Engineers began assembling two giant tunnelling machines this month that will dig the final 4.5 miles of underground high-speed railway – carrying HS2 trains into the heart of London.
Weighing an incredible 1,250 tonnes, each of the tunnel boring machines (TBMs) will launch from an underground box at one end of the project’s Old Oak Common station. They are expected to take around one and a half years to reach the railway’s final southern terminus at London Euston.
In keeping with tradition, the TBMs have been given female names after prominent women in history. 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.
The unveiling of the TBMs – the final set to be launched for the railway between London and the West Midlands – marks another significant milestone for HS2.
Rail Minister Lord Hendy said: “The sheer scale of the progress at Old Oak Common – and the benefits we will realise from taking HS2 to Euston, as confirmed in the Budget – reminds us that this is a once-in-a-generation project. HS2 will not only improve transport links for millions but unlock huge potential for growth, jobs and regeneration right across the country.”
The Mayor of London, Sadiq Khan, said: “After so much uncertainty, it’s great to finally be working with a government that recognises the huge importance of this scheme to London and the UK, and understands why terminating HS2 at Euston is so important. It’s exciting that these tunnel boring machines will soon be getting work underway.”
Huw Edwards, stations delivery director, HS2 Ltd, said: “We welcome the commitment from the government to build HS2 all the way to Euston and preparations to tunnel from Old Oak Common are now well underway.
“A terminus station for HS2 in the heart of the capital will provide a vital gateway for passengers travelling to and from the Midlands and North, and will undoubtedly lead to the transport-led regeneration of Euston – supporting growth in the local economy and creating thousands of new homes and jobs.”
The two 190m-long TBMs were manufactured by tunnelling experts Herrenknecht AG in Germany and were transported to Old Oak Common in pieces before being reassembled on site.
This summer, the HS2 team lifted the machines into the underground station box using a 750-tonne crane. They are now being reassembled at the eastern end of the station, ready to bore to Euston.
The cutterhead of the machine, which has been optimised to cut through London clay, is 8.53m across with the inner diameter of the tunnel set to be 7.55m.
Underground factories on the move
The TBMs are like underground factories, excavating the tunnels using a turning cutterhead, lining them with pre-cast concrete tunnel segments, grouting them into place before moving forward at an average speed of 16 metres per day. Teams work around the clock below ground on the along with teams on the surface supporting them.
HS2’s London tunnels contractor, Skanksa Costain STRABAG (SCS) joint venture, is leading construction of the twin-bored tunnel. In January 2024 the team completed a logistics tunnel from a nearby site at Atlas Road, which will provide access for materials, and a route to remove the London Clay being excavated by the machine.
It has also built two spray concrete-lined tunnels from which the TBMs will be launched Eastwards towards Euston. Boring is expected to begin in 2025.
SCS is already well advanced with construction the Northolt tunnel – a separate structure running west out of Old Oak Common towards West Ruislip, outer London.
James Richardson, managing director, SCS JV, said: “We are delighted that the government has given the green light to delivering HS2 through to Euston, ensuring the full benefits of the new railway are realised. Over the past two years, we have continued with critical activities to ensure that we can deliver the Euston Tunnel and approaches quickly, and at the lowest possible cost.
“We are also making excellent progress on the tunnelling between West Ruislip and Old Oak Common, with four TBMs in operation, and the works well over 50% complete.”
The machines will complete their journeys underground and the outer can of the machine will be left to form part of the tunnel. The mechanical components of the TBMs will be pulled back through the 4.5 mile tunnel and disassembled.
The approach to Euston will be completed by spray concrete lined tunnels which will then transition into three tunnels, which will enable trains to be fed into and from platforms at Euston station.
Main image: Rail minister, Lord Peter Hendy, and new HS2 chief executive Mark Wild
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