The massive project started in 2021 and has now been called to an end.

Aerial view of Chiltern tunnel

The Chiltern tunnel is 10-miles-long. (Image: HS2)

Construction has been completed on HS2's longest railway tunnel. The 10-mile-long Chiltern tunnel stretches beneath the Chiltern Hills and will see trains travel at 200mph through it. Two tunnel boring machines, Florence and Cecilia, were used to excavate the twin-bore tunnel.

Each impressive machine was designed specifically for the geology of the Chilterns and housed a self-contained underground factory. They dug the tunnel and lined it with concrete wall segments as they went along, grouting them into place. The TBMs were run by a crew of 17 operatives working in shifts, supported by more than 100 people at the main compound.

Each of the separate northbound and southbound tunnels was built with 56,000 precision-engineered, fibre-reinforced concrete wall segments, HS2 says, all made at the south portal of the tunnel. Around 2.7million cubic metres of material were removed during the construction of the tunnels, which were then used for landscaping around the south portal site.

Inside the Chiltern tunnel

The tunnel was bored by massive machines. (Image: HS2)

Now, officials say that the closure of work at two of the line’s Chiltern tunnel vent shafts at Chesham Road and Little Missenden has brought the project to an end.

They added: "Five deep ventilation and access shafts were sunk to depths of as much as 78 metres along its route and completed before the TBMs’ arrival. The understated design of each shaft’s headhouse, by Grimshaw architects, is intended to blend unobtrusively with the surrounding landscape of the Chiltern hills.

"The machines progressed north at an average speed of 16 metres per day before breaking through near Great Missenden, Buckinghamshire, in early 2024.

"Since then, work has continued to build the porous extensions to its north and south portals, install its internal walkways and fit out 40 cross passages."

The head of civil engineering for Chiltern tunnel, Mark Clapp, said: “Multi-faceted 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.

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"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.”