Giant HS2 ‘moles’ reach halfway point of Chiltern tunnels

HS2 Chiltern tunnel curving out of sight in November 2022.

HS2 has confirmed that two huge 2,000 tonne machines have reached the halfway point in excavating the 10-mile long Chiltern tunnels.

Two giant tunnel boring machines (TBMs) – named Florence and Cecilia – have excavated 1.3 million cubic metres of chalk and flint, which is enough to fill more than 500 Olympic-sized swimming pools.

Florence and Cecilia have spent the last 18 months excavating the twin tunnels between the M25 and South Heath in Buckinghamshire. 

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.

The materials excavated will be used as part of an ambitious chalk grassland restoration project at the south portal of the tunnel. The project will create 127 hectares of new landscaping, wildlife habitat and biodiverse chalk grassland.

Once completed, the Chiltern tunnels will carry high speed trains between London and the north at speeds of up to 200mph (320km/h), helping to improve connections, free up space for more freight and local trains on the existing railway and provide zero carbon journeys for people across the UK.

Martyn Noak, HS2 Ltd’s head of tunnel engineering, said: “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 above ground as well as significantly reducing disruption to communities during construction and operation.”

HS2’s main works contractor, Align, is responsible for the Chiltern tunnel and nearby Colne Valley Viaduct. Align is a joint venture between Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick. TGT, Align’s supply chain partner, is manning the TBMs.

Didier Jacques, Align’s underground construction director, said: “Florence and Cecilia passing the halfway point, on what is a long drive for the TBMs, is a great achievement.

“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 though our slurry treatment plant and landscape the chalk produced across the site. Overall, a great team effort.”

The two identical TBMs have been designed specifically for the mix of chalk and flints under the Chilterns and 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 hours a day. They are supported by more than 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.

HS2 has 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.

Click here to watch a video of HS2 tunnelling in action.

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