In a landmark moment for Britain's new high-speed railway, the main construction of HS2's longest tunnel has been completed. The structural work on the 10-mile twin-bore Chiltern tunnel concludes a major civil engineering project that began almost five years ago.
A Five-Year Engineering Feat
The significant milestone was reached with the finishing of work at two key ventilation shafts at Chesham Road and Little Missenden. Main construction on the tunnel itself commenced in May 2021 with the launch of two colossal, 2,000-tonne tunnel boring machines (TBMs) from a site near the M25 at Maple Cross in Hertfordshire.
The TBMs, named Florence and Cecilia, progressed northwards at an average speed of 16 metres per day. They completed their journey, breaking through near Great Missenden in Buckinghamshire in early 2024. Prior to their arrival, five deep ventilation and access shafts were sunk along the route, with the deepest reaching 78 metres.
Blending into the Landscape
Attention to the environmental and visual impact has been a key consideration. The headhouses for the shafts, designed by Grimshaw architects, feature an understated design intended to blend unobtrusively with the surrounding Chiltern Hills Area of Outstanding Natural Beauty.
Since the TBMs broke through, work has continued to finalise the structure. This has included building porous extensions at the north and south portals, installing internal walkways, and fitting out 40 cross passages that connect the two parallel tunnels for safety and maintenance access.
Progress Amid a Project Reset
The Chiltern tunnel is the second of HS2's five twin-bore tunnels to be declared structurally complete, following the one-mile Long Itchington Wood tunnel in Warwickshire last year. Once fitted with tracks and overhead electrical equipment in later project phases, trains will race through the tunnel at 200 miles per hour, traversing its entire length in just three minutes.
While this marks clear progress on the 140-mile line between London and Birmingham, HS2 Ltd Chief Executive Mark Wild is leading a comprehensive reset of the wider project. The aim is to ensure the remainder of the route is delivered as efficiently as possible and for the lowest reasonable cost, with significant work still ahead.
