HS2 lifts fourth TBM after Northolt breakthrough
HS2 engineers have lifted the final tunnel boring machine (TBM) from the 8.4-mile Northolt twin tunnels in west London, marking a major milestone in the capital’s underground construction programme.
The TBM, named ‘Anne’, was lifted to the surface at Green Park Way in Greenford on Sunday 24 August, after completing a 3.4-mile (5.5 km) drive. The machine had been in operation for 16 months, excavating London Clay while installing thousands of concrete tunnel segments to form the permanent lining.
Named after educational reformer Lady Anne Byron, the 170 m long, 1,700-tonne machine was removed in sections using a large gantry crane. Its 9.11 m diameter cutterhead was the first part to be lifted from the shaft, followed by the shields and back-up gantries.
Anne is one of four TBMs used to excavate the Northolt tunnels, which run from West Ruislip to Old Oak Common. TBMs launched from both ends of the route and met in the middle. Across the four machines, the Northolt drives:
- excavated over 4 million tonnes of clay, and
- installed nearly 100,000 precast concrete segments to form the tunnel rings.
Each segment weighs up to 7 tonnes, and segments were delivered deep into the tunnel using multi-service vehicles capable of running in both directions on the tight underground route.
A hidden concrete operation
Each TBM acts as a mobile underground factory, cutting forward while installing precast concrete ring segments, then grouting them in place to form a watertight seal. A crew of 17 workers operate the TBM around the clock, supported by surface teams monitoring pressure, steering, and logistics.
As tunnels extend, the distance to the TBM increases, creating growing logistical complexity. For example, on the Chiltern Tunnel — the longest on the route — it took over an hour to reach the TBM from the south portal by the end of its drive.
As with other HS2 tunnels, each ring is made from seven precast concrete segments, produced off-site and stockpiled near the portal. The ring build cycle is critical to TBM progress, with any disruption in segment supply risking delays.
What’s next
With the Northolt tunnelling complete, work now shifts to cross passages, tunnel fit-out, and surface connections. TBMs that have completed their drives are being disassembled and returned to their manufacturers, where components will be reused or recycled for future tunnelling projects.
The next big tunnelling milestone will be the launch of the Euston TBMs in 2026, which will excavate the final section of tunnel bringing HS2 trains into the heart of central London.
Tunnel progress across HS2 in 2025
- HS2 will see five TBMs complete their drives this year — making 2025 a breakthrough year for the megaproject.
- Over 82% of tunnelling is now complete across the route.
- The 10-mile Chiltern Tunnel and 1-mile Long Itchington Wood Tunnel are now structurally finished, with base slabs, cross passages, and walkways installed.
- TBM Elizabeth remains the last active machine on site, continuing the second drive of the 3.5-mile Bromford Tunnel in Birmingham.
- Preparations are underway for the final Euston Tunnel drives, with TBMs due to launch in 2026.
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