[6][7] During 2017, Contract C1 (Central 1), which covers the 21.6km section of the line that the Chiltern tunnels comes within, was awarded to the Align JV joint venture, comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

[5] Each TBM weighs around 2,000 tonnes, has a length of 170 m (560 ft), and has been specially customised to suit the local geology, which primarily consists of chalk and flint.

Staff are conveyed between the surface and the TBM using people carriers, which will have up to a one-hour transit time towards the later years of the boring.

While both TBMs are intended to be operational simultaneously throughout the majority of the work, due to sensitivities surrounding the M25 motorway, only one TBM will be active at a time while within close proximity to this key trunk route.

These vents are all to be fitted with headhouses, produced through engagement with both the Chilterns AONB Review Group and Buckinghamshire Council.

[23] The tunnel will have porous portals (a form of tunnel entrance hood) that extends for up to 220 metres (720 ft) beyond the end of the bored section in the hillside - this is to dissipate the pressure wave built up by trains at line speed, thus reducing the adverse effects of audible "sonic boom" in line with environmental commitments.

[38] The Chalfont St Giles ventilation shaft was reached in October 2022,[39] Amersham in March 2023,[40] Little Missenden in August,[41] and Chesham Road in December the same year.

[42] The chalk excavated during the boring of the tunnels is to be used to create a nature reserve covering 127 hectares (310 acres) in the vicinity of the southern portals; for this purpose, measures for the temporary storage and treatment of up to 3,000,000 cubic metres (110,000,000 cu ft) of chalk slurry were provisioned onsite prior to excavation commencing.

[49] The resulting inquiry found that the local water supply was not contaminated, and the sinkhole was filled with 165 cubic metres (5,800 cu ft) of chalk in September 2023.

[53] Florence's progress was hampered significantly during the last few weeks of her drive by a geology change, as the final 140m to the north portal replaced the usual chalk with a more clay-like substance, causing several blockages in the slurry pipes.