The 2000t TBM, named ‘Florence', is one of 10 machines excavating the 32 miles of tunnels on the new railway linking London Euston with Birmingham and was the first to launch in May 2021.
The breakthrough is a major milestone for the HS2 project, which will almost halve journey times between Britain's two largest cities while freeing up space for more local services on the most crowded part of the existing West Coast Main Line.
Two identical TBM were used to create the twin-bore Chiltern tunnel, which stretches from the South Portal near the M25 to South Heath in Buckinghamshire. Parallel tunnels will carry north and south-bound trains with a second TBM, named ‘Cecilia', due to a breakthrough in the coming weeks.
Designed specifically for the geology under the Chilterns, each TBM is an underground factory - excavating the tunnel, lining it with 56,000 pre-cast concrete segments and grouting them into place as it moves forward at an average speed of 16m per day.
Four similar TBM are being used for the London approach tunnels, while another two will work on Birmingham's Bromford tunnel. Preparations are also underway for the launch of two more machines to excavate the Euston tunnels.
HS2's executive chairman, Sir Jon Thompson, said: "Today is an incredible day for HS2, and I'd like to thank the hundreds of people who've worked so hard over many years to make it happen. Once complete, HS2 will dramatically improve journeys between our two largest cities and also free up space on the existing mainline for more local trains.
"We've still got a lot of work to do, but historic moments like today really underline the huge amount of progress that's been made and the fantastic engineering skills we have on the project."
Both machines launched from the South Portal and are operated by HS2's main works contractor, Align - a joint venture formed of three international infrastructure companies: Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick. The machines are made in Germany by TBM specialists Herrenknecht.
Each TBM is operated by a crew of around 17 people, working in shifts to keep the machines running 24/7. They are supported by over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.
The challenges in completing a 16km drive should not be underestimated
In total, more than 450 people have worked on the tunnels and in support teams on the surface over the last three years. This includes a dedicated team that produced 112,000 precision-engineered, fibre-reinforced concrete tunnel wall segments at a purpose-built temporary factory at the South Portal who completed their work just before Christmas - and a team processing the spoil from the tunnels.
Align's project director, Daniel Altier, said: "The typical drive for a TBM is 5-6km and, therefore, the challenges in completing a 16km drive should not be underestimated.
"Florence and her sister TBM Cecilia were designed in partnership with Herrenknecht. They incorporate a number of innovations and technologies that have been introduced on TBM in the UK for the first time, to enhance performance and safety. This includes ‘semi-continuous boring', allowing our TBM to build the rings that line the tunnels without pausing.
"The mining of the tunnel is a fantastic achievement for not only the Align tunnelling team but also the earthworks team, who have managed chalk excavated from the tunnels and placed on site, along with many other supporting functions.
"I would also like to acknowledge our supply chain partners, including TGT that provided the teams to operate the TBM and MS with whom we designed and operated the slurry treatment plant, with 24 filter presses, it is the largest in the world."
The three million cu.m of chalk and other material removed during the tunnelling is being used to create an ambitious grassland restoration project at the south portal, which will include 127 hectares of new landscaping, wildlife habitat and biodiverse chalk grassland.
The chalk, which is pumped back through the tunnel in a slurry mixed with water, is processed at a slurry treatment plant at the south portal site where the flint is removed and water extracted before it can be reused in landscaping.
At its deepest point, the tunnel is 80m beneath the Chilterns and passes under the M25 motorway, local railway lines and twice under the River Misbourne. Extensive water quality, groundwater level and surface water flow monitoring was put in place prior to the start of construction, and there has been no significant change to water quality during the tunnelling work.
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