HS2 makes tunnel progress under the Chilterns

Two giant tunnelling machines are more than three-quarters of the way through a 10-mile drive under the Chilterns – HS2’s longest tunnels.

The two machines, named Florence and Cecilia, have reached the Little Missenden ventilation shaft.

The enormous 2,000 tonne machines 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 170m long self-contained underground factory, digging the tunnel, lining it with 56,000 concrete segments to form rings and grouting them into place as it moves forward.

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 two million cubic metres 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 35m 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 project client, said: “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 two TBMs are operated by Align – a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

Each machine has a crew of around 15 people, working in shifts and supported by more than 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.

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.