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HS2’s TBM Sushila completes Northolt Tunnel milestone

HS2's TBM Sushila has completed the Northolt Tunnel West drive.

Tunnel boring machine (TBM) Sushila has completed a significant section of HS2’s Northolt Tunnel, breaking through into a reception area beneath Green Park Way in Ealing.

The breakthrough, achieved on 19 December 2024, marks a key step in constructing the high-speed railway connecting West Ruislip and Old Oak Common in west London.

The Northolt Tunnel spans 13.5 kilometres (8.4 miles) and is being excavated by four TBMs—Sushila, Caroline, Emily, and Anne. Each machine operates continuously, lining the tunnel with pre-cast concrete segments and moving forward at an average rate of 16 m (52 ft) per day. TBM Sushila launched in October 2022 from West Ruislip and completed its 8-kilometre (5-mile) journey after excavating over 1.2 million tonnes of earth and installing 4,217 concrete rings.

Sushila’s breakthrough was facilitated by an innovative foam concrete-filled reception can, designed to handle the high water pressure in the ground. The method maintains the TBM’s pressure, enabling sealant to be applied from within the tunnel to prevent water ingress. Once the can is depressurised, the machine will be dismantled and removed.

James Richardson, managing director of the Skanska-Costain-STRABAG (SCS) joint venture overseeing the tunnel’s construction, noted, “The arrival of Sushila at Green Park Way marks a huge step forward for the Northolt Tunnel. This achievement reflects the team’s incredible effort and global expertise in delivering such a complex project.”

HS2’s Northolt Tunnel’s TBMs, including Sushila, were manufactured by German firm Herrenknecht AG and weigh approximately 2,050 tonnes. The machine’s cutterhead has a diameter of 9.84 m (32 ft), and the TBM itself measures 160 m (525 ft) in length.

Excavated material has been repurposed locally, avoiding the need for lorry transport and contributing to HS2’s Green Corridor initiative, which will create wildlife meadows and wooded areas.

Malcolm Codling, HS2’s client director for London tunnels, stated: “This breakthrough for the Northolt Tunnel is a testament to years of hard work by our tunnelling contractor. We are on track to complete this section by the end of 2025, advancing HS2’s progress into central London.”

The Northolt Tunnel’s completion is part of HS2’s broader efforts to connect the capital with the Midlands and northern England via a high-speed rail network. Other recent milestones include completing the 16-kilometre (10-mile) Chiltern Tunnel and a logistics tunnel at Old Oak Common earlier this year.

Sushila Hirani, a local teacher after whom the TBM is named, expressed her pride in the project: “It has been an honour to witness this engineering feat. I look forward to taking my first HS2 journey through the tunnel.”

Construction of the HS2 route through London continues, with additional TBMs preparing to launch for the Euston Tunnel, connecting Old Oak Common to central London.

 

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