Diabolo Project Zaventem

The Diabolo Project, led by Belgian railway operator Infrabel, was Belgium’s first PPP (Public-Private Partnership) project. The underground station at the airport was connected to the rail network to make Brussels Airport directly accessible from the north. The shape of the connection resembles a diabolo, hence the name.

  • Client: Dialink
  • Period: 2008
  • Location: Brussels Airport, Zaventem, Belgium
  • Solution: Tunnelling
  • Technique: Tunnel boring using segments

Behind the Scenes

Smet Tunnelling carried out the tunnel boring as a partner within the international THV Dialink consortium. The rail link is 5.17 km long and consists of two machine-bored tunnel tubes, each 1,070 meters long. The tunnel lies entirely beneath the airport grounds, under taxiways, aircraft parking areas, and even the critical runway. These areas were able to continue operating without disruption during our work thanks to our ongoing coordination with Infrabel and Brussels Airport.

The work was carried out under exceptionally complex conditions:

  • The tunnel tubes are completely submerged in groundwater, just 3.5 meters away from the foundation piles of the nearby maintenance hangars
  • The maximum allowable settlement was only 15 mm, which made techniques such as jet grouting arches and fiberglass-reinforced “soft eyes” necessary
  • The pipe jacking had to proceed without closing the runway, under strict airport security, with limited working heights and escorted transports

A Sustainable Approach to the Project

  • SDG 9 – The project involves the construction of a new 5.17-kilometer rail link, including two bored tunnel tubes measuring 1,070 meters, a 480-meter cut-and-cover tunnel, and a 400-meter crossover tunnel. It is Belgium’s first PPP project, involving complex construction methods beneath an operational airport.
  • SDG 11 – The project improves access to Brussels Airport, reduces traffic congestion, and enhances safety. By constructing underground beneath Runway 07L–25R, the taxiways, and the aircraft parking areas, the impact on the surrounding area is minimized. The runway could not be closed, so the construction method ensures both environmental and operational continuity.
  • SDG17 – The project is a prime example of international cooperation between contractors, investors, Infrabel, and Brussels Airport.

Innovation

Our process is as efficient as possible thanks to our proprietary SKID designs and prefabricated materials. This enables us to build and commission installations in less than a month.