HS2’s tunnelling team in London is one step closer to completing the capital’s Northolt Tunnel after the latest in a series of complex tunnelling breakthroughs.

The tunnel boring machine (TBM), named Emily, has completed its 3.4-mile drive at Green Park Way in Greenford, west London – the third of four machines to complete its work on the tunnel.

It was launched in February 2024 and has completed one bore of the eastern section of the tunnel – removing 775,000 tonnes of London clay and installing 17,514 tunnel segments.

In keeping with tradition, the machine was given a female name after Emily Sophia Taylor who helped establish the Perivale Maternity Hospital in 1937 before becoming Ealing’s first female mayor in 1938.

The 8.4-mile Northolt Tunnel is being built using four TBMs in total – two building the western section from West Ruislip and two building the eastern section from Victoria Road in North Acton.

The machines all finish at Green Park Way where they will be lifted out from the underground shaft. The final machine is expected to complete its drive later this year.

The twin bore tunnel will eventually take HS2’s brand 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 in the ground at the site at Green Park Way, the TBMs are finishing their drives in a reception can in the shaft. The reception can method allows the TBM to maintain pressure while sealant can be applied from the tunnel lining, preventing water ingress. It is then depressurised before it is opened and the TBM is lifted out.

Malcolm Codling, project client director for HS2, 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 tunnelling experts Herrenknecht in Germany, TBM Emily weighs 1,700 tonnes and has a 9.11m diameter cutterhead. The TBM is an earth pressure balance machine, which was designed specifically for the soft London clay that it extracted from the ground.

The machine was manned by three teams of 48 as it worked around the clock to create the tunnel. HS2’s London Tunnels contractor, Skanska Costain STRABAG JV (SCS JV), completed the work.

At peak production, the TBM advanced at around 38 metres per day. The crew operating the machine ensured the cutterhead cut through the earth, taking the clay out of the tunnel on a conveyor system, and then reinforcing the excavated tunnel with concrete segments – each weighing up to seven tonnes. The segments were made in Hartlepool by STRABAG.

The London clay extracted has been taken away from Victoria Road Crossover Box, near Old Oak Common station, by conveyor to the London Logistics Hub at Willesden Euro Terminal. From there, it was transported by rail and reused 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 broke through on 6 June, while the fourth Northolt TBM, Anne, is due to complete its drive in the summer.