Excavation of HS2’s Northolt Tunnel was completed yesterday when the final TBM arrived at Green Park Way in Greenford, west London.
TBM Anne, the fourth of four TBMs used to dig the 13.5km twin-bore tunnel, installed the final concrete ring underground before emerging into a vent shaft.
The Northolt Tunnel will take HS2’s high-speed trains between West Ruislip at the outer edge of the capital and the new Old Oak Common super-hub station in west London. It is one of five deep twin-bore tunnels on the 225km route and the second longest behind the Chiltern Tunnel, which is 16.1km long.
The Northolt Tunnel, which is 35m below ground at its deepest point, was bored by HS2’s London tunnels contractor Skanska Costain Strabag JV (SCS JV). Two TBMs built the 8km western section from West Ruislip and two built the 5.5km eastern section from Victoria Road in North Acton.
TBM Anne, named after Lady Anne Byron, has now finished the second bore of the 5.5km eastern section, completing the tunnel excavation.
All four Herrenknecht machines finished their journey at Green Park Way where they are to be lifted out from the underground shaft. The third TBM, Emily, finished its tunnel bore earlier this month, and will be lifted out of the ground later this summer. The first two machines to arrive, from West Ruislip, were lifted out earlier this year.
In a complex engineering operation, high water pressure in the ground at the site at Green Park Way has meant that the TBMs are finishing their drives in steel reception cans installed in two underground shafts. The reception can method allows the TBM to maintain pressure while sealant can be applied from the tunnel lining, preventing water ingress. It is then depressurised before it is opened and the TBM is lifted out.
TBM Anne was launched in April 2024 and has spent 14 months mining the tunnel. Three teams of 48 workers have operated the machine around the clock to complete the work. At peak, the 1,700-tonne, 9.11m-diameter TBM advanced at around 38m per day.
The excavated London clay was taken to HS2’s London Logistics Hub where it was taken away by rail for beneficial reuse across the UK.
In total, the four TBMs excavated 4.16 million tonnes of London clay and lined the tunnel with 14,300 concrete tunnel rings, made up of 94,233 concrete segments. The tunnel segments used by TBM Anne were made in Hartlepool by Strabag and transported to site by rail.
The inside width of the tunnel varies between the eastern and western section – 8.10m in the east and 8.80m in the west. The tunnels for HS2 are wider for the sections of the route where the trains run faster given aerodynamic needs.
SCS JV managing director Richard Adam said TBM Anne’s arrival was a major milestone for both HS2 and SCS.
“This is the fourth and final TBM that completes the 8.4-mile twin-bore Northolt Tunnel. TBM Anne’s journey has been one of efficiency and precision, delivered safely and on schedule,” he said.
“As we move forward, work continues at pace on the cross-passages, tunnel fitout and surface structures and we are preparing to launch our final two TBMs to Euston early next year.”
SCS JV will also continue to build the tunnel invert where the high- speed tracks will be laid and complete the five headhouses.
