real-world issues that strengthens their confidence and determination to succeed.
“Opportunities like this help bring learning to life, give young people insight into the world of work, and show them the range of careers that could be open to them in the future. As an educator, it’s a delight to see students enthusiastic about the projects they are doing and keen to engage further, and to hear how they reflect on their future pathways through such experiences.”
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