HS2 TBMs pass Amersham shaft

3 March 2023


The two TBMs digging HS2’s longest tunnels have completed more than half of their 16.1km drive.

Florence and Cecilia have spent almost two years excavating the twin tunnels under the Chiltern hills, between the M25 and South Heath in Buckinghamshire. The two Herrenknecht TBMs have now bored nearly 9km and passed the 44m-deep shaft near Amersham.

Designed specifically for the geology of the Chilterns, the 10.26m diameter TBMs were launched six weeks apart in summer 2021 from a site near the M25. So far they have excavated more than 1.8 million m3 of chalk and flint.

As well as digging and lining the tunnels with 2x4m rings, HS2 main works contractor Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – has completed the excavation of five shafts that will provide ventilation and emergency access near Chalfont St Peter, Chalfont St Giles, Amersham, Little Missenden and an intervention shaft at Chesham Road.

A ‘headhouse’ will be built on top of the Amersham shaft to house safety equipment, with a flint-faced boundary wall and a pre-patinated zinc roof to help match the surrounding landscape.

Align tunnel manager Coralie Peroux said the TBMs reaching the third shaft at Amersham was a great achievement for not only the tunnelling team and the team involved in excavating and preparing the shaft, but also the supporting teams on the surface at the South Portal, manufacturing the concrete segments and processing the spoil.

“In particular I would like to pay credit to Align Shafts team, working with our supply chain partners KVJV and Keltbray, who have been working tirelessly over the last few months to ensure the shaft is ready for the arrival of Florence and Cecilia,” she said.

Approximately 2.7 million m3 of material – mostly chalk and flint – will be excavated during the construction of the tunnels and used for landscaping. Once construction is complete, the temporary buildings at the south portal will be removed and around 90ha of grassland created.

HS2 currently has five TBMs in the ground, with another five due to be launched over the coming years. Together they will create 103km of tunnel between London and the West Midlands, including major tunnels on the approach to London and Birmingham.

  • The project update comes as HS2 CEO Mark Thurston said the project’s timing and phasing are under review to curb rising costs. The price tag has risen from the £33bn outlined in 2010 to at least £71bn. Thurston said the impact of inflation had been significant over the past year.