HS2 completes civil engineering works on longest tunnel project
posted on 21st Jan 2026 09:52
Construction works on HS2’s longest tunnel have been completed – marking an important civil engineering milestone for the new high-speed railway. HS2 Ltd said that close of work at two of the line’s Chiltern tunnel vent shafts at Chesham Road and Little Missenden brings to an end a project that began almost five years ago. Once fitted out with tracks and overhead electrical equipment as part of later phases of the project, the tunnel will carry trains travelling at 200 mph (320 km/h), enabling them to traverse its 16 km in just three minutes.
Main construction of the twin-bore tunnel began in May 2021 with the staggered launch of two 2,000-tonne tunnel boring machines (TBMs) from a site next to the M25 near Maple Cross in Hertfordshire. Five deep ventilation and access shafts were sunk to depths of as much as 78 m along its route and completed before the TBMs’ arrival. The understated design of each shaft’s headhouse, by Grimshaw architects, is intended to blend unobtrusively with the surrounding landscape of the Chiltern hills.
The machines progressed north at an average speed of 16 metres per day before breaking through near Great Missenden, Buckinghamshire, in early 2024. Since then, work has continued to build the porous extensions to its north and south portals, install its internal walkways and fit out 40 cross passages. It is the second of HS2’s five twin-bore tunnels to be structurally complete after the same achievement was marked for the one-mile Long Itchington Wood tunnel in Warwickshire last year.
However, while clear progress is being made to construct the 140-mile line between London and Birmingham, there is still significant work to do. Mark Wild, HS2 Ltd Chief Executive, is now leading a comprehensive reset of the project to ensure the remainder of the route is delivered as efficiently as possible and for the lowest reasonable cost. The Chiltern tunnel’s construction was led by HS2 Ltd’s main works contractor for this section of the line - Align JV, a joint venture between Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick.
The project team organised a highly efficient and carefully sequenced operation to ensure the TBMs’ round-the-clock operation. Each one received a continuous supply of 56,000 eight-tonne tunnel lining segments from a purpose-built factory near the tunnel’s southern portal. Named Florence and Cecilia after modern nursing’s founder, Florence Nightingale and astronomer Cecilia Payne-Gaposchkin, who both had connections with the local area, the duo excavated three million tonnes of chalk during their 33-month journey.
Rather than being taken away on the public highway, the excavated material has been used on site to create more than 120 hectares chalk grasslands around the tunnel’s southern portal on the Herts/Bucks border – similar to the environment that existed there before it was farmed. The initiative has expanded Hertfordshire’s remaining biologically diverse grassland habitat by nearly a fifth and recreated environment for flora and fauna to thrive.
Completion of civil engineering works will be followed by the installation of mechanical, electrical and plumbing equipment, with the design-phase already underway and enabling works beginning this year. Rail systems including track and overhead electrical equipment to power the new high-speed trains will follow once this work is completed. Civil works continue elsewhere on HS2.