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Break through on HS2’s longest tunnel [– with video]

27 Feb After nearly three years of drilling, TBM Florence has completed her 10-mile journey under the Chilterns.

Florence breaking through
Florence breaking through

Florence, a 2,000-tonne tunnel boring machine, set off from the Chilterns tunnel south portal site next to the M25 in Hertfordshire on 13th May 2021. On 27th February 2024, she broke through into the north portal at South Heath in Buckinghamshire, completing her drive.

Florence is one of 10 TBMs excavating the 32 miles of tunnel on the new London-Birmingham rail line and was the first to launch. At 10 miles, the Chilterns tunnel is the longest.

Sister machine Cecilia, who set off two months after Florence, is boring a parallel twin tunnel for south bound trains.

Rail minister Huw Merriman said: “Today’s breakthrough of HS2’s longest tunnel highlights the momentum behind the project and the achievement is testament to the hard work and dedication of the 450-strong team helping deliver the line that will transform rail travel for generations to come.”

HS2 Ltd executive chairman Sir Jon Thompson said: “We’ve still got a lot of work to do, but historic moments like today really underline the huge amount of progress that’s been made and the fantastic engineering skills we have on the project.”

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Both machines are operated by HS2’s main works contractor, Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick. The machines are made by Herrenknecht, in Germany.

Each TBM is operated by a crew of around 17 people, working in shifts to keep the machines running 24/7. They are supported by more than 100 people on the surface, managing the logistics and maintaining the progress of the tunnelling operation.

Align project director Daniel Altier said: “The typical drive for a TBM is 5-6km and therefore the challenges in completing at 16km drive should not be underestimated.  Florence and her sister TBM Cecilia were designed in partnership with Herrenknecht. They incorporate a number of innovations and technologies that have been introduced on TBMs in the UK for the first time, to enhance performance and safety. This includes ‘semi-continuous boring’, allowing our TBMs to build the rings that line the tunnels without pausing.”

Align is also building the Colne Valley viaduct, which will be the longest railway bridge in the UK, with construction of the deck now more than two-thirds complete.

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