Tunnelling case study

Tunnelling example: Alaskan Highway road tunnel, Seattle, USA

A highway goes underground

Creating space for new and safe traffic routes: The world's largest tunnel boring machine has been eating its way through the ground under Seattle, Washington since summer 2013. Its job: To build a tunnel extending 3 km with a diameter of 15.8 meters replacing the two-level Alaskan Highway as an urban arterial road. The high outputs of 2,800 tons per hour are carried off by a H+E conveyor system specially coordinated to the local requirements featuring a tunnel belt conveyor of 2,987 meters in length. A horizontal belt storage unit with a capacity of 500 meters guarantees continuous extension of the belt.

The Alaskan Highway has been part of State Route 99 for the past 50 years. As of Seattle city center, State Route 99 was extended southward as a highway and within the city center, it runs as the elevated Alaskan Way Viaduct along the steep coastline. The remainder of the elevated highway is to be demolished 2016. Construction of the tunnel was triggered by an earthquake in 2001 which caused damage to the elevated highway.


  • Tunnel diameter: 15.8 m
  • Min. radius: 480 m
  • Excavated material: EPB
  • TBM supplier: Hitachi
  • Tunnel belt length: 2,987 m
  • Belt width: 1,600 mm
  • Delivery rate: 2800 t/h
  • Power installed: 3 x 362 kW
  • Belt storage capacity: 1 x 500 m / horizontal
  • Installation: 2013

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