The ALIGN consortium (Bouygues Travaux Public / Sir Robert McAlpine / VolkerFitzpatrick) is in charge of the design and construction of section C1 of HS2, a high-speed rail line that will provide a non-stop link between London and Birmingham. This 21.6-kilometre section north-west of London includes a 3.4 kilometre-long viaduct (Colne Valley Viaduct) and two 16.04 kilometre-long tunnels (Chilterns Tunnels). Variable-density TBMs, a technology already used by Bouygues Travaux Public in Hong Kong and on the Grand Paris Express project, will be used to adapt excavations to the specific geological conditions of the terrain. The work also includes the construction of cross passages between the two tunnels, as well as five ventilation shafts.