Transportation crews plan to route more than cars when constructing new portions of Interstate 90 over Snoqualmie Pass in the Cascade Mountains. They’ll be directing avalanches too. When—not if in the high-elevation pass that separates western and eastern Washington—the avalanches occur, Washington State Dept. of Transportation officials want the snow and debris to flow under bridges, now blocked by snowsheds, in a 15-mile corridor improvement project.
Originally plans called for wider snowsheds as part of the $353 million project, but recently approved designs by the Federal Highway Administration allow for two elevated bridges in lieu of tradition.
The design change came after several months of collaboration between WSDOT and contractor Guy F. Atkinson Construction. When complete, the new bridges—some as high as 70 ft.—will take traffic up and over a series of engineered avalanche paths designed to direct future sliding snow, rock and debris between the piers and toward Keechelus Lake. The design will reduce the number of closures of I-90 do to avalanche control work.
The cost of the eastbound bridges, about $71 million apiece, is the same as the original snowshed design costs, not requiring a change in the project budget. But the reduction in maintenance and operation expenses should save WSDOT about $650,000 annually.
“The innovative design of the new bridges will keep I-90 open during the winter,” says Don Whitehouse, WSDOT regional administrator.
With over 300 inches of snowfall each year, I-90 succumbs to over 20 closures each year as the snowsheds built in the 1950s need regular help to ease the snow load around them. If engineers prove successful in routing the snow, the snowshed could be a design of the past.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He has also written for TIME, Popular Mechanics, Popular Science and more. You can follow him on Twitter at @tdnewcomb or visit his website here.