Construction of the Chiltern tunnel – the longest on the HS2 high‑speed line – has reached a major milestone with the completion of its civil engineering works, HS2 Ltd has announced.
The twin‑bore tunnel, which will carry one track in each bore, stretches about 16km beneath the Chiltern Hills between Hertfordshire and Buckinghamshire. Once fitted with track and overhead electrification in later phases, trains will be able to pass through at up to 320kmh, traversing the bore in roughly three minutes.
Main tunnelling began in May 2021 when two 2,000t tunnel‑boring machines (TBMs) launched near the M25 at Maple Cross. They progressed northwards at an average of about 16m a day. The TBMs – named Florence and Cecilia (after Florence Nightingale and astronomer Cecilia Payne‑Gaposchkin) completed the drive in early 2024 – the former in February and the latter in March. In total, they excavated some 3Mt of chalk.
Work to finish the tunnel has continued since then: teams built porous portal extensions at either end (specially designed to reduce pressure waves of air and mitigate “sonic booms”), installed internal walkways and completed about 40 cross passages between the bores.
The south portals of the Chiltern tunnel extend beyond the tunnel bore and are porous to mitigate sonic booms
Two vent and access shafts at Chesham Road and Little Missenden have now been closed out, concluding a project that began almost five years ago. Five deep shafts in total were sunk during construction, some to depths approaching 78m. The headhouses for the shafts, designed by Grimshaw architects, are deliberately low‑profile to reduce visual impact on the Chilterns.
The tunnelling contract for this section was led by Align JV, a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick. To keep the TBMs operating continuously, a purpose‑built factory near the southern portal supplied precast lining segments: each machine received a steady stream of 56,000 8t segments over the course of the drive.
Rather than removing the excavated chalk by road, project teams repurposed the material on site to create more than 120ha of chalk grassland around the southern portal on the Hertfordshire‑Buckinghamshire border. HS2 Ltd says the work has expanded the county’s remaining biologically diverse grassland habitat by nearly a fifth.
The Chiltern tunnel is the second of HS2’s five twin‑bore tunnels to be structurally completed, following last year’s completion of the 1.6km Long Itchington Wood tunnel in Warwickshire.
Nevertheless, substantial work remains before the line opens. HS2 Ltd has said civil completion will be followed by installation of mechanical, electrical and plumbing systems, then rail systems such as track and overhead electrification. Design work for the fit‑out is already under way and enabling works are due to begin this year.
The wider HS2 programme has faced cost and timetable pressures in recent years; Mark Wild, chief executive of HS2 Ltd, is leading a project “reset” intended to deliver the remainder of the route as efficiently and cheaply as possible, the company has said.
Civil works continue on other parts of the 225km route between London and Birmingham as HS2 progresses through its next phases of construction.
‘Exemplary’ team
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.”
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Source: www.newcivilengineer.com
