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State Route 99 Tunnel, United States of America

The realization of starter units which form the foundation for the tunnel-in-tunnel. This area is formed with custom steel formwork. The two support areas – one for the rising wall, another for the prefabricated panel which is mounted at the end of all reinforced concrete work and subsequently forms the bottom carriageway slab – are clearly visible.
The rising walls are concreted directly on the starting units which in turn form the support for the upper roadway. Two MAXIMO Formwork sets ensure fast, efficient concreting of the 4.50 m high walls.
The slab was subsequently concreted by means of a total of six formwork carriages – constructed using the back-step construction method.
Shuttering and striking processes for the slab are supported by hydraulic components. All formwork carriages were designed with corresponding passageway openings for transporting materials in the direction of the tunnel boring machine.
The view from the north towards the already completed southern tunnel entrance.
Within the drilled tunnel tube with a diameter of around 17.50 m, a double-story road tunnel is being realized – mainly constructed using in-situ concrete together with a comprehensive PERI solution. (Rendering: Washington State Department of Transportation)

Project data

Location: Seattle, Washington, United States

  • New construction of a 3.2 km long tunnel to replace the 1950 vintage Alaskan Way Viaduct; daily traffic volume of 110,000 vehicles
  • Drilled tunnel with a 17.50 m diameter, covered with 60 cm thick concrete ring segments, tunnel realized using one of the world´s largest tunnel boring machines (TBM) Bertha
  • Subsequent development with a double-storey road tunnel, mostly built using in-situ concrete construction


  • Realization of the foundation, walls and supported slab as a carriageway construction within the tunnel tube previously drilled
  • Detailed planning of the concreting sections with mobile formwork solutions for corbels, walls and slab; all formwork carriages following the TBM moved on one and the same rail construction through the tunnel
  • Taking into account the winding tunnel route and the resulting lateral inclinations of the slab


Seattle Tunnel Partners, Seattle, Washington, United States

Customer's benefit

  • Coherent overall concept due to the close cooperation of all parties and early planning
  • Efficient construction of the structure under difficult boundary conditions
  • Cost-effective execution by maximizing the proportion of system components from the PERI rental park
  • Fast working through the use of hydraulic solutions for shuttering and striking
  • Maintaining the tight construction schedule for all forming operations due to flexible geometrical adaptations

PERI solution

  • Development of an overall concept for forming foundations, walls and slabs within the previously drilled tunnel tube
  • Planning and delivery of project-specific mobile formwork carriage solutions for creating starter units, 4.50 m high side walls as well as a supported slab
  • Formwork carriage designed for 16.45 m long sections – based on the expected average drilling rate of the tunnel boring machine
  • Shuttering and striking concept with hydraulic components if required
  • Configuration of all formwork carriages with corresponding passageway openings for transporting materials in the direction of the tunnel boring machine