Construction has been completed on HS2’s longest tunnel, marking a significant milestone for one of the UK’s most complex civil engineering projects.
Civil engineering works have now concluded on the 10-mile twin-bore Chiltern tunnels, following the completion of the final vent shafts at Chesham Road and Little Missenden. The achievement brings to a close nearly five years of intensive construction activity through the Chiltern Hills and clears the way for the next phase of fit-out works.
With the structural works finished, the tunnels are now ready to be equipped with track, overhead power systems and mechanical and electrical installations. Enabling works for mechanical, electrical and plumbing systems are expected to begin later this year, with rail systems to follow once that stage is complete.
Main construction of the tunnels was carried out by Align JV, a joint venture between Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick. Works began in May 2021 with the staggered launch of two 2,000-tonne tunnel boring machines from a site near the M25 at Maple Cross in Hertfordshire.
Ahead of tunnelling, five deep ventilation and access shafts were constructed along the route, reaching depths of up to 78 metres. Each shaft headhouse was designed by Grimshaw Architects to sit sensitively within the Chilterns landscape.
The tunnel boring machines advanced north at an average rate of 16 metres per day before completing their drives near Great Missenden in early 2024. Since then, teams have focused on constructing the porous tunnel extensions at the north and south portals, installing internal walkways and completing 40 cross passages linking the twin bores.
HS2 Ltd’s head of civil engineering for the Chiltern tunnel, Mark Clapp, described the project as a rare opportunity for engineers to work on a scheme of such scale and complexity. He praised the collaboration between HS2 Ltd, Align JV and its supply chain, highlighting the quality and commitment shown throughout delivery.
Align JV project director Adrien Baudard said completing the civil works was a point of pride for the entire team, noting advances in safety, sustainability and technical performance achieved during the programme. He added that the project’s approach to carbon reduction, safety innovation and skills development would influence tunnelling best practice for years to come.
The Chiltern tunnel is the second of HS2’s five twin-bore tunnels to reach structural completion, following the one-mile Long Itchington Wood tunnel in Warwickshire, completed last year.
Despite progress on individual elements, the wider HS2 programme remains behind schedule and over budget. HS2 Ltd chief executive Mark Wild continues to work on a reset plan aimed at delivering the remaining London to Birmingham route more efficiently and at the lowest reasonable cost.
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