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

HS2 said 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 progress is being made to construct the 140-mile line between London and Birmingham, there is still significant work to do. Mark Wild, HS2 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. 

The Chiltern tunnel’s construction was led by HS2 Ltd’s main works contractor for this section of the line - Align JV, a joint venture between Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick.

Align JV project director, Adrien 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.”

The project team organised a highly efficient and carefully sequenced operation to ensure the TBMs’ round-the-clock operation. Each one received a continuous supply of 56,000 eight-tonne tunnel lining segments from a purpose-built factory near the tunnel’s southern portal.

The news comes as the HS2 project has also hit two significant employment milestones.

More than 2,000 people have secured an apprenticeship on HS2 and over 5,000 unemployed have progressed into jobs.

Latest data shows an additional 99 people started an apprenticeship between September and December 2025. This takes the total number of new apprentice starts on the project to 2,032 – exceeding the original target set before construction started.  

In May 2025, HS2 confirmed that it had reached its goal of supporting 5,000 unemployed people into work – and today also confirmed that this figure has climbed to 5,645.

This comes as a new report from HS2 – Mind the Skills Gap – shows that the combination of apprenticeships and workless job starts delivered on the project to date is worth £315m to the UK economy.

HS2’s 2,000th apprentice is Mohammed Sharif from Bordesley Green in Birmingham. He lives just minutes away from the vast network of viaducts that are being built on the approach to HS2’s new terminus station at Curzon Street.

Seeing HS2’s construction from his bedroom window inspired him to explore a career in civil engineering. Now, just two years on, he’s part of the team helping to build HS2 in his home city.

“I knew I wanted to work on HS2, so I applied to BMet college to study for a T-Level in Construction and secured a work placement with HS2’s construction partner, Balfour Beatty VINCI,” he said.

“I worked hard at college and put 100% into my placement - and it paid off.  When I finished my studies, Balfour Beatty VINCI offered me a civil engineering apprenticeship.

“I used to look out of my bedroom window and see all the work taking place on HS2 – now I’m helping to build it. This project is going to open up so many opportunities for young people like me, I couldn’t be prouder to be a part of it.”