On 3rd April 2025 HS2 has completed a 5-mile tunnel drive under London – using one of four TBMs being deployed to build the railway’s Northolt tunnel. The TBM named Caroline, is the second machine to reach the Green Park Way vent shaft in Ealing. The 2,050-t 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 - Emily and Anne - 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. Later this year they will finish their journeys, which will complete the 8.4-mile Northolt tunnel between Old Oak Common and West Ruislip.
In total 93% of the Northolt tunnel has now been built.
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.
