HS2’s second tunnel boring machine (TBM), named Caroline, has completed a 5-mile tunnel drive under London. This is the second machine to reach the Green Park Way vent shaft in Ealing, and one of four giant tunnelling machines being deployed to build the railway’s Northolt Tunnel. 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. This method 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.
Four TBMs are being used to build the Northolt Tunnel – an 8.4-mile twin bore tunnel that will take 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.
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.