HS2 tunnel breakthrough delivers concrete milestone

The third of four tunnel boring machines (TBMs) constructing HS2’s Northolt Tunnel has completed its 5.5km drive beneath west London, marking a major concrete milestone on the high-speed rail route.

TBM Emily broke through at Green Park Way in Greenford earlier this month after launching in February 2024. The 1,700-tonne machine installed 17,514 precast concrete segments along its route, forming a key section of the 13.5km twin-bore tunnel connecting West Ruislip to Old Oak Common.

Each segment, weighing up to 7 tonnes, was manufactured by STRABAG at its Hartlepool facility and transported to site for installation. The segmental lining was reinforced and placed using earth pressure balance tunnelling techniques, designed specifically for London Clay.

The breakthrough marks the penultimate drive on the Northolt Tunnel, which will be fully excavated later this summer when the final TBM, Anne, completes its journey. Once the structural works are finished, crews will focus on casting the tunnel invert, building walkways and access points, and completing 34 concrete-lined cross passages between the two tunnel bores.

“This 5.5km drive is a major achievement for our team,” said Malcolm Codling, project client director for HS2 Ltd. “With tunnelling nearing completion, we’re focusing on delivering the next phase of civil works safely and efficiently.”

The Northolt Tunnel is being delivered by the Skanska Costain STRABAG joint venture (SCS JV). As with other HS2 tunnel drives, London Clay excavated from the route has been removed by conveyor and rail to minimise road traffic, and is being reused at sites in Kent, Cambridgeshire and Warwickshire.

“Precast concrete continues to play a central role in delivering safe, high-performance tunnels to tight tolerances,” said Dave Hannon, project director for tunnels and routeway at SCS JV. “Our teams are now pressing ahead with fit-out and supporting structures to get the tunnel ready for track installation.”

With tunnelling works also advancing at Bromford and Long Itchington Wood, 2025 is shaping up to be a landmark year for concrete progress on HS2’s subterranean infrastructure.