In the late 1960s, Wyoming’s highway engineers had a novel idea.
As the Wyoming Department of Transportation planned Highway 22 over Teton Pass not much could be done with the road where it passed under the Twin Slides avalanche path because of the narrow constriction at the top of the pass.
Around the corner, though, engineers wanted to pull the road out from the bottom of Glory Bowl, an even larger slide path. So WYDOT decided to build a suspension bridge over the gully that leads from Glory Bowl to Crater Lake, the thinking being that slides would pass beneath the bridge.
“Avalanche bridge is first of its kind,” read a headline in the Oct. 7, 1969, Casper Star-Tribune, the article touting it as a practical alternative to boring a tunnel or building a snowshed.
Just four months later the same newspaper reported that a historic slide came from Glory Bowl, uprooting trees and warping the deck of the bridge. Construction had been halted for the winter without installing the concrete roadbed, so the structure didn’t have the torsional strength to withstand the forces created by the slide’s powder cloud and roiling mass of snow.
It was a total loss. Abandoning the idea, WYDOT put the road against the bottom of the slide path.
“We still give the engineers a hard time for that,” WYDOT avalanche forecaster John Fitzgerald said.
The thinking behind the bridge was sound. Once complete it would have been strong enough to withstand the forces from the powder cloud and high enough to let slides travel below. But it wasn’t meant to be.
The failure didn’t deter other places from considering the concept. Fifty years later, in 2018, contractors with the Washington State Department of Transportation completed an avalanche bridge project on Interstate 90 over Snoqualmie Pass near Seattle, successfully realizing the vision of those WYDOT engineers.
Abandoning the snowshed
Unlike Teton Pass, Snoqualmie Pass isn’t a bastion of backcountry skiing.
“We have nothing even close to what you have,” WSDOT avalanche program manager John Stimberis said. “We have one area where lift access brings people within range of skiing to the highway, but with the divided highway being an interstate it’s a very difficult endeavor to ski to the highway.”
Leaving Seattle, I-90 wends its way through a series of suburbs before beginning a gentle rise toward the pass. Strip malls and neighborhoods give way to forested hills shrouded in mist. Even in March, very little snow clung to the hills.
At the top of the pass it was a different story. The snowline appeared almost out of nowhere, with runnels and gouges carved in the wet, low-elevation snow by avalanches.
At 2,600 feet the pass levels out, traversing the shores of Keechelus Lake, which forces the road against craggy slopes. Just east of the pass’ eponymous hamlet, the road rises a bit on one of the avalanche bridges.
Even if you’re looking for them, the bridges are easy to miss because they just blend into the road. They were part of a highway improvement project that took 20 years, a partnership that involved the state transportation department working with Atkinson Construction and designer Jacobs Engineering Group, among other companies.
When the original two-lane highway was built, a snowshed protected the most avalanche-prone part, but a subsequent widening of the highway added two lanes of travel outside of it. That created a situation in which avalanche control work would close the highway because the two unprotected lanes would be covered in snow.
Like Teton Pass, Snoqualmie Pass is a major artery, but instead of connecting a single town with the bulk of its workforce, I-90 is a major shipping route.
“We’re a trade-dependent state. We’ve got the deepwater ports in the Seattle-Tacoma area. That’s where trucks are going,” said Jason Smith, of Jacobs Engineering. “On the east side, we’re big agriculture. We haul our large stuff to Seattle to the ports.
“So any closure is just not acceptable.”
When state highway officials started planning to replace the 50-year-old roadbed in the late 1990s, they had to weigh replacing the snowshed. If they did, closures were still likely because of the potential that a big slide could run off the sides of the shed and because another slope, Slide Curve, had no protective infrastructure.
So they lifted the roadway, putting it on big columns and excavating beneath it. Instead of needing a structure specifically designed for an avalanche path, they simply constructed the same kind of bridge the highway department had across the state, separating the pillars wide enough that avalanches would pass between them.
By applying old technology to a new problem, they kept their job easy.
“We built very, very simple bridges that we build all the time,” said Phil Larson, of Atkinson Construction.
Somewhat counterintuitively, Stimberis, the avalanche coordinator, sees fewer slides now that reach the old roadbed.
Frequent protective work like he had to do before the bridges were built fills the middle of the slide path with debris, pushing the toe of subsequent slides farther down, which meant lots of avalanches hit the road before the bridge was in place. The bridge effectively separates the road from the slide path, eliminating a lot of work and allowing the path to return to a natural state.
“Over the last four years now that we’ve operated with the bridges fully in place, we’ve recorded 20 avalanches that reached the old road bed,” Stimberis said. “And my estimate is we would probably have done control about 65 times.”
An expensive fix
On a rainy day in early March, Larson, Smith and some of the other engineers from the project gathered in a conference room at the Hyak Maintenance Facility just a couple miles from the bridges. They were eager to talk.
In almost two decades of planning and building, the project included a bevy of public comment and planning, wildlife overpasses and, of course, avalanche mitigation. The men shared intricate, almost esoteric, details of the construction and waxed about the way public departments and private companies worked together on a scope that eclipsed other projects, blending elements like the bridges, which they had done before, with snow nets, a concept foreign to them before the I-90 renovation.
They’re confident the infrastructure they built could be replicated elsewhere, though they stopped short of saying exactly where it would work because the spatial and geotechnical elements of any project are site-specific. For example, they said the snow nets on Slide Curve were difficult to install, with some footings being nearly 20 feet deep.
“Our rock up here is angled in several different ways,” Larson said, “and it’s not very competent.”
Rock in the Tetons might be more competent (engineer-speak for solid), he said, so the slopes above Highway 22 could be a good fit for the lines of braided steel nets. Nearly a mile of the nets hold the slope above Slide Curve in place, adding compressive support to the snowpack.
Stimberis said the slope hasn’t slid since they were installed.
Despite the clear success of the bridges and nets, this kind of infrastructure doesn’t come cheap. A paper Stimberis wrote in 2010 said the overall highway project carried a $550 million price tag, and Atkinson’s website says the contract to widen the highway and build the bridges was for $177 million.
In comparison, WYDOT’s annual budget for the entire state is around $250 million, money that covers road maintenance and construction, snow removal and other projects for the 10th-largest state by area. WYDOT’s avalanche team, Fitzgerald and his colleague, Brenden Cronin, don’t expect the state to shell out tens of millions of dollars for a single project in a remote corner of the state when budgets are already contracting.
Despite their avowed opinion that infrastructure like a bridge would make their lives easier in the long run, the forecasters are wary of how construction would disrupt life on the pass.
“You can’t just build a bridge or build a snowshed,” Cronin said “You’re gonna have to shift the entire roadway, and that’s a huge impact.”
Teton Pass’ harsh winters mean building would take place only in the summers and shoulder seasons, the times when Jackson’s economy booms from an influx of tourists, some of whom drive over the pass. Though Snoqualmie Pass is much lower in elevation, the builders faced the same problem, with a local twist.
“You basically have a six-month window where we can do the construction,” Larson said. “Of that six months, you only have three months when the lake’s down low enough to do the work in the lake area.”
With a similar construction season in the Tetons, especially at elevation, the WYDOT forecasters anticipate any project would take years. That presents challenges both for drivers, who would be inconvenienced for several summers, but also for the construction crews, since incomplete projects are less able to hold up to avalanches.
For evidence of that, one need look no farther than the failed bridge on Teton Pass.
A world without avalanche closures
In the winter of 2011-12, avalanche operations closed Snoqualmie Pass for a total of 108 hours. Closures like that hold up entire parts of Washington’s economy. It’s not a commuter pass, but on average between 3,500 and 4,000 large trucks pass over it each day, and the average number of daily vehicles, 25,000, can balloon to 50,000 over a holiday weekend.
The alternate route from the agricultural communities of eastern Washington to Seattle’s ports goes down into Oregon, where I-84 skirts the state border and meets up with I-5 in Portland. The route would add hours, if not an entire day, onto a shipping route.
“No one really does that,” Smith said, emphasizing the need to keep the pass open.
The bridges — and the closures they prevent — save the state an untold amount of economic activity. Atkinson Construction also says the project saved the state $37 million, money that would have been spent to maintain the snowsheds. Stimberis said they have eliminated 75% of his control and cleanup work, allowing him and his crews to focus on other slopes, or simply work less.
From 1992 to 2016, he said, the area had 370 closures, an average of about 15 per year. Each one might be at least four hours of work, but the bridges essentially eliminated much of that work on those slopes, drastically reducing the number of closures.
That helps the road maintenance and snow plowing crews, which have their hands full considering the wet, heavy snow the pass receives and its high level of traffic.
“The time they have to spend clearing an avalanche after closing the highway for an avalanche control mission, that’s time they can’t spend elsewhere,” Stimberis said. “If it’s snowing 2 inches an hour, that’s a lot of lane miles that aren’t getting plowed.”
While Teton Pass might not have completed the first avalanche bridge, 50 years later, the state of Washington has shown that the idea was sound. The proof is in the way the bridges (and the nets) have changed Stimberis’ life for those particular slopes.
“It’s eliminated the avalanche problem for us,” he said.
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