Second TBM’s breakthrough marks excavation completion of HS2’s Chiltern tunnels

The second of the two tunnel boring machines (TBM) that were used for the excavation of HS2’s longest Chiltern twin-bore tunnel marked a major milestone for the project, as it made its final breakthrough near South Heath, Buckinghamshire, on Thursday March 21, 2024.

Furthermore, the first TBM, named Florence, made its final breakthrough at the same place roughly a month prior to the second one, which is named Cecilia.

Both TBMs have identical specifications, weighing about 2,000 tons and measuring 10.25 meters in diameter.

Boring under the Chiltern Hills began in June 2021, while the project consisted of excavating the 10 mile (16 kilometer) long tunnels at an average speed of 16 meters per day and reaching as deep as 80 meters.

Moreover, a total of three million cubic meters of chalk and other materials were excavated from the site.

The TBMs used innovative techniques, implemented for the first time in the UK, such as semicontinuous boring, which allowed the machines to lay tunnel rings behind them without pausing.

To be more specific, about 112,000 concrete rings were grouted in place for the construction of the tunnels’ linings.

Finally, each machine was operated by crews of 17 people, while a total of some 450 people have worked on the project these past three years.