HS2’s tunnelling team in London is now one step closer to completing the capital’s crucial Northolt Tunnel, following the latest in a series of complex tunnelling breakthroughs.
The Tunnel Boring Machine (TBM), Emily, successfully completed its 3.4-mile drive at Green Park Way in Greenford, west London. This marks the third of four machines to finish its work on the 8.4-mile Northolt Tunnel.
Emily, launched in February 2024, has completed one bore of the eastern section of the tunnel. During its operation, the TBM removed an impressive 775,000 tonnes of London Clay and installed 17,514 concrete tunnel segments. In a nod to tunnelling tradition, the machine was named after Emily Sophia Taylor, who helped establish the Perivale Maternity Hospital in 1937 and later became Ealing’s first female mayor in 1938.
The Northolt Tunnel is being constructed using four TBMs in total: two boring the western section from West Ruislip, and two (including Emily) building the eastern section from Victoria Road in North Acton. All machines are designed to conclude their drives at the Green Park Way shaft, where they will be lifted out. The final machine is expected to complete its drive later this year.
This twin-bore tunnel will eventually carry HS2’s new high-speed trains between the outer edge of the capital and the new Old Oak Common super-hub station in west London. Due to high water pressure at the Green Park Way site, the TBMs finish their drives within a “reception can” in the shaft. This method allows the TBM to maintain pressure while sealant is applied from the tunnel lining, preventing water ingress, before the machine is depressurised and lifted out.
Malcolm Codling, Project Client Director for HS2 Ltd, said: “HS2’s tunnelling drives in London for the Northolt Tunnel are nearing completion and it is a real achievement for our team to complete this 3.4-mile section on the tunnel safely and with such efficiency. There is still much more to do in the tunnel to get it HS2 ready and we will be focusing on productivity to continue with our work safely to get the HS2 route in London ready for track to be laid, and systems installed.”
Manufactured by world-leading tunnelling experts Herrenknecht in Germany, TBM Emily weighs 1,700 tonnes and features a 9.11-metre diameter cutterhead. The TBM is an earth pressure balance machine, specifically designed for the soft London Clay it extracted. The machine was operated by three teams of 48, working around the clock, with the work completed by HS2’s London Tunnels contractor, Skanska Costain STRABAG JV (SCS JV).
At peak production, TBM Emily advanced at approximately 38 metres per day. The crew ensured the cutterhead efficiently cut through the earth, conveying the clay out of the tunnel, and then reinforcing the excavated tunnel with concrete segments, each weighing up to 7 tonnes. These segments were manufactured in Hartlepool by STRABAG.
The excavated London Clay was transported by conveyor from the Victoria Road Crossover Box, near Old Oak Common station, to the London Logistics Hub at Willesden Euro Terminal. From there, it was moved by rail for reuse at sites in Cambridgeshire, Kent, and Warwickshire.
Two other TBMs, Sushila and Caroline, bored the western section of the tunnel from West Ruislip, breaking through in December 2024 and April 2025, respectively. TBM Emily completed its breakthrough on Friday 6th June, while the fourth Northolt TBM, Anne, is expected to finish its drive in the summer.
This marks a bumper year for tunnelling on the HS2 project, following the breakthrough of TBM Mary Ann – the first of two machines excavating the Bromford Tunnel into Birmingham – in early May.
Dave Hannon, Project Director, Tunnels & Routeway, Skanska Costain STRABAG JV, said: “The arrival of TBM Emily at Green Park Way marks a significant milestone in the programme. We are now approaching completion of the Northolt Tunnel – an exciting achievement and a testament to the collaboration, dedication, and technical capabilities of our teams. With our continued focus we are pressing ahead at pace to complete the remaining works, including cross passages, tunnel walkways, ventilation shafts, headhouses, and portals, as we deliver on part of the HS2 project.”
Once the full tunnel structure is complete, the SCS JV team will finish building 34 cross passages between the two tunnel bores. The tunnel invert, a flat surface forming the base of the tunnel, will also be completed, ready for track laying. Additionally, the tunnel shafts, which provide ventilation and emergency access, will be finalised.
