TBM breaks through on longest HS2 tunnel

28 February 2024

HS2’s first TBM – which launched almost three years ago – has broken through at the end of her 16km journey under the Chilterns, excavating the longest tunnel on the rail project.

The 2,000-tonne TBM, named Florence, is one of 10 machines excavating the 51.5km of tunnels on the new railway linking London Euston with Birmingham and was the first to launch, in May 2021.

Florence made an average daily advance of 16m and lined the tunnel with 56,000 pre-cast concrete segments.

Two identical Herrenknecht TBMs were used to create the twin-bore Chiltern tunnel, which stretches from the South Portal near the M25 to South Heath in Buckinghamshire. TBM Cecilia is due to break through in the coming weeks.

Florence and Celia were both launched from the South Portal and are operated by HS2’s main works contractor, Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

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 progress of the tunnelling operation.

In total, more than 450 people have worked on the tunnels and in support teams on the surface over the past three years. This includes a team that produced 112,000 precision-engineered, fibre-reinforced concrete tunnel wall segments at a temporary factory at the South Portal who completed their work just before Christmas, and a team processing the spoil from the tunnels.

Four similar TBMs are being used for the London approach tunnels, while another two will work on Birmingham’s Bromford tunnel. The two TBMs supplied for the Euston Tunnel will be launched this year and stored under Old Oak Common Station until the government gives approval for the line to be extended to Euston Station in central London.

Align project director Daniel Altier said the typical drive for a TBM was 5-6km and so 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 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,” said Altier.

“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 TBMs and MS with whom we designed and operated the slurry treatment plant; with 24 filter presses it is the largest in the world.’”

HS2 Ltd executive chairman Sir Jon Thompson said there was still 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”.

At its deepest point, the tunnel is 80m beneath the Chilterns and passes under the M25, 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 HS2 said there had been no significant change to water quality during the tunnelling work.

The 3 million m3 of chalk and other material removed during the tunnelling is being used to create a grassland restoration project at the south portal, which will include 127ha of new landscaping, wildlife habitat and chalk grassland.

Align is also delivering the record-breaking Colne Valley Viaduct which will be the longest railway bridge in the UK, with construction of the deck now more than two-thirds complete.