In the muddy banks of the River Thames, a piece of machinery weighing as much as a jumbo jet has begun a slow, relentless push eastwards. Its mission: to carve out a vital artery for Britain’s future electricity supply, part of the largest overhaul of the national grid in generations.
The 271.5-tonne tunnel boring machine (TBM), named ‘Caroline’, is excavating a new 2.2-kilometre crossing between Gravesend in Kent and Tilbury in Essex. The tunnel, with an internal diameter of four metres—comparable in size to a Tube train tunnel—will house high-voltage 400kV cables. It is designed to replace the ageing Thames Cable Tunnel, which was commissioned in 1970 and is now approaching the end of its operational life.
The Legacy in a Name
The TBM’s name honours a pioneer who helped shape the modern electrical world. Caroline Haslett was a trailblazing electrical engineer and a formidable campaigner for women in engineering during the 20th century. She co-founded the Electrical Association for Women in 1924, an organisation that educated women on using electricity safely and promoted its potential to liberate them from domestic drudgery.
According to a research briefing on the project, Haslett was also the first secretary and later president of the Women’s Engineering Society. Her influence extended to safety standards; she was the only woman on a key post-war committee and contributed to developing the BS 1363 standard for the three-pin fused plugs still used in British homes today. The TBM was named after her following a competition with local schools.
A Cornerstone of the ‘Great Grid Upgrade’
This ‘Grain to Tilbury’ project is a critical component of National Grid’s ‘Great Grid Upgrade’, a nationwide effort to strengthen the network and connect vast amounts of new renewable energy, including 50GW of offshore wind by 2030. Energy Minister Michael Shanks launched the TBM, highlighting the project’s strategic importance for infrastructure and economic growth.
The new tunnel is being delivered by National Grid with tunnelling specialists Ferrovial BEMO, a joint venture with a track record on projects like the Thames Tideway and Crossrail. The TBM itself, a Herrenknecht Mixshield model, was manufactured by the German firm that also supplied machines for HS2 and the Elizabeth Line.
Engineering Beneath the Tideway
The engineering undertaking is formidable. Before ‘Caroline’ began her journey, two deep shafts—one 15 metres wide, the other 12 metres—were sunk 35 metres down at each bank using a Vertical Shaft Sinking Machine (VSM) named ‘Verena’, after another pioneering female engineer, Verena Holmes. National Grid states the 15-metre shaft is the widest ever created globally using this precise VSM technology, which allows for simultaneous excavation and lining from the surface.
Atop these shafts, two new ‘headhouse’ buildings are being constructed. These will act as access points for engineers and, crucially, as ventilation shafts. Because the electricity cables generate significant heat, air will be pumped from the Thurrock side through the tunnel to Gravesend to provide cooling.
The concrete rings lining the tunnel are being made locally by Pacadar on the Isle of Grain, which also produced segments for HS2. National Grid confirmed that the material excavated by ‘Caroline’ is expected to be beneficially reused in future local developments.
Community Roots and Future Plans
The project has significant local footprint. So far, 76 people from the surrounding area are working on it, with 15 local businesses in the supply chain. National Grid’s project director, Mark Farmer, said the launch marked a significant milestone. The scheme also plans to support 22 apprenticeships, graduate roles, and work placements, and is working with schools to promote STEM careers.
Looking ahead, the new cable tunnel is scheduled for completion in 2029. Subsequently, National Grid plans to refurbish the overhead line between Tilbury, Kingsnorth, and the Isle of Grain in 2028, with local communities to be consulted. The government has also indicated that households near new or upgraded transmission infrastructure may receive energy bill discounts.
Once the new tunnel is operational, the historic 1970 tunnel—one of the first in the UK to use pre-cast concrete segments, built at a cost equivalent to £59 million today—will be decommissioned. It will not be opened to the public.
As ‘Caroline’ advances metre by metre beneath the river, it does so carrying a dual legacy: enabling a cleaner, more resilient energy grid for the decades ahead, while paying tribute to a visionary engineer who fought to plug women into the power industry of her own time.
