HS2 Ltd has announced that construction work has now been completed on the line’s longest tunnel, bringing to an end a project that began nearly five years ago.
The Chiltern tunnel, which will soon be fitted with tracks and overhead equipment during a later phase of the project, is set to carry trains at 200mph, allowing them to traverse a 10 mile stretch in three minutes.
The tunnel will be the longest in HS2’s network
© HS2 Ltd
Main construction of the twin-bore tunnel began in May 2021 as two 2,000-tonne tunnel boring machines (TBMs) started their journey from a site next to the M25 near Maple Cross, Hertfordshire. In order to accommodate the two TBMs; Five deep ventilation and access shafts were sunk to depths as far as 78 metres along its route before their arrival.
Each shaft’s headhouse has been designed by Grimshaw architects to blend in with the surrounding landscape of the Chiltern hills.
Once operational; the two TBMs progressed north at an average speed of around 16 metres per day, breaking through near Great Missenden, Buckinghamshire, in early 2024.
Work has since continued to build a set of porous extensions to both its north and south portals, install internal walkways and fit out 40 cross passages.
Porous extensions have now been built at both ends of the tunnel
© HS2 Ltd
The structure is the second of HS2’s five twin-bore tunnels to be structurally completed after the successful installation of the one-mile Long Itchington Wood tunnel in Warwickshire last year.
Project 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.
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.
In order to enable round-the-clock operation; each TBM received a continuous supply of 56,000 eight-tonne tunnel lining segments from a purpose-built factory near the tunnel’s southern portal.
During their 33 month journey, the two TBMs – named Florence and Cecilia after modern nursing’s founder, Florence Nightingale and astronomer Cecilia Payne-Gaposchkin – excavated three million tonnes of chalk, all of which has subsequently been used on site to create 120 hectares of chalk grasslands around the tunnel’s southern portal along the Herts/Bucks border.
The completion of civil engineering works will soon be followed by the installation of mechanical, electrical and plumbing equipment, with the design-phase now underway. Rail systems, including track and overhead electrical equipment, will follow upon completion of these works.