High Speed 1 – North Downs Tunnel

Re-engineered tunnel invert design

£5 million saved

Delivered five months early

The North Downs Tunnel is an award-winning 3.2 kilometre tunnel with an internal finished diameter of 12.8 metres, built to carry the Channel Tunnel Rail Link (CTRL). One of the largest and deepest twin-track rail tunnels ever constructed in the UK, it carries trains at up to 300 kilometres per hour at depths reaching 100 metres beneath the chalk hills of the North Downs.

UnPS designed the primary lining and temporary works for this project, working collaboratively with the customer’s engineer to develop an innovative solution for the secondary lining.

As a consequence of a highly collaborative partnership between the customer and project delivery teams, UnPS led an extensive value engineering process with the customer and its designers that resulted in savings of £5 million.

  • Customer: Union Railways Ltd
  • Project value: £75m
  • Partners: Morgan Vinci JV
  • The project won the Large Project Award category at the Tunnelling Industry Awards 2002
  • The redesigned and flattened tunnel invert led to significant material savings by reducing both the volume of excavated material and the thickness of the concrete base
  • The new invert profile allowed the concreting works to be carried out as a continuous pour leading to substantial programme savings
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