HS2, the UK’s High Speed 2 project developer, has finished the civil engineering stage of its longest tunnel, the 16km Chiltern tunnel, marking a major step in the high-speed rail project connecting London and Birmingham.
The completion follows nearly five years of work, during which construction teams have recently finalised projects at two ventilation shafts located at Chesham Road and Little Missenden.
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The tunnel, constructed by Align Joint Venture (JV), a partnership between Bouygues Travaux Publics, Sir Robert McAlpine, and Volker Fitzpatrick, will eventually carry trains at speeds up to 320km/h.
Each tunnel bore holds a single track for trains travelling in opposite directions.
The main tunnelling operation started in May 2021 with the launch of two 2,000t tunnel boring machines (TBM) from a site near Maple Cross in Hertfordshire.
These machines averaged 16m per day and completed their journey near Great Missenden in early 2024.
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By GlobalDataConstruction included sinking five ventilation and access shafts along the route to depths of up to 78m before the arrival of the tunnelling machines.
The design of each shaft’s above-ground structure aims to minimise visual impact on the Chiltern Hills landscape.
The TBMs, named Florence and Cecilia, excavated about three million tonnes of chalk over a period of almost three years.
This material was used onsite to create over 120ha of chalk grassland habitat next to the southern portal, expanding the region’s grassland by nearly a fifth.
Following the structural works, teams have built porous extensions at both ends of the tunnel, installed internal walkways, and completed work on 40 cross passages connecting the twin bores.
The Chiltern tunnel is now structurally complete, becoming HS2’s second finished twin-bore tunnel after the Long Itchington Wood tunnel in Warwickshire.
Further phases will involve the installation of mechanical, electrical, and plumbing infrastructure within the tunnel. Design activities are currently underway, and enabling works commenced earlier this year.
Track systems and overhead power equipment will be added once these tasks are complete.
HS2 Chiltern tunnel civil engineering head Mark Clapp said: “The team we assembled at HS2, 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.”
HS2 chief executive Mark Wild is overseeing a review with the aim of delivering the remaining sections efficiently and at controlled costs.
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.
“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.”
Although considerable progress has been achieved on this section of the route, significant construction work is still required before the full completion of the 225km line.
