Two enormous HS2 tunnelling machines marked a milestone in the journey through Chilterns - the longest tunnel in the entire high-speed railway project. 

Named after famous figures, the two giant tunnelling machines Florence and Cecilia measuring 170m in length are now almost halfway through the 10-mile journey under the Chilterns more than a year after they were launched last summer.

The two-tonne machines - named after Florence Nightingale and Cecilia Payne-Gaposchkin – passed the second of five ventilation shafts four miles into the journey between the south portal by M25 near Chalfont St Peter and HS2's north portal in Great Missenden. 

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HS2 Ltd’s Head of Tunnel Engineering Martyn Noak said: “The Chiltern tunnel will take HS2 underneath the hills and safeguard the woodlands and wildlife habits above ground as well as significantly reducing disruption to communities during construction and operation of the new railway.

South portal of the Chiltern tunnel after launch of the tunnel boring machines in summer 2021 (Credit: HS2)

“It’s great to see how much progress has been made by Florence and Cecilia – and the teams excavating the five shafts – and I’d like to thank everyone involved in getting us this far.”

Around 2.7 million cubic metres of material – mostly chalk and flint - will be excavated during the construction of the tunnels and used for landscaping, and so far more than 1m cubic metres have been placed on site.

The 46m deep shaft at Chalfont St Giles is one of four that will provide ventilation and emergency access to the twin tunnels.

A fifth will just provide emergency access. Once complete, the shaft will be covered by a headhouse designed to resemble farm buildings.

A CGI visualisation of the HS2 Chalfont St Giles headhouse, which houses emergency and ventilation access to the Chiltern Tunnel (Credit: HS2)

Once construction is complete, the temporary buildings at the south portal will be removed and the site landscaped with around 90 hectares of wildlife-rich chalk grassland habitats.

Florence and Cecilia were designed specifically for the geology of the Chilterns – an area known for its rich chalk and flint land and precious chalk streams.

Chalfont St Giles headhouse aerial view (Credit: HS2)

First launched in the summer of 2021, each machine is a self-contained underground factory for digging the tunnel and lining it with more than 100,000 concrete segments to form a ring, which it binds into place as it inches forward for another two years. 

A Chalfont St Giles vent shaft looking up in summer 2022 (Credit: HS2)

Each machine has a crew of 17 people working in shifts with support from more than 100 people on the surface, who manage the logistics and operation of the tunnelling.

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