Our Northwest

How the Denny Creek Viaduct preps for winter

Wednesday, November 26, 2014by  Jon Osterberg

Story No. 6 in a series
   Just as you winterize your home, Northwest landmarks get prepped for winter, too.
   That includes the Denny Creek Viaduct, looming far above the forest floor just west of Snoqualmie Pass.
   Or is it truly winterized? A structure that large, spanning a chasm at 2,600 feet elevation, must require winter maintenance, right? Read on.
   You might not know it by its name, but you’re probably familiar with the Denny Creek Viaduct. To I-90 travelers it’s “that long, downhill bridge near the ski area.”
   To summer hikers on their way to Denny Creek‘s granite waterslides, it’s “that huge overpass above the trail.”
   Construction on the viaduct began in 1976. It was, and remains, an engineering marvel. The viaduct comprises a unique concrete box-girder design that minimizes impact on the environment. Literally within the viaduct’s shadow, hikers amble up the Denny Creek Trail to enter the Alpine Lakes Wilderness Area.
   The 3,620-foot-long viaduct stretches above old-growth tree tops, in places soaring 160 feet above the ground. Its roadway is 52 feet wide with three lanes, plus shoulders.
   Because of its height, some predicted it would be a death trap. “It's an elevated ice-skating rink,” some said. “Drivers will skid and crash over the edge!”
   The Spokane Chronicle even reported on July 31, 1981, the day after the viaduct opened, “The Washington State Patrol is concerned that unwary motorists could skid off the span during icy conditions.”
   Such fears turned out to be unfounded.
   “From what I recall, we’ve had no more accidents there than anyplace else on the freeway,” said Trooper Chris Webb of the Washington State Patrol.
   There’s an urban myth circulating that the viaduct is equipped with heating coils that keep it from freezing in winter. Not true.
   “There are no specific winterizing requirements for the Denny Creek Viaduct,” said Barbara LaBoe, WSDOT Communications. “While very long, because of its location it was designed and built for adverse weather conditions. It has few moving parts, no painting requirements, and no operational preventative requirements.”
   LaBoe added that the viaduct is regularly cleaned, swept, inspected, and repaired during the summer, which prepares it for winter.
   However, there is one element of winterization – so to speak – being tested nearby on I-90 right now. WSDOT installed 4,600 solar-powered LEDs this fall in a five-mile test section west of Snoqualmie Summit.
   If you’ve ever crossed the pass after dark, especially during rain or snow, you know how I-90's white lane markers nearly disappear. It’s no wonder, if you think about how the white lines and reflectors over Snoqualmie Pass get abused: snowplows, de-icer, chains, studded tires.
   It can be difficult for drivers to gauge, "Am I in my own lane? Straddling the centerline? Or on the shoulder?"
   I enjoyed the new LEDs last month. They clearly marked my lane. It felt a bit odd, because it looked like I was driving down an airport runway lined with landing lights.
   But it was a welcome kind of odd.
   WSDOT will monitor the LEDs over the next three years to test their durability and longevity. If they hold up, we could see a lot more of them.

(Bottom photo courtesy WSDOT)

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