Tunnel progress as HS2’s ‘Florence’ and ‘Cecilia’ hit Chiltern tunnel milestone

HS2 has confirmed ‘Florence’ and ‘Cecilia’ – two giant tunnelling machines digging the project’s longest tunnels - have reached the Chesham Road intervention shaft, on track to complete their mammoth 10-mile journey under the Chilterns next year.

The enormous 2,000 tonne machines are now 90% of the way through their two-and-a-half-year drive between the M25 and South Heath in Buckinghamshire to create the twin-bore tunnel.

In total, HS2 will require 64 miles of tunnelling, with five tunnel boring machines (TBMs) already in the ground, three more ready to go and another two due to be delivered next year. 

Once complete, HS2 will help improve connections between London, Birmingham and the North while creating space on the most congested part of the existing West Coast Main Line.

Each Chiltern TBM 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 machines were launched in summer 2021 from a site near the M25 and have already excavated approximately 2.8 million cubic metres of chalk and flint.

As well as digging and lining the tunnels, engineers 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 with work now happening to create the internal structures and the headhouses on the surface.

Mark Clapp, HS2 head of delivery, said: “Once complete, HS2 will transform journeys between London and the West Midlands and free up space on the busiest part of the West Coast Main Line.

“We’re making great progress in the Chilterns, with 90% of the tunnel excavation now complete. That’s an incredible engineering achievement and I look forward to the breakthrough, next year.”

The 42m deep shaft near Great Missenden – which the TBM has now reached – is largely hidden behind a hedge and line of mature oak trees next to the B485.

A ‘headhouse’ will be built on top of the shaft designed to resemble local farm buildings with new planting to help blend it into the surrounding landscape. 

The Chesham Road shaft will only be used for emergency access, while the others will also contain ventilation equipment.

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 over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.

Alongside the Chiltern tunnels, 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.

Two TBMs which will excavate the Euston tunnels are due to be delivered to site next year.