Go to content
Architect's impression of the Chiltern Tunnel South Portal in Hertfordshire.
Architect's impression of the Chiltern Tunnel South Portal in Hertfordshire.

Chiltern tunnel

The Chiltern tunnel is the longest tunnel on the HS2 route. The twin-bore tunnel is 10 miles long and will carry passengers under the Chiltern Hills. Two tunnel boring machines, Florence and Cecilia, will be used on this part of the route.

The two enormous 2,000 tonne machines were delivered to their launch site at the Chiltern tunnel South Portal, next to the M25 in Buckinghamshire, meticulously reassembled, tested and commissioned by an expert team of tunnelling engineers, before they started work on the high speed rail project’s first tunnels.

TBM Florence was launched in May 2021 and TBM Cecilia launched in July 2021.

The 10-mile-long ‘twin bore’ Chiltern tunnel will be the longest on the project with its two TBMs, launched in the summer of 2021, set to be underground, working 24/7 for more than three years.

Track Florence and Cecilia’s progress as they tunnel beneath the Chiltern Hills

Photos of HS2's first pair of tunnelling machines, Florence and Cecilia, shortly before they started their journeys under the Chiltern Hills.

After construction of the tunnel has been completed, we will reuse construction materials to create 127 hectares of new chalk grassland, woodland, wood pasture and wetland habitats that will substantially enhance the local natural environment.

HS2’s ambitious Chiltern tunnel chalk grassland project

Find out more about Colne Valley Western Slopes