Construction works on the Chiltern tunnel – HS2’s longest – have been completed, marking an important civil engineering milestone for the new high-speed railway.
The completion of two of the line’s Chiltern tunnel vent shafts, at Chesham Road and Little Missenden, has brought to an end a project that began almost five years ago.
The Chiltern tunnel’s construction was led by Align JV, HS2’s main works contractor for this section of the line and a joint venture between Sir Robert McAlpine, Bouygues Travaux Publics and VolkerFitzpatrick.
Once fitted out with tracks and overhead electrical equipment, the tunnel will carry trains travelling at 200mph, enabling them to traverse its 10 miles in just three minutes.
Main construction of the twin-bore tunnel – each to carry a single track for trains travelling north and south – began in May 2021 with the staggered launch of two 2,000-tonne tunnel boring machines (TBM’s) 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 metres 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 in 2025.
However, while clear progress is being made to construct the 140-mile line between London and Birmingham, there is still significant work to do. HS2 chief executive Mark Wild 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.
Align JV project director Adrian Baudard said: “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.
“Whether it was advancing safety standards, setting new TBM records, or helping to train future engineers, we’ve had the privilege of learning from and working with some of the top talent in our field.
“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.”
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