HS2 site at Chalfont Lane gets ready for tunnelling to start next year

The South Portal face at Chalfont Lane. Here, the tunnel boring machines will start their drive under the Chilterns next year.

HS2 has published images of the work at its site at Chalfont Lane, between junctions 16 and 17 on the M25.

This is where two giant tunnel boring machines (TBMs) will be ‘launched’ early next year to dig the 10-mile-long Chiltern tunnel to near South Heath in Buckinghamshire.

As they dig, the tunnel will be lined behind them using more than 118,000 concrete segments, which will be cast on site. The twin-bore Chiltern tunnel will be the longest and deepest tunnel bores on route, with separate northbound and southbound tunnels and five ventilation shafts.

The ‘arisings’, or spoil from the digging, will be mixed with water into a slurry so that it can be pumped out of the tunnels quickly and efficiently. At the surface, this slurry will be dried out and used for landscaping, so avoiding the need for the material to be taken out by road.

The Chalfont Lane site will include the factory making tunnel lining segments and the construction HQ of Align JV.

The South Portal Chalfont Lane site will, at 135 acres (55 hectares), be the biggest construction site on the project. Teams from HS2’s main works contractor Align JV – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick – will work with its subcontractors to deliver both the tunnelling and the construction of the adjacent Colne Valley viaduct.

Colne Valley viaduct crossing Harefield Lake.

Designed by Align’s partners Jacobs and Ingerop-Rendel, along with architect Grimshaw, this will be the longest railway viaduct in the UK. At 2.1 miles (3.4km) long, it will carry HS2 across the Grand Union Canal and surrounding lakes. Further south, the line will go into tunnel again before arriving at Old Oak Common and London Euston.

Around 350 staff from Align JV are now permanently based on site. At the peak of construction, around 1,200 people are expected to be employed in the design and construction of the Chiltern tunnel and the viaduct, with 50 opportunities for apprentices.

David Bennett, HS2

Dedicated motorway slip-roads have been built to link the new site with the M25 and take construction traffic off local roads.

HS2 delivery director David Bennett welcomed the progress on site. “The south portal site will have one of the most important parts to play in the story of HS2,” he said. “From here, we’ll manage the construction of the Chiltern tunnel and the iconic Colne Valley viaduct.

“It’s great to see how much progress has been made on site – and I look forward to working with the team to deliver what will be two of the most exciting and ambitious engineering challenges on the project.”

Daniel Altier, Align JV.

Align JV project director Daniel Altier added: “Significant planning and preparation have been completed by the Align team, working in close cooperation with HS2, and it’s great to see developments at the south portal. We now have a significant presence on-site and this is set to increase over the weeks and months ahead. 

“This is a complex project and it’s both rewarding and exciting for the team to see the component parts coming together after all their detailed planning. Align is now actively recruiting to build our team to address the complex engineering challenges associated with the delivery of this iconic project.”

The site at Chalfont Lane will include:

  • Site Offices – The construction HQ for Align JV, HS2’s main works contractor responsible for the Chiltern tunnel and Colne Valley viaduct.
  • Viaduct precast factory – This is where concrete sections for the Colne Valley viaduct will be cast before being assembled on location, just to the south of the main site.
  • Batching plant – Where the concrete will be mixed.
  • Tunnel precast factory – The concrete tunnel segments will be cast here on site before being taken down the tunnel to the TBMs. The factory has to start work three montha before tunnel boring commences so as to stockpile enough segments.
  • TBM launch area – Two tunnel boring machines will launch from this point to dig the twin 16km tunnels which will carry the trains in either direction.
  • Slurry treatment plant – Here, the slurry from the TBMs, mostly chalk, will be dried out so it can be reused in landscaping the site.

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