TBM Caroline finishes 2.2 km power tunnel under the Thames early

What happened

The tunnel runs under the Thames estuary from Tilbury in Essex to Gravesend in Kent, east of London. It replaces the existing Thames Cable Tunnel, which opened in 1969 and has reached the end of its service life. The new tunnel is part of National Grid's Grain to Tilbury project, one piece of a wider programme the company calls the Great Grid Upgrade. It will strengthen the transmission of electricity between the two counties.

Caroline was launched from a shaft at Tilbury in early 2026, with excavation under way by spring. By August, National Grid said the machine had passed the halfway point. On Sunday 27 September, it broke through at Gravesend. Over the full drive, the machine installed more than 10,000 precast concrete segments, which formed almost 1,700 lining rings. In its best month, it advanced 926 m and built 712 rings. Its best day was 41 m and 31 rings.

Several companies shared the work. Ferrovial and BeMo delivered the tunnel as a joint venture, Mott MacDonald designed it, and Herrenknecht supplied the machine and specialist tunnelling services. Mark Farmer, National Grid's project director, credited the early finish to the skill and teamwork of the people involved. The Gravesend shaft is now being drained so the machine can be lifted out and taken apart. Fit-out and electrical testing will follow, and New Civil Engineer reported that the new 400 kV circuits are expected to be powered up in 2028.

The machine was named after Caroline Haslett, an early twentieth-century electrical engineer who campaigned for women in engineering. The UK energy minister, Michael Shanks, attended its launch at Tilbury. At that time, New Civil Engineer reported that the project employed 76 local people and had worked with 15 local businesses. These figures are a reminder that a tunnel project also affects the towns at each end, through jobs, traffic, noise and the movement of spoil, and that engineers must plan for those effects as well as for the tunnel itself.

The engineering behind it

Caroline is a Herrenknecht Mixshield, a type of closed-face tunnel boring machine. It cut a tunnel with an internal diameter of 4 m through chalk, about 15 m below the riverbed. In general, a closed-face machine keeps the ground at the cutting face under pressure, often using a slurry of water and fine clay. This supports the face and holds back groundwater, which matters greatly under a river. The excavated material is pumped out as slurry and separated at a treatment plant on the surface. New Civil Engineer reports that a dedicated separation plant handled the spoil on this project.

Behind the cutting head, the machine builds the tunnel lining as it goes. Each ring is made of several precast concrete segments, lifted into place by an erector arm and bolted together. The machine then pushes forward against the finished ring using hydraulic jacks. Gaskets between the segments keep water out. Because the segments are made in a factory, their shape and concrete quality can be closely controlled.

The shafts at each end are deep. According to New Civil Engineer, the Tilbury launch shaft is 48.7 m deep and 15 m in diameter, and the Gravesend reception shaft is 46.3 m deep. Both were sunk with a vertical shaft sinking machine, also made by Herrenknecht, which excavates under water while the shaft lining is lowered from above. The full tunnel boring machine was about 100 m long, too long for the shaft, so it was started with a shorter section and extended as the drive went on.

The breakthrough itself used a careful method. The Gravesend shaft was deliberately flooded before the machine arrived. This balanced the water pressure inside the shaft with the pressure in the surrounding chalk, which reduced the risk of groundwater rushing in as the machine broke through the shaft wall. Engineers also used an underwater camera for remote inspection.

What it means in Nepal

The sources do not mention Nepal, so this section is about the work and skills involved. The first point is that tunnels are not only for roads and railways. Power companies, water utilities and drainage authorities also need them to carry cables, pipes and flows under rivers and cities. A civil engineer who understands tunnelling may work for an electricity company as easily as for a road agency.

The second point is about planning. This drive finished early because, as the contractors said, the ground was favourable and the work was well organised. Steady progress in a tunnel depends on a supply of segments arriving on time, the removal of spoil, maintenance of the machine and good communication between shifts. These are construction management skills, and they decide whether a project finishes early or late.

The third point is that ground conditions set the method. Here, the engineers knew the tunnel would be in chalk under a river, and they chose a closed-face machine and a flooded reception shaft to handle the water. Any tunnel design begins with understanding the ground and the groundwater, and then choosing a machine, lining and method that suit them.