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State Route 99 Tunnel, United States of America

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