A tunnel boring machine (TBM) has completed excavating a second bore of the 5-mile-long Northolt Tunnel for the HS2 high-speed railway between London and Birmingham.

Four tunnel boring machines are being used to excavate Northolt Tunnel, and earlier today, Thursday, 3rd April, TBM Caroline became the second machine to emerge at the Green Park Way vent shaft in Ealing, West London.

Caroline broke through into an underground reception can filled with foam concrete, where it installed the 4,217th concrete tunnel ring needed to complete the tunnel.

Because there is high water pressure in the ground at this site, a special method called a “reception can” method was used to enable Caroline to break through.

The reception can method allows pressure to be maintained, which prevents water ingress while sealant is applied to the tunnel lining.

Each of HS2’s tunnel boring machines is named, with Caroline being named after the 18th-century astronomer Caroline Herschelfor.

The name was chosen by pupils from Brentside Primary Academy in Ealing.

Including the Northolt Tunnel and yet-to-be-constructed Euston Tunnel, there will be 12.9 miles of twin-bore tunnels from Old Oak Common station in West London to HS2’s central London terminus at Euston.

They are being constructed by Skanska Costain STRABAG joint venture (SCS), which will also construct eight vent shafts and headhouses to provide ventilation and emergency access to the tunnels.

Further work to complete Northolt Tunnel includes the construction of 20 cross-passages between the two tunnel bores to allow passengers to cross to the other side of the tunnel in an emergency and the installation of tunnel inverts that will provide a flat surface to support the high-speed track.

“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.”

Richard Adams, Managing Director for Skanska Costain STRABAG joint venture