Longest HS2 tunnel structurally complete

HS2 Ltd has confirmed the completion of civil engineering works on its longest tunnel to date, marking a major milestone in delivery of the UK’s new high-speed railway. The close-out of works at the Chiltern tunnel ventilation shafts at Chesham Road and Little Missenden brings to an end nearly five years of intensive underground construction.

The twin-bore Chiltern tunnel extends for approximately 10 miles and will ultimately carry trains travelling at up to 200 mph, allowing the section to be traversed in around three minutes once rail systems are installed. Main tunnelling began in May 2021 with the launch of two 2,000-tonne tunnel boring machines from Maple Cross, near the M25, progressing northwards beneath the Chiltern Hills.

Five deep ventilation and access shafts, reaching depths of up to 78 metres, were completed ahead of tunnelling and designed with low-profile headhouses to minimise visual impact on the landscape. The TBMs advanced at an average rate of 16 metres per day, breaking through near Great Missenden in early 2024. Subsequent works included construction of portal extensions, internal walkways, and installation of 40 cross passages connecting the two bores.

The works were delivered by Align JV, a joint venture between Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick. Over 33 months, the TBMs excavated approximately three million tonnes of chalk, with lining installation supported by a dedicated segment factory producing more than 56,000 precast units.

Excavated chalk was reused on site to create over 120 hectares of chalk grassland near the southern portal, expanding locally scarce habitat and avoiding extensive road haulage. With civil works now complete, the project will move into mechanical, electrical and plumbing installation, followed by track and overhead electrification works as part of later HS2 phases.