Chiltern Tunnel

The Chiltern Tunnel is the longest tunnel on the HS2 route, stretching 10 miles (16km) beneath the Chiltern Hills. Two TBMs, Florence and Cecilia, were used to excavate the twin-bore tunnel. TBM Florence completed her journey in February 2024, followed by TBM Cecilia in March 2024.

Tunnel construction

The enormous 2,000 tonne tunnel boring machines were built in a factory by world-leading German tunnelling specialists Herrenknecht. After completion the machines were disassembled before beginning their long journey to England. On arrival at site, they were reassembled by the entrance to the tunnels, where a specially prepared head wall allowed them to safely begin their excavation.

Designed specifically for the geology of the Chilterns, each machine is a self-contained underground factory; digging the tunnel, lining it with concrete wall segments and grouting them into place as it moves forward. A crew of 17 operatives keep the machines running, working in shifts, and supported by over 100 people at the main compound, managing the logistics and maintaining the smooth progress of the tunnelling operation.

Each of the separate northbound and southbound tunnels will require 56,000 precision engineered, fibre-reinforced concrete wall segments – which are all being made at the south portal of the tunnel, next to the M25.

Approximately 2.7 million cubic metres of material will be excavated during the construction of the tunnels and used for landscaping around the south portal site. Once construction is complete, this will help create around 90 hectares of wildlife-rich chalk grassland habitats. Chalk grassland used to be widespread across the hills of southeast England and are considered habitats of international conservation significance with just 700 hectares left across the Chilterns.

Chilterns South Portal Chalk Grassland Project

Cross passage construction

In August 2022, construction of HS2’s longest tunnels begun a new phase with the start of work on thirty-eight cross passages, between 15 and 20 metres in length, linking the northbound and southbound tunnels.

An expert team of miners used a remotely controlled excavator to break out of and excavate from one tunnel to the adjacent tunnel. As the excavation is progressed the ground is supported using a sprayed concrete lining (SCL). Once this is complete, a water-proof membrane is installed followed by a secondary concrete lining.

Once a passageway is complete, the safety doors can be installed at either end. Each running tunnel has an emergency walkway that allows passengers to safely evacuate the train and walk through the cross passages into the other tunnel where they can be rescued on a passenger train. Access for emergency services is from either the portal or one of the five ventilation / emergency access shafts.