A five-year construction project on the Chiltern tunnel, the longest tunnel on HS2, has been completed with the close of work at vent shafts at Chesham Road and Little Missenden.

Two 2,000-tonne tunnel boring machines (TBMs) started construction of the twin-bore tunnel in May 2021 from a site near Maple Cross in Hertfordshire. Before they started work, five deep ventilation and access shafts up to 78 metres deep were sunk along the tunnel’s route.

Excavation progressed at an average speed of 16 metres per day until early 2024, when the TBMs broke through near Great Missenden in Buckinghamshire. Work then continued to build porous extensions to the tunnel’s north and south portals, install internal walkways, and fit out 40 cross passages.

The TBMs, named Florence and Cecilia after Florence Nightingale and astronomer Cecilia Payne-Gaposchkin, operated round the clock through a highly efficient, carefully sequenced operation. A continuous supply of 56,000 eight-tonne tunnel lining segments was supplied from a purpose-built factory near the tunnel’s southern portal.

Three million tonnes of chalk were excavated during the TBMs’ 33-month journey, with the excavated material being used to create more than 120 hectares of chalk grasslands around the tunnel’s southern portal.

Although civil engineering works have been completed, mechanical, electrical, and plumbing equipment still need to be installed. Design of these is already underway, and enabling works will start later this year. Once that is complete, the installation of the rail systems, including the track and overhead electrical equipment, can start.

Mark Clapp, HS2 Ltd head of civil engineering for Chiltern tunnel, commented that multi-faceted projects of the Chiltern tunnel’s scale and complexity don’t often come along. The team we assembled at HS2 Ltd, and with Align JV, the main civil works contractor, and all its sub-contractors, was exemplary. He paid tribute to everyone involved and felt certain that their hard work would stand the test of time.

Special portals have been built to eliminate the possibility of a sonic boom when high-speed trains enter the tunnel.

“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.”

Adrien Baudard, Align JV project director