I-90 Snoqualmie Pass

Mitigating the Effects of Avalanches
Hyak, Washington
Washington State Department of Transportation
KPFF Consulting Engineers
Year Completed: 

Scheduled for completion in 2016, the I-90 Snoqualmie Pass project is a 1,100-foot bridge over the lakeshore, a 170,000 square-foot mechanically-stabilized earth wall, and 4,600 linear feet of drainage. Atkinson will blast and excavate 400,000 cubic yards of rock, as well as place 116,000 tons of base course, 31,000 cubic yards of Portland cement concrete pavement, and 38,000 tons of hot mix asphalt.

In addition to the lakeshore bridge, the I-90 Snoqualmie Pass project includes four single-span bridges to replace existing culverts and improve drainage, wildlife, and fish passage. The twin 1,200-foot-long bridges rise over avalanche chutes and can accommodate 100-year avalanche events. In addition, Atkinson will widen the mountain pass from two to three lanes in each direction, virtually eliminating road closures due to avalanche and snow removal, and straightening three miles of roadway to mitigate several unsafe curves.

Atkinson proposed a Cost Reduction Incentive Proposal, which substitutes elevated side-by-side bridges for the contract snowshed. It benefits the project and Washington State Department of Transporation in a number of ways, including eliminating long term operations and maintenance costs associated with the snowshed and minimizing the impacts to the public.