HS2 has completed the final 3.4-mile stretch of excavation of the main 8.4-mile Northolt Tunnel under London, hitting a major milestone in the construction of infrastructure for the service.
Completed by the fourth of four tunnel boring machines (TBMs) on 26 June; the twin-bore tunnel is the second longest on the HS2 route between London and the West Midlands, coming in just behind Chiltern Tunnel, which runs 10 miles long.
Following the installation of the final concrete ring underground, the TBM, christened Anne, emerged into a vent shaft at Green Park Way, Greenford.
Anne was first launched in April 2024 and completed its journey in 14 months with the help of three teams of 48 workers, with its peak excavation period seeing it move at roughly 38 metres per day.
The Northolt Tunnel will carry the service’s high-speed trains between West Ruislip and the Old Oak Common super-hub station in West London.
Construction work on the tunnel was carried out by HS2 London tunnels contractor Skanska Costain STRABAG JV (SCS JV) and used four TBMs, with two responsible for the 5-mile western section from West Ruislip and two responsible for the 3.4-mile eastern section from Victoria Road, North Acton.
All four TBMs concluded their journey at Green Park Way, where they are to be lifted from the underground shaft. The third TBM, Emily, finished its tunnel bore earlier this month, and will be lifted from the ground later this summer.
The initial two TBMs (Caroline & Sushila) were lifted out earlier this year, with all four machines having finished their drives in steel reception cans in order to tackle issues with high water pressure in the ground on-site. This allows for the TBMs to maintain pressure while sealant is applied from the tunnel lining, preventing the ingress of water before depressurisation and removal.
Alan Morris, Construction Delivery Director, HS2 Ltd, said:Completing the excavation of this 8.4 mile-long tunnel on HS2 is a real achievement and one the team should be immensely proud of.
We’re building HS2 for the future, to increase capacity on our rail network and improve journeys for millions of rail users.
The construction of HS2 is already bringing benefits, with £20bn economic benefit already being delivered at either end of the line.
The HS2 project is currently undergoing a reset led by Mark Wild to ensure its delivery at the lowest reasonable cost whilst continuing to progress with major structures such as tunnels and viaducts.
The four machines, manufactured by Herrenknecht AG, excavated a total of 4,160,000 tonnes of London clay, simultaneously lining the tunnel with 14,300 concrete tunnel rings made up of 94,233 precision-designed concrete segments, which were in turn manufactured in Hartlepool by STRABAG.
Richard Adam, Managing Director, SCS JV, said:The arrival of TBM Anne today marks a major milestone for both HS2 and SCS. This is the fourth and final TBM that completes the 8.4-miles twin-bore Northolt Tunnel. TBM Anne’s journey has been one of efficiency and precision, delivered safely and on schedule.
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.
Following the completion of the tunnel’s excavation, the team responsible will continue to construct the flat tunnel invert on the base, creating a surface for the laying of all high-speed tracks going forward, as well as the mine 34 cross passages between the two bores of the tunnel.
Five headhouses will also be installed on the surface of the tunnel, with each covering the ventilation shaft connecting HS2 to the open air in order to regulate both air quality and temperature within the tunnel.
