HS2 engineers have completed an 8km tunnelling drive on the Northolt Tunnel under London.
TBM Sushila, reached the Green Park Way vent shaft in Ealing yesterday, breaking through into a reception can filled with foam concrete.
The innovative method was adopted because of high water pressure in the ground on the site. It allowed the TBM to maintain its pressure while sealant could 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 13.5km twin-bore Northolt Tunnel is being constructed by four Herrenknecht machines: 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 teacher, was the first to start and is the first to complete the journey. The four TBMs each make an average daily advance of 16m and will all finish their journeys at Green Park Way, arriving in a main and satellite shaft.
The Northolt Tunnel is being built by the Skanska, Costain and Strabag (SCS) joint venture. The group will also construct the Euston Tunnel eastward from Old Oak Common to the centre of London.
Launched in October 2022 from West Ruislip, 9.84m-diameter TBM Sushila has excavated over 1.2 million tonnes of earth and installed 4,217 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.
Malcolm Codling, HS2’s client director for the London Tunnels, said the breakthrough was the result of many years of hard work by SCS.
“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,” he said.
SCS JV managing director James Richardson said Sushila’s arrival at Green Park Way marked a huge step in the delivery of the Northolt Tunnel and HS2.
“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,” he said.
This is the fourth major tunnel breakthrough for HS2 this year. Tunnelling for both bores of the 16.1km Chiltern Tunnel[https://www.tunnelsandtunnelling.com/news/final-breakthrough-made-on-hs2s-longest-tunnel/], the longest on the route, was completed in March. The Atlas Road Logistics[https://www.tunnelsandtunnelling.com/news/tbm-breaks-through-on-hs2s-london-logistics-tunnel-11459696/] tunnel which will be used to help facilitate construction of the Euston Tunnel from Old Oak Common station to Euston was completed in January.
In the budget in October, the Chancellor announced that the construction of HS2’s Euston Tunnel would go ahead[https://www.tunnelsandtunnelling.com/news/uk-government-commits-funding-for-hs2-tunnelling-to-euston-station/]. The two TBMs, named Karen and Madeline, are being assembled in the underground box at Old Oak Common station site.
