HS2’s longest tunnel 90% complete

TBMs Florence and Cecilia have reached the Chesham Road intervention shaft on the 16km twin-bore tunnel under the Chilterns, marking 90% completion of their two-and-a-half-year drive.

The 2,000-tonne, 170m-long Herrenknecht machines were launched in summer 2021 from a site near the M25 motorway and so far, have excavated approximately 2.8 million m³ of chalk and flint.

As well as digging and lining the tunnels, engineers from Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – have also completed the excavation of four other shafts that will provide ventilation and emergency access near Chalfont St Peter, Chalfont St Giles, Amersham and Little Missenden. Work is now under way to create the internal structures and the headhouses on the surface.

The Chesham Road shaft will be used only for emergency access.

HS2 Ltd head of delivery Mark Clapp said Chiltern tunnel was making good progress, with 90% of tunnel excavation complete.

“That’s an incredible engineering achievement and I look forward to the breakthrough next year,” he said.

Align’s underground construction director, Didier Jacques, said: "With our first TBM Florence having reached our fifth shaft at Chesham Road and our second TBM Cecilia due to reach the shaft shortly, this a great achievement for not only the tunnelling team, but also the supporting teams on the surface at the South Portal, manufacturing the concrete segments required to line the tunnels and processing the spoil from the tunnels.”

In total, HS2 will require 103km of tunnelling, with five TBMs already in the ground, three ready to go and another two due to be delivered next year. In addition to Florence and Cecilia, two TBMs are currently excavating the London tunnels with two more due to launch soon. One TBM is excavating the Bromford tunnel in Birmingham, with another ready to go.

The two TBMs that will excavate the Euston tunnels are due to be delivered to site next year. In August, it was announced they will be stored under Old Oak Common Station until the government gives the go-ahead for the Euston Tunnel.

At the time, HS2 told PA Media that while HS2 passenger services were not due to begin for at least six years, the two TBMs needed to be lowered into place next year to allow for additional train tracks to be built over the top, with the Great Western main line to be widened as part of the Old Oak Common development.