Halfway point beneath the Chilterns is met seeing HS2 achieve its longest tunnels

HS2 Ltd is celebrating the vast project's longest tunnels as ‘Florence' and ‘Cecilia' which are two huge Tunnel Boring Machines have now passed the halfway point under the Chilterns.

The tunnelling machines way in at a massive 2,000 tonnes and have been hard at work excavating over the last 18 months the twin tunnels from the M25 to South Heath in Buckinghamshire. The tunnels are a fantastic solution for protecting the environment with the added advantage of providing improved connectivity between London, Birmingham and the North.

Alongside digging and lining the tunnels, engineers have now finalised the excavation of five ventilation shafts and an emergency access shaft near Chalfont St Peter, Chalfont St Giles, Amersham, Little Missenden and Chesham Road. The engineers also broke through a cross passageway to a shaft at Chalfont St. Peter recently.

Over 1.3 million cubic metres of chalk and flint have been removed and will use in a progressive chalk grassland restoration project located at the tunnel's south portal. The site will see a vast 127 hectares of landscaping completed as well as the creation of wildlife habitat and biodiverse chalk grassland.

When finished, the Chiltern tunnels will see high-speed trains travelling through them connecting London and the North at huge speeds of up to 200mph which will not only improve travel experience but will also provide space for more freight and local services to use the current railway.

Martyn Noak, HS2 Ltd's Head of Tunnel Engineering, 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. Once complete, HS2 will offer low carbon journey options linking London with the major cities of the north and releasing capacity for more freight and local trains on our existing mainlines.”

Align is HS2's main works contractor which is a joint venture created from three international infrastructure companies that will be responsible for the Chiltern tunnel and Colne Valley Viaduct:

• Bouygues Travaux Publics
• Sir Robert McAlpine
• VolkerFitzpatrick

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. I would like to pay credit to TGT, our supply chain partner, whose personnel are 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.'

The two identical Tunnel Boring Machines were specially designed in order to cope with the mix of chalk and flint which are under the Chilterns. The machines are excavating separate tunnels for both north and southbound trains, during the process, the excavated material is mixed with water in order to form a slurry which is then pumped to the surface.

Both of the machines work as a self-contained factory beneath the ground, working ever forward in digging the tunnel, lining it with concrete wall segments and grouting them into place. The impressive machines operate at approximately 15 metres per day and have used over 56,000 precision-engineered, fibre-reinforced segments each of which is made on-site at a temporary pre-cast factory, in order to create the tunnel walls.

17 People are needed to operate each of the Tunnel Boring Machines and work in shifts in order to keep them working around the clock. On the surface, the project is supported by more than 100 people who manage logistics and make sure the progress of the tunnels moves forward smoothly.

Further to the Chiltern tunnels, HS2 also revealed that two Tunnel Boring Machines have recently started working on the projects London tunnels achieving a first breakthrough and the completion of the first of two tunnels beneath Long Itchington Wood in Warwickshire.