South portal of the Chiltern tunnel after launch of the TBMs summer 2021

‘Florence’ and ‘Cecilia’ reach four-mile depth

HS2 have reached a major milestone in their Chiltern tunnel project as its has been revealed that their two enormous 2,000 tonne tunnelling machines have passed the second of five ventilation shafts this week, 4 miles into their 10-mile drive.

These tunnel boreing machines (TBM), named Florence and Cecilia, were deployed originally in Summer 2021 from the southern portal of the tunnels. The 170m long TBM's work to dig out the tunnel, whilst lining it with concrete segments, forming rings as they are grouted into places through the machine bores on.

These concrete rings have been constructed through more than 44,000 segments, with an estimated 68,000 set for further installation before the northern end of the tunnel has been bored in two years’ time.

Chalfont St Giles’ 46m deep shaft comes as one of four, which will provide ventilation and emergency access to the ten-mile-long twin tunnels. These will be accompanied by a fifth shaft, only designed to supply emergency access when needed.

The north and southbound tunnels will require 56,000 precision engineered, fibre-reinforced concrete segments. To ensure the project is supporting UK business, these segments are all being made in purpose-built factories at a local site in the M25.

These two TBM’s host a crew of 17 people each, seeing shift work implemented with over 100 people on the surface to support through the management of logistics and operational maintenance. Being the largest infrastructure project in Europe, HS2 and its subsidiary schemes support a plethora of jobs, equating to over 27,000 roles across the UK directly. Upon its completion, HS2 will support job creation further as it creates transport links between major cities, opening new opportunities for local services and freight passage.

Martyn Noak, HS2 Ltd’s Head of Tunnel Engineering, said:

“The Chiltern tunnel will take HS2 underneath the hills and safeguard the woodlands and wildlife habits above ground as well as significantly reducing disruption to communities during construction and operation of the new railway.

“It’s great to see how much progress has been made by Florence and Cecilia – and the teams excavating the five shafts – and I’d like to thank everyone involved in getting us this far.”

Throughout a project of this size, material excavation becomes a major component, seeing approximately 2.7 million cubic metres of material – mostly chalk and flint – excavated during the construction. To ensure the environmental sustainability of the project is maximised, these materials will be used for landscaping.

Upon project completion, the temporary buildings at the south portal will then be removed to allow landscaping of the site with 90 hectares of wildlife-rich chalk grassland habitats. These materials used to be widespread across the hills of south east England, but are now considered of international conservation significance due to just 700ha remaining across the Chilterns. HS2’s conservation efforts significantly aid in the fight for sustainability in UK rail, whilst also incorporating local heritage to their construction which garners key support from affected areas.

Didier Jacques, Align’s Underground Construction Director, said:

“Florence and Cecilia reaching our second shaft at Chalfont St Giles is a great achievement for the tunnelling team and I would like to pay credit to TGT, our supply chain partner, whose personnel are manning the TBMs.

“It’s also important to acknowledge the work by the construction team involved in excavating and preparing the shaft. In particular I would like to pay credit to KVJV, VolkerLaser and Keltbray our supply chain partners, who have been working tirelessly over the last few months to ensure the shaft is ready for the arrival of Florence and Cecilia, learning from their experience of excavating and preparing our first shaft at Chalfont St Peter.”  

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