HS2’s Northolt Tunnel Reaches Breakthrough After 14 Months of Continuous Excavation

The excavation of the 8.4-mile Northolt Tunnel under London marks a pivotal achievement in the HS2 project, now recognised as Britain’s second-longest tunnel. This twin-bore tunnel, which stretches between West Ruislip and the new Old Oak Common interchange, was constructed using four tunnel boring machines (TBMs). The final TBM, named Anne, completed its drive in June 2025 after 14 months of non-stop underground operations.

Excavated to a depth of up to 35 metres, the tunnel required a high level of precision and coordination. Each TBM was supported by three rotating shifts of skilled workers, ensuring continuous progress. Anne alone installed over 14,000 concrete rings, each formed by segments weighing up to 7 tonnes. Throughout the excavation, approximately 4.16 million tonnes of London clay were extracted and transferred by conveyor to HS2’s London Logistics Hub, then removed by rail to reduce pressure on road networks.

The excavation process involved complex ground conditions, especially at the reception point in Green Park Way, where high water pressure necessitated the use of steel reception cans. These allowed the TBMs to maintain internal pressure and prevent water ingress until safely removed.

With the tunnel excavation now complete, construction will shift to the tunnel base or invert, where high-speed rail tracks will be laid. Additionally, 34 cross passages will be built between the twin bores, alongside five surface-level headhouses. These structures will house ventilation and fire control systems critical to operational safety.

Following, a video discussing the remarkable achievement of Northolt Tunnel breakthrough.