On 19th December 2024, TBM Sushila - which is 160-m in length, with a cutterhead of 9.84 m in diameters and a weight of 2,050 t, reached Green Park Way vent shaft in Ealing, breaking through underground into a reception can filled with foam concrete. This 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.
It's the first breakthrough for the 13.52 km (8.4-mile) twin-bored Northolt tunnel. The tunnel is being constructed by 4 TBMs - made by Herrenknecht AG and named Sushila, Caroline, Emily and Anne - advancing each at an average speed of 16 metres per day. They will complete by the end of 2025 the first 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. All four will finish their journeys at Green Park Way, arriving in a main and satellite shaft.
The Northolt tunnel is being built by the joint venture SCS, formed of the companies Skanska, Costain and STRABAG. The same JV will also construct the Euston tunnel eastward from Old Oak Common to the centre of London.
TBM Sushila has been named after a local school teacher. It was the first to be launched in October 2022 from West Ruislip and is the first to complete the journey. It 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.
The contractor is 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.
This is the fourth major tunnel breakthrough for HS2 this year. Tunnelling for both bores of the 16.09 km (10-mile) Chiltern tunnel, the longest on the route, was completed in March this year. A 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.
At the budget in October, the Chancellor announced that the construction of HS2’s Euston tunnel would go ahead. On 2 December HS2 revealed the two 190m-long and 1,250 t weight machines, named Karen and Madeline, are being assembled in the giant underground box at Old Oak Common station site. One machine is called Karen after Karen Harrison, the first female train driver in the UK who was based out of Old Oak Common depot. The second is named Madeleine, after Madeleine Nobbs, the former president of the Women’s Engineering Society.
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