HS2: milestone reached on project’s longest tunnels

Work to construct HS2’s longest tunnels has almost reached the halfway point.

This week two tunnel boring machines (TBM) operating at the site passed through the second ventilation shaft under the Chilterns.

The machines, Cecilia and Florence, have now completed the first 4 miles of their drive from the Chiltern tunnel’s south portal to the second ventilation shaft at Chalfont St Giles, Buckinghamshire.

Work on this section of the mega-rail project started in the summer of 2021. It will see the machines carry out a 10-mile journey in total.

Each machine is a 170m long self-contained underground factory, digging the tunnel, lining it with concrete segments to form rings and grouting them into place as it moves forward.

Five ventilation shafts will be created along a 10-mile stretch, and the work is expected to take more than two years to excavate and create the next three ventilation shafts.

The shafts will provide ventilation and emergency access to the line. A headhouse will be built over the shafts.

The operation is being carried out by Align, a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick. Its underground construction director Didier Jacques described the complexity of the milestone reached.

He said: “It’s important to acknowledge the work by the construction team involved in excavating and preparing the shaft.

“In particular I would like to pay credit to KVJV, VolkerLaser and Keltbray, our supply chain partners, who have been working tirelessly over the past few months to ensure the shaft is ready for the arrival of Florence and Cecilia, learning from their experience of excavating and preparing our first shaft at Chalfont St Peter.”

Last week HS2 launched another of its giant TBM in Ruislip, which began excavating the London tunnels for the high-speed rail project linking the capital with the Midlands and the North.

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