HS2’s longest tunnel reaches major civil works milestone

Construction of HS2’s longest tunnel has reached a major milestone, with civil engineering works now completed on the 10-mile Chiltern Tunnel.

HS2 Ltd confirmed that the closure of works at two ventilation shafts at Chesham Road and Little Missenden marks the end of a project that began nearly five years ago. Once fully fitted out in later phases with track and overhead electrical equipment, the tunnel will carry high-speed trains at up to 200mph, allowing journeys through the tunnel in around three minutes.

Main tunnelling works began in May 2021, when two 2,000-tonne tunnel boring machines were launched from a site near the M25 at Maple Cross in Hertfordshire. Five deep ventilation and access shafts, reaching depths of up to 78 metres, were completed along the route in advance. The headhouse designs by Grimshaw architects were developed to blend into the Chiltern hills landscape.

The machines advanced north at an average rate of 16 metres per day and broke through near Great Missenden in early 2024. Since then, teams have completed internal walkways, 40 cross passages, and portal extensions at both ends.

The Chiltern Tunnel is the second of HS2’s five twin-bore tunnels to be structurally complete, following the Long Itchington Wood tunnel in Warwickshire last year.

Work was delivered by Align JV, a joint venture between Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick. HS2 civil engineering lead Mark Clapp praised the project team’s performance, while Align JV’s Adrien Baudard highlighted achievements in safety, training and sustainability.

More work remains across the HS2 route, with Mark Wild, HS2 Ltd Chief Executive, now leading a project-wide reset to improve delivery efficiency and cost control.