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Prospectus Blog Cascadia-Arup Report: Deep Bore Tunnels @ $200M-$700M Per Mile

A report titled “Large Diameter Soft Ground Bored Tunnel Review” has just been released by the transportation think tank Cascadia Center and global engineering and consulting firm Arup. It strongly suggests that a new state cost estimate for a deep bored tunnel of approximately two miles to replace the aging Alaskan Way Viaduct could be greatly inflated.
The state’s Alaskan Way Viaduct Stakeholders Committee (SAC) pegs the cost of the one-mile tunnel at $3.5 billion, but the Cascadia-Arup report surveyed deep-bored vehicle tunnels worldwide and found costs typically fall in the range of $200 million-$700 million per mile, for large diameter soft ground bored tunnels, created with assistance of a tunnel boring machine (pictured below at right).
A Nov. 20 Cascadia letter with 2 summary charts on tunnel costs, plus the Arup report, were distributed to SAC members at the SAC’s Nov. 20 meeting, where SAC cost estimates for eight different Viaduct replacement options, including the deep bored tunnel, were unveiled.
In its letter to the SAC, Cascadia Center notes:

  • Surface street and transit improvements will help replace the Viaduct, but 70 percent of the traffic is bypass or freight;
  • Deep bored tunnel technology is widely used, and provides lower full-life cycle costs than above-ground roadways;
  • Environment and urban design are enhanced with a tunnel;
  • A deep bored tunnel to replace the viaduct would likely cost in the range of $200-$700 million per mile.
  • SAC is expected to winnow down the eight options to three recommended for further consideration. In December, Governor Christine Gregoire, Seattle Mayor Greg Nickels and King County Executive Ron Sims are to recommend one option. The state legislature will make the final decision.
    Cascadia Center reiterates its recommendation made to the SAC in March 2008 that an independent expert review panel assess the costs of a deep bore tunnel to replace the Viaduct.
    Cascadia has learned from sources close to the proceedings that one quarter of the $3.5 billion SAC estimate for the deep bore tunnel is for design fees. These can be dramatically reduced, along with risk management and construction timeline costs, through a properly detailed “design-build-operate-maintain” contract with a project management consortium. In addition, time-variable tolling in the SR 99/I-5 corridor, including the proposed tunnel, could help fund the project.
    RELATED:
    A Deep Bore Tunnel to Replace The Alaskan Way Viaduct,” Cascadia Prospectus.