HS2 has confirmed that ‘Florence’ and ‘Cecilia’ – the two TBMs digging the project’s longest tunnels – have reached the Chesham Road intervention shaft, on track to complete their mammoth 16km journey under the Chilterns next year.
The 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 102km of tunnelling, with five 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 170m long, lining the bores with 56,000 concrete segments to form rings.
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 2.8M cubic metres of chalk and flint.
As well as boring 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.
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..
Didier Jacques, Align’s Underground Construction Director said: “With our first TBM Florence having reached our fifth shaft at Chesham Road and our second TBM Cecilia due to reach the shaft shortly, this a great achievement for not only the tunnelling team, but also the supporting teams on the surface at the South Portal, manufacturing the concrete segments required to line the tunnels and processing the spoil from the tunnels.
“We are looking forward to continuing the good progress with the TBMs, which are due to complete their drives early next year.”
Approximately 3 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.
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.