Skanska Costain STRABAG engineers celebrate successful five-mile tunnelling drive under the capital.
Marking a major milestone for the HS2 London Tunnels project, the giant Tunnel Boring Machine (TBM), named Sushila, reached the Green Park Way vent shaft in Ealing on Thursday 19th December 2024 breaking through into a reception can filled with foam concrete.
The innovative method has been adopted due to high water pressure in the ground on the site. It allows the TBM to maintain its pressure while sealant can be applied from the tunnel lining to prevent water ingress.
Once the reception can is depressurised, it will be opened and the TBM will be lifted out.
The 8.4-mile twin-bored Northolt Tunnel is being constructed by a quartet of machines named Sushila, Caroline, Emily and Anne. They will complete the tunnel that will carry high-speed trains between HS2’s super-hub station at Old Oak Common, West London, and the outskirts of the capital at West Ruislip.
Sushila, named after a local school teacher, was the first to start and first to complete the journey. All four will finish their journeys at Green Park Way, arriving in a main and satellite shaft.
The Northolt Tunnel is being built by our joint venture, Skanska, Costain and STRABAG (SCS). The joint venue will also construct the Euston Tunnel eastward from Old Oak Common to the centre of London. The tunnel boring machines are made by world leading tunnelling experts Herrenknecht AG.
Each machine operates like an underground factory, excavating the tunnels, lining them with pre-cast concrete tunnel segments, grouting them into place before moving forward at an average speed of 16 metres per day. Teams work around the clock below ground on the TBMs along with teams on the surface supporting them.
Launched in October 2022 from West Ruislip, TBM Sushila has excavated over 1.2 million tonnes of earth and installed 4217 tunnel rings. All the earth excavated by the machine has been placed in two areas west of the tunnel, eliminating the need to use public roads for lorry removal. These areas will be turned into wildlife meadows and wooded areas as part of HS2’s Green Corridor.
Macolm Codling, HS2’s Client Director for the London Tunnels, said: “This is our first breakthrough for the Northolt Tunnel and is the result of many years of hard work from our tunnelling contractor. We are on schedule to complete our first tunnel for HS2 trains under the capital by the end of 2025, just as we prepare to bring HS2 tunnels into the heart of the London at Euston.”
The TBM used for the excavation is 160 metres in length, with a cutterhead 9.84 metres in diameter. It weighs approximately 2,050 tonnes, roughly the weight of 10 blue whales.
James Richardson, Managing Director of SCS JV, said: “The arrival of Sushila at the Green Park Way ventilation shaft marks a huge step in the delivery of the Northolt Tunnel and the new railway. This achievement reflects the tremendous efforts of the entire team, which has drawn together global capability to deliver this incredibly complex section of work. We are making great progress on the remaining tunnelling, with three further TBMs in operation on the Northolt Tunnel and the two Euston Tunnel TBMs currently being prepared for launch.”
Sushila Hirani, a local school teacher after whom the TBM was named, said: "To have had the opportunity to witness the progress of the TBM over the last two years has been a real honour. I am in awe of the sheer scale and incredible work of the engineers and excited for the day I will be able to take my first HS2 train journey through the tunnel".
This is the fourth major tunnel breakthrough for HS2 this year. Tunnelling for both bores of the 10-mile Chiltern Tunnel, the longest on the route, was completed in March this year. A logistics tunnel which will be used to help facilitate construction of the Euston Tunnel from Old Oak Common station to Euston was completed in January this year.