Civil engineering works have been completed on HS2’s Chiltern Tunnel, the longest tunnel on the UK high-speed rail project, marking a major milestone in the construction of the London–Birmingham route.
HS2 Ltd confirmed that completion of works at the Chesham Road and Little Missenden ventilation shafts brings to a close nearly five years of construction activity on the 16km twin-bore tunnel. Once fully fitted out during later phases, the tunnel will carry high-speed trains operating at up to 320km/h, allowing the route to be traversed in approximately three minutes.
Construction of the twin tunnels began in May 2021 with the staggered launch of two 2,000t tunnel boring machines from a site adjacent to the M25 near Maple Cross, Hertfordshire. Each bore will accommodate a single track for northbound and southbound services.
Ahead of tunnelling, five ventilation and access shafts were constructed along the alignment, reaching depths of up to 78m. The headhouses, designed by Grimshaw Architects, were developed to minimise visual impact within the Chilterns Area of Outstanding Natural Beauty.
The TBMs advanced northwards at an average rate of 16m per day, breaking through near Great Missenden, Buckinghamshire, in early 2024. Since completion of the drives, works have focused on construction of the porous portal structures, installation of internal walkways, and fit-out of 40 cross passages.
The Chiltern Tunnel is the second of HS2’s five twin-bore tunnels to reach structural completion, following the 1.6km Long Itchington Wood Tunnel in Warwickshire.
The project was delivered by Align JV, a joint venture comprising Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick, under contract to HS2 Ltd.
To maintain continuous tunnelling operations, the project team implemented a tightly sequenced logistics strategy, supplying each TBM with precast lining segments from a dedicated factory near the southern portal. In total, approximately 56,000 eight-tonne segments were manufactured and installed.
The TBMs, named Florence and Cecilia, excavated around 3 million tonnes of chalk during the 33-month tunnelling programme. Excavated material was reused on site to create more than 120ha of chalk grassland near the southern portal, avoiding transport by road and contributing to habitat restoration in the area.
With civil works complete, the project will now move into mechanical, electrical and plumbing installation, with design activities already underway and enabling works scheduled to commence this year. Installation of track and overhead line equipment will follow as part of subsequent phases.
Construction activity continues across other sections of the HS2 programme, while HS2 Ltd undertakes a wider reset of the project to optimise delivery and cost efficiency for the remaining route. For further information on HS2 please click here and UK/65. 03/26.
