HS2 Celebrates Final Breakthrough for Chiltern Tunnel

Tunnel boring machine (TBM) Cecilia has completed the final breakthrough for HS2’s longest tunnel under Chiltern Hills.

This breakthrough completes a 10-mile journey that commenced in June 2021.

Final breakthrough for HS2’s longest tunnel
Final breakthrough for HS2’s longest tunnel

Cecilia is the second TBM to reach the northern portal of the high-speed rail tunnel under Chiltern Hills, joining TBM Florence which arrived last month after completing an adjacent tunnel drive.

The two 2,000-tonne machines have built the twin-bore tunnel at depths of up to 80 metres, excavating a total of three million cubic metres of chalk.

While excavating the tunnels, the machines have also lined them with 56,000 pre-cast concrete tunnel segments, which are grouted into place. This is accomplished while moving forward at an average speed of 16 metres per day.

Once HS2 is operational, trains will travel through the Chiltern tunnel at speeds of up to 200mph. This will significantly reduce journey times between London and Birmingham, as well as reducing traffic on the West Coast Mainline and freeing up space for more local services.

HS2 Ltd’s head of civil works for this section of the route, Mark Clapp, said:

“Today is a remarkable achievement for all concerned. Cecelia’s arrival completes the excavation of HS2’s 10-mile twin-bore tunnel, but it’s only the end of the first chapter in the story of building Britain’s high-speed railway under the Chiltern Hills.

“Several years of intensive, world class engineering lie ahead of us to complete the tunnels’ four ventilation shafts plus headhouses; install the mechanical and electrical plant and machinery, slab track and overhead electrical wires before trains begin passing through at 200mph.”

Both Cecilia and Florence were operated by HS2’s main works contractor, Align – a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick. The machines were made in Germany by TBM specialists Herrenknecht.

Each TBM was operated by a crew of around 17 people working in shifts to keep the machines running. These teams were supported by over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling.

Align’s Underground Construction Director, Didier Jacques said:

“Ten miles is a long drive for a TBM, with a typical average being around three miles. The complexity and technical challenges of completing such a long drive for both our TBMs has been significant, however I am delighted that as a team, we have risen to the challenge.

“Both TBMs were designed in partnership with Herrenknecht and incorporate innovations and technologies that have been introduced on TBMs in the UK for the first time, to enhance performance and safety. This includes ‘semi-continuous boring’, allowing our TBMs to build the rings that line the tunnels without pausing.”

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