Northolt tunnel
From Old Oak Common to West Ruislip
Northolt tunnel is the second longest on the HS2 route at 8.4 miles (13.5km) long beneath Hillingdon and Ealing. It will carry HS2’s trains between the new Old Oak Common station in west London to West Ruislip, on the outer edge of the capital. Four tunnel boring machines (TBMs) – Sushila, Caroline, Emily and Anne – were used to excavate the twin-bore tunnel.
TBMs Sushila and Caroline completed their 5 mile (8km) western section in December 2024 and April 2025, followed by TBMs Emily and Anne, who finished the 3.4 mile (5.5km) eastern section in June 2025.
Tunnel construction
The 1,700-tonne TBMs were built by German tunnelling specialists Herrenknecht. After they were built, the TBMs were disassembled and transported to England. On arrival at site, they were reassembled near the entrance to the tunnels, where a specially prepared head wall meant they could safely begin their excavation work.
Designed specifically for the geology of the London Clay, each machine was a self-contained underground factory. At peak construction, the TBM advanced about 38 metres every day.
The internal width of the tunnel varies between the eastern and western sections: 8.10 metres in the east and 8.80 metres in the west. The tunnels are wider in the sections where trains go faster, to manage the changes in air pressure and flow around them.
The London Clay was taken out of the tunnel using a conveyor system and transported to HS2’s London Logistics Hub, where it was then removed by rail for reuse across the UK.
The four machines excavated 4,160,000 tonnes of London Clay and lined the tunnel with 14,300 concrete tunnel rings. The rings were made of 94,233 precision-designed concrete segments.
The tunnel segments used in the eastern section were made in Hartlepool by STRABAG. The ones used in the western section were made in Kent by PACADAR UK. Both were transported to site by rail, reducing the traffic on London’s busy road network.
TBM reception chamber
All four machines finished their journey at Green Park Way.
Because of the high water pressure at the Green Park Way site the TBMs finished their drives in steel reception cans installed in two underground shafts. This allowed the TBMs to maintain pressure while sealant was applied from the tunnel lining to prevent water getting in. Once they were sealed, the can was depressurised and then opened to lift out the TBM.
Support infrastructure
Victoria Road headhouse
Located in North Acton, Victoria Road headhouse is a bold take on the local industrial style. It mixes heritage with a fresh, modern feel to reflect the area’s past and future alike.
Westgate headhouse
Westgate headhouse is between the River Brent and West Gate. Its green roof and soft planting will help the design settle into its setting and support local wildlife as it grows over time.
Green Park Way headhouse
Set by the railway, Green Park Way headhouse has been shaped to sit gently in its setting. We gave careful thought to views and form in Brent and Ealing.
Mandeville Road heahouse
Mandeville Road headhouse is in Brent and Ealing. Its design materials shift with place, from brick to metal to wood, echoing the changes in landscape from rural edges to urban centres.
South Ruislip heahouse
South Ruislip headhouse is between Ruislip Gardens and South Ruislip underground stations, close to Cineworld on Victoria Road in Hillingdon. It forms part of the green corridor, linking new habitats, trees and shared spaces to help nature and people thrive along the route.
West Ruislip portal
The West Ruislip portal sits beside the Chiltern line, set within new habitat creation areas across West London, known as the Sustainable Placement Areas (SPAs).
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