HS2 has completed civil engineering works on its longest tunnel, marking a milestone for UK infrastructure.
Civil engineering works on HS2’s longest tunnel have now been completed, representing a key milestone in the delivery of the UK’s new high-speed railway. The close of works at the Chiltern tunnel’s final ventilation shafts at Chesham Road and Little Missenden concludes a complex programme that began nearly five years ago.
Once fully fitted with track, overhead electrical equipment and rail systems, the 10-mile Chiltern tunnel will carry high-speed trains travelling at up to 200mph. This will enable journeys through the tunnel in approximately three minutes, supporting faster, more reliable rail services between London and Birmingham.
Twin-bore tunnel construction and engineering approach
The Chiltern tunnel comprises two parallel bores, each designed to carry a single track for northbound and southbound services. Main tunnelling 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.
Prior to the arrival of the TBMs, five deep ventilation and access shafts were constructed along the route, reaching depths of up to 78 metres. The headhouses, designed by Grimshaw Architects, were developed to integrate sensitively with the surrounding Chiltern Hills landscape, reflecting planning and environmental commitments made during the project’s approval process.
The TBMs advanced northwards at an average rate of 16 metres per day, completing their drives near Great Missenden in Buckinghamshire in early 2024. Subsequent works have included the construction of porous concrete extensions at the tunnel portals, installation of internal walkways, and completion of 40 cross passages to support safety and operational resilience.
Position within the wider HS2 programme
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. Together, these achievements demonstrate continued progress across the 140-mile HS2 route between London and Birmingham.
However, HS2 Ltd has acknowledged that significant work remains across the programme. Chief Executive Mark Wild is leading a comprehensive reset of the project, with the objective of ensuring the remaining sections are delivered as efficiently as possible and at the lowest reasonable cost.
Align JV and collaborative delivery
Construction of the Chiltern tunnel was led by Align JV, HS2’s main works contractor for this section of the route. Align JV is a joint venture comprising Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick.
Mark Clapp, HS2 Ltd’s Head of Civil Engineering for the Chiltern tunnel, highlighted the scale and complexity of the project and the collaboration required to deliver it successfully. He said: “Multi-facetted projects of the Chiltern tunnel’s scale and complexity don’t often come along; as a civil engineer, you’re lucky to be involved in anything like it. The team we assembled at HS2 Ltd, and with Align JV – our main civil works contractor – and all its subcontractors, to deliver this part of the new high-speed railway was exemplary.
“I pay tribute to everyone involved. They can all feel certain that their hard work will stand the test of time.”
Adrien Baudard, Project Director at Align JV, also reflected on the broader impact of the works: “Being part of the successful delivery of the Chiltern tunnel has been a source of immense pride for everyone at Align and our partners across the supply chain. Their unwavering commitment, technical skill, and collaborative spirit have been truly impressive.
“With the completion of the Chiltern tunnel’s civil works, the achievements of this integrated project team as well as the sustainable benefits such as carbon reduction and safety innovations will set a standard in tunnelling for generations to come.”
TBM operations, logistics and environmental outcomes
The project required a highly coordinated logistics operation to keep both TBMs operating continuously. Each machine was supplied with 56,000 eight-tonne precast concrete lining segments, manufactured at a purpose-built factory near the southern portal.
Named Florence and Cecilia, after Florence Nightingale and astronomer Cecilia Payne-Gaposchkin, the TBMs excavated approximately three million tonnes of chalk during their 33-month journey. Rather than transporting this material by road, the excavated chalk was reused on site to create more than 120 hectares of chalk grassland near the Hertfordshire and Buckinghamshire border.
This approach has expanded Hertfordshire’s remaining chalk grassland habitat by nearly 20 percent, supporting biodiversity and re-establishing landscapes similar to those that existed prior to agricultural use.
Next phases: digital construction and rail systems
With civil engineering works complete, the next phase will involve installation of mechanical, electrical and plumbing systems, supported by digital construction methodologies during the design and enabling stages. Track installation and overhead line equipment will follow, forming the integrated rail systems required to operate high-speed services safely and reliably.
Civil engineering works continue across other sections of HS2 as the programme progresses towards operational readiness.
Completion of the Chiltern tunnel’s civil engineering works demonstrates HS2’s engineering capability, collaborative delivery model and digital construction potential as the programme advances towards full railway systems installation.
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