HS2 removes second TBM after 8km London tunnel drive

HS2 has successfully removed its second giant tunnel boring machine (TBM) from beneath west London, marking another key milestone in the construction of the high-speed rail line’s twin-bore Northolt Tunnel.

The machine, named Caroline, was lifted in sections from the Green Park Way site in Greenford over the late May bank holiday weekend. Its 9.48-metre diameter cutterhead, along with the front and middle shield, weighed around 850 tonnes and required a large gantry crane for removal.

Caroline completed her 8-kilometre drive from the West Ruislip portal to Greenford in April, having excavated over 1.2 million tonnes of earth and installed 4,217 concrete tunnel rings. Each ring was formed from precast concrete segments manufactured off-site to precise specifications, demonstrating the scale and importance of concrete infrastructure in HS2’s delivery.

The machine is one of four TBMs working on the 13.5km Northolt Tunnel, which will carry high-speed trains from Old Oak Common to the edge of London. It follows the earlier removal of TBM Sushila in March, also at the Green Park Way site.

The tunnel lining segments form one of the most concrete-intensive elements of the project, driving demand for high-performance precast concrete and supporting skilled jobs in manufacture, logistics, and installation.

Caroline was named by local schoolchildren after astronomer Caroline Herschel. Her removal represents not only an engineering milestone but a significant logistical achievement, underscoring the role of concrete in one of the UK's largest infrastructure programmes.