HS2 has today (3rd April 2025) completed a 5-mile tunnel drive under London – using one of four giant tunnelling machines being deployed to build the railway’s Northolt Tunnel.
The tunnel boring machine (TBM), named Caroline, is the second machine to reach the Green Park Way vent shaft in Ealing. The 2,050-tonne TBM broke through into an underground reception can filled with foam concrete, and installed the 4,217th concrete tunnel ring to complete the 5-mile stretch of tunnel.
Due to high water pressure in the ground at the site, HS2 is using the reception can method for the breakthrough of the machines. The reception can allows the TBM to maintain pressure while sealant can be applied from the tunnel lining, preventing water ingress. The reception can is then depressurised before it is opened and the TBM is lifted out.
A quartet of TBMs are being used to build the Northolt Tunnel – an 8.4-mile twin bore tunnel that will take brand new high-speed trains from the outer edge of the capital at West Ruislip into the new Old Oak Common super-hub station in west London. Two machines, including Caroline, have been used to build the western section of the tunnel while two more excavate the eastern leg – all being lifted out of the ground at Green Park Way.
Launched in October 2022, TBM Caroline has completed her drive with teams working around the clock to operate the machine. It excavates the earth and installs the concrete segments that make up the rings to form the tunnel. The TBM was named by school children from Brentside Primary Academy in Ealing after 18th century astronomer Caroline Herschel.
The first machine to be launched for this stretch of the HS2 route, Sushila, completed her journey in January and has subsequently been lifted out from the ground at Green Park Way using a gantry crane. Two further machines are currently building the eastern section of the tunnel from HS2’s Victoria Road Crossover Box just outside of the Old Oak Common station site. In total 93% of the Northolt tunnel has now been built, and the two further machines will complete their journeys later this year.
Work on construction of the new high-speed continues as Mark Wild, the Chief Executive of HS2 Ltd, undertakes a comprehensive review of the project. His work will lead to a fundamental reset the programme – ensuring it is delivered efficiently and for 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, Skanska Costain STRABAG joint venture (SCS), is responsible for building the HS2 route through London. In total, this involves 12.9 miles of twin-bore tunnels, including the Northolt Tunnel and the Euston Tunnel, which will eventually take the railway from Old Oak Common station into HS2’s central London terminus at Euston. Excavation of the Euston Tunnel is expected to start in the next 12 months. SCS will also be building eight vent shafts and headhouses – providing ventilation and emergency access to the tunnels.
Each tunnel boring machine operates like an underground factory, excavating the tunnels, lining them with pre-cast concrete tunnel segments, grouting them into place before moving forward at an average speed of 16 metres per day. The tunnel boring machines are made by world leading tunnelling experts Herrenknecht AG.
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.”
Now that the two western TBMs have completed their journeys, SCS JV will continue to work in the 5-mile stretch of the Northolt Tunnel to construct 20 cross-passages which run between the two tunnel bores. Cross passages are a safety mechanism allowing passengers to cross to the other side of the tunnel in an emergency. The team will also install tunnel inverts – the flat surface on which the high-speed track will be laid – and complete the tunnel walkways.
In total, five deep, twin-bore tunnels are being built for HS2 between London and the West Midlands. HS2 has already completed the excavation of of two of its tunnels – the 10-mile Chiltern Tunnel and the 1-mile Long Itchington Wood Tunnel in Warwickshire. 2025 is set to be a bumper year for TBMs completing their journeys across the route, with two drives now complete in London, and four more to go – two in the West Midlands and two in London.
HS2 is also preparing two further TBMs to build the Euston Tunnel after the Chancellor gave the green light in the budget last year.
