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HS2 Tunneling Reaches Milestone

HS2 on Aug. 8 confirmed that ‘Florence’ and ‘Cecilia’ – the two TBMs digging HS2’s longest tunnels – have reached the Little Missenden ventilation shaft, more than three quarters of the way through their 10 mile drive under the Chilterns.

HS2 is the UK’s biggest rail investment ever made in the North of England and is Europe’s largest infrastructure project. The new high-speed line will run between the North West and the South East, stopping at Manchester, Birmingham and London with trains continuing on the existing network to Scotland and elsewhere. In total, over 250 miles of new high-speed line is planned across the country

The 2,000 tonne TBMs have spent two years excavating the twin tunnels between the M25 and South Heath in Buckinghamshire that will help improve connections between London, Birmingham and the North.

Each machine is a 170 m long and will place 56,000 concrete segments along its journey. Designed specifically for the geology of the Chilterns, the first TBMs were launched in summer 2021 from a site near the M25 and have already excavated approximately 2 million cubic meters of chalk and flint.

As well as digging and lining the tunnels, engineers have also completed the excavation of five shafts that will provide ventilation and emergency access near Chalfont St Peter, Chalfont St Giles, Amersham and Little Missenden with an intervention shaft at Chesham Road.

The 35 m deep shaft near Little Missenden – which the TBMs have now reached – is next to the A413 about half a mile from the Buckinghamshire village. A ‘headhouse’ will be built on top of the shaft to house ventilation and safety equipment, designed to resemble local farm buildings with new planting to help blend it into the surrounding landscape.

The news comes just weeks after planning consent was granted by Buckinghamshire Council for the North Portal of the tunnel under Schedule 17 of the HS2 Act, meaning that all major design elements now have consent.

David Emms, HS2 Ltd’s Project Client, said: “HS2 will transform rail journeys between London the midlands and the north, and free up space on the existing network for more freight and local services. We’re seeing great progress on the tunnel, which will take our trains deep under the Chiltern hills, safeguard the woodlands and wildlife habits above and significantly reducing disruption to communities.

“The huge progress that Florence and Cecilia have made would not have possible without a huge team in support – manufacturing the tunnel segments, excavating the vent shafts and I’d like to thank everyone involved.”

The two TBMs are operated by, Align – a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

Approximately 2.7 million cubic metres of material – mostly chalk and flint – will be excavated during the construction of the tunnels and used for landscaping. Once construction is complete, the temporary buildings at the south portal will be removed and the site landscaped with around 90 hectares of wildlife-rich chalk grassland habitats. Chalk grassland used to be widespread across the hills of southeast England and are considered habitat of international conservation significance with just 700ha left across the Chilterns.

HS2 currently has five TBMs in the ground, with a further five due to be launched over the coming years. Together they will create 64 miles of tunnel between London and the West Midlands including major tunnels on the approach to London and Birmingham.

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