HS2 completes construction of its longest tunnel after five years of work

Construction works on HS2’s longest tunnel have been completed – marking an important civil engineering milestone for the new high-speed railway.

HS2 Ltd said that close of work at two of the line’s Chiltern tunnel vent shafts at Chesham Road and Little Missenden brings to an end a project that began almost five years ago.

Once fitted out with tracks and overhead electrical equipment as part of later phases of the project, 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 (TBMs) 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 last year.

However, while clear progress is being made to construct the 140-mile line between London and Birmingham, there is still significant work to do.

Mark Wild, HS2 Ltd Chief Executive, 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.