HS2 tunnelling machines reach half way in 10-mile route under Chilterns to Bucks

The massive tunnelling machines boring their way under the Chilterns in preparation for the HS2 rail line have passed half way on their 10-mile excavation, HS2 Ltd confirmed today.

The enormous 2,000 tonne machines, nicknamed Florence and Cecilia, have spent the last 18 months excavating the twin tunnels between the M25 and South Heath in Bucks.

As well as digging and lining the tunnels, engineers have also completed the excavation of 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 cubic metres of chalk and flint – enough to fill more than 500 Olympic swimming pools - has been excavated and will be used as part of an ambitious chalk grassland restoration project at the south portal of the tunnel, which will see the creation of 127 hectares of new landscaping, wildlife habitat and biodiverse chalk grassland.

Once complete, the Chiltern tunnels will carry high-speed trains between London and the north at speeds of up to 200mph.

Designed specifically for the mix of chalk and flint under the Chilterns, the two identical tunnel boring machines (TBMs) are excavating separate tunnels for north and southbound trains. The excavated material is mixed with water to form a slurry, before being pumped back to the surface.

Each machine operates as a self-contained underground factory - digging the tunnel, lining it with concrete wall segments and grouting them into place at a speed of around 15 metres a day.

More than 56,000 precision-engineered, fibre-reinforced segments have been installed to create the walls of the tunnels – with each one made on site in a dedicated temporary pre-cast factory.

A crew of 17 people operate each TBM, 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.

Martyn Noak, head of tunnel engineering for HS2 Ltd, said: “It’s great to be able to celebrate the halfway point of this 10-mile long tunnel. The whole team has put in a fantastic effort over the last year and a half and I’d like to congratulate the crews of both Florence and Cecilia for passing this important tunnelling milestone and thank them for all their hard work.

“This tunnel will take HS2 underneath the Chiltern hills, safeguarding the woodlands and wildlife habits above ground as well as significantly reducing disruption to communities during construction and operation.”

Didier Jacques is underground construction director for Align, the consortium of international infrastructure companies responsible for the Chiltern tunnel and nearby Colne Valley Viaduct.

He said: “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.”

HS2 also recently launched two TBMs to begin the project’s London tunnels and completed the first of two tunnels under Long Itchington Wood in Warwickshire.