Major milestone reached as HS2 finishes construction of 10-mile Chiltern tunnel

Construction work on HS2’s longest tunnel has now been completed, marking a significant civil engineering milestone for the high-speed rail project.

HS2 Ltd confirmed that finishing work at two ventilation shafts on the Chiltern tunnel, at Chesham Road and Little Missenden, brings to a close a programme that has been under way for almost five years. The tunnel is a key part of the new railway and, once later phases install track and overhead power equipment, it is designed to carry trains at speeds of up to 200mph. At that speed, trains would pass through the 10-mile tunnel in around three minutes.

The twin-bore tunnel, with separate bores for northbound and southbound services, entered its main construction phase in May 2021. Two 2,000-tonne tunnel boring machines were launched in stages from a site near the M25 at Maple Cross in Hertfordshire. Ahead of their arrival, five deep access and ventilation shafts were excavated along the route, some reaching depths of up to 78 metres. The headhouses, designed by Grimshaw architects, were created with a low-profile appearance intended to sit discreetly within the Chiltern hills landscape.

Boring progressed northwards at an average rate of 16 metres a day before the machines emerged near Great Missenden in Buckinghamshire in early 2024. Since that breakthrough, work has continued on constructing porous tunnel extensions at both portals, installing internal walkways and completing 40 cross passages linking the two bores.

The Chiltern tunnel is the second of HS2’s five twin-bore tunnels to reach structural completion, following the Long Itchington Wood tunnel in Warwickshire, which achieved the same milestone last year. While the completion highlights progress on the 140-mile route between London and Birmingham, HS2 Ltd has acknowledged that substantial work remains. The company’s chief executive, Mark Wild, is overseeing a comprehensive reset of the programme aimed at delivering the remaining sections as efficiently as possible and at the lowest reasonable cost.

Construction of the tunnel was led by Align JV, HS2’s main works contractor for this section of the route, a joint venture comprising Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick.

HS2 Ltd head of civil engineering for Chiltern tunnel, Mark Clapp 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.”

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.”

Image: HS2