The two 10.2m diameter Herrenknecht machines, Florence and Cecilia, were launched 18 months ago to dig the twin tunnels between the M25 and South Heath in Buckinghamshire, creating the longest tunnels on the new high-speed rail line.

Engineers have also excavated five shafts that will provide ventilation and emergency access near Chalfont St Peter, Chalfont St Giles, Amersham, Little Missenden and Chesham Road. They also recently completed the first breakthrough from a cross passageway to the shaft at Chalfont St Peter.

More than 1.3 million m3 of chalk and flint have been excavated and will be used as part of a chalk grassland restoration project at the south portal of the tunnel.

Once complete, the Chiltern tunnels will carry high-speed trains between London and the north at speeds of up to 320km/h.

HS2’s main works contractor, Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – is responsible for the Chiltern tunnel and nearby Colne Valley Viaduct.

Align's underground construction director, Didier Jacques, said passing the halfway point, on what was a long drive for the TBMs, was a great achievement and he paid credit to supply chain partner TGT whose personnel were manning the TBMs.

“This significant progress would not have been possible without the supporting teams on the surface at the south portal, who supply the thousands of 8.5-tonne segments required to line the tunnels, process the spoil pumped back as slurry through our slurry treatment plant and landscape the chalk produced across the site. Overall, a great team effort,” said Jacques.  

Designed for the mix of chalk and flints under the Chilterns, the two identical TBMs are excavating separate tunnels for north and southbound trains. They have advanced around 15m a day and have installed 56,000 fibre-reinforced concrete segments.

HS2 recently launched two TBMs to begin the project’s London tunnels and celebrated the first breakthrough with the completion of the first of two tunnels under Long Itchington Wood in Warwickshire.