Another major milestone has been reached in the construction of the High Speed Two (HS2) railway, as the second giant tunnelling machine deployed for the Northolt Tunnel has successfully completed its 5-mile underground journey beneath the capital.
The massive 2,050-tonne Tunnel Boring Machine (TBM), named Caroline, broke through into a specially constructed underground reception chamber filled with foam concrete at the Green Park Way vent shaft in Ealing. This breakthrough marks the installation of the 4,217th concrete tunnel ring, completing the significant 5-mile stretch of the twin-bore tunnel.
Due to high water pressure in the ground at the Green Park Way site, HS2 engineers employed a “reception can” method for the TBM’s arrival. This technique allows the machine to maintain pressure while sealant is applied to the tunnel lining, effectively preventing water ingress. The reception can will now be depressurised before being opened, allowing Caroline to be safely lifted out of the ground.
Caroline is one of a quartet of TBMs excavating the 8.4-mile Northolt Tunnel, a crucial section of the HS2 route that will carry high-speed trains from West Ruislip on the outskirts of London into the new Old Oak Common super-hub station in west London. Two TBMs, including Caroline, have been responsible for the western section of the tunnel, while the remaining two are currently digging the eastern leg. All four machines will eventually be extracted at the Green Park Way site.
Launched in October 2022, TBM Caroline operated around the clock, excavating the earth and simultaneously installing the concrete segments that form the tunnel rings. The machine was named by local schoolchildren from Brentside Primary Academy in Ealing in honour of 18th-century astronomer Caroline Herschel.
The first TBM to be launched for this section, Sushila, completed its drive in January and has already been lifted out of the ground using a gantry crane. The two remaining TBMs, Emily and Anne, are currently progressing with the eastern section of the tunnel from HS2’s Victoria Road Crossover Box, located just outside the Old Oak Common station site. Impressively, 93% of the entire Northolt Tunnel has now been completed, with Emily and Anne expected to finish their drives later this year.
While construction continues apace, HS2 Ltd Chief Executive Mark Wild is currently undertaking a comprehensive review of the project, aiming to reset the programme to ensure efficient delivery and the lowest possible cost.
Malcolm Codling, Client Director for HS2 Ltd, said: “We’re immensely proud of the work we have done to complete the second breakthrough for the Northolt Tunnel underneath the capital. As we focus on delivering Britain’s new high speed railway between London and the West Midlands, we are gearing up for our most productive year to date on this stretch of the line.”
HS2’s London tunnels contractor, the Skanska Costain STRABAG joint venture (SCS), is responsible for the 12.9 miles of twin-bore tunnels being built through the capital, including the Northolt Tunnel and the future Euston Tunnel. Excavation of the Euston Tunnel, which will connect Old Oak Common to HS2’s central London terminus at Euston, is anticipated to commence within the next year. SCS will also be constructing eight ventilation shafts and headhouses along the route.
Each of the Herrenknecht AG-built TBMs operates as a sophisticated underground factory, excavating the tunnels, lining them with pre-cast concrete segments, and grouting them into place, advancing at an average speed of 16 metres per day.
Richard Adams, Managing Director for Skanska Costain STRABAG joint venture, said: “The arrival of TBM Caroline at Green Park Way marks a significant milestone – the completion of the western section of the Northolt Tunnel – and is a testament to the dedication and skill of all our teams. Later this year two more TBMs, Emily and Anne, will finish their journeys, which will complete the 8.4-mile Northolt tunnel between Old Oak Common and West Ruislip.
“Work on the cross passages, tunnel walkways, ventilation shafts, headhouses and portals continues at pace as our team works at peak productivity to deliver our part of HS2, that will bring better journeys and boost economic growth.”
With the two western TBMs now finished, SCS JV will focus on constructing 20 cross-passages within the 5-mile stretch of the Northolt Tunnel. These cross-passages are vital safety features, allowing passengers to move between the two tunnel bores in the event of an emergency. The team will also install the tunnel inverts, the flat surface on which the high-speed tracks will be laid, and complete the tunnel walkways.
In total, five deep, twin-bore tunnels are being constructed for HS2 between London and the West Midlands. HS2 has already completed the excavation of the 10-mile Chiltern Tunnel and the 1-mile Long Itchington Wood Tunnel in Warwickshire. 2025 is shaping up to be a significant year for tunnelling progress, with two drives now complete in London and four more – two in the West Midlands and two in London – still to come.
Furthermore, HS2 is actively preparing two additional TBMs for the construction of the Euston Tunnel, a project given the green light by the Chancellor in last year’s budget.
