Netherlands, 2000-2005
To address technical and environmental challenges including an enormous polder, a tunnelling area below sea level, waterlogged subsoil, all in a protected region, Bouygues Travaux Publics came up with a decisive original solution: the construction of a single tube tunnel (7,160 metres) using the largest slurry pressure balance tunnel boring machine at that time (14.87 metres). The solution chosen by the Dutch Ministry of Transport, involving the building of a central partition wall, enabled the number of passages between the two tracks to be doubled. It also dispensed with cross passages, which are very difficult to build in sandy ground. Lastly, the area required for the access portal works and the volume of excavated material were significantly reduced, thus limiting the impact on the environment.