JACKSON — To reopen Teton Pass in the next two weeks, the Wyoming Department of Transportation plans to build a temporary road on the inside of the curve that collapsed in the Big Fill Slide …
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JACKSON — To reopen Teton Pass in the next two weeks, the Wyoming Department of Transportation plans to build a temporary road on the inside of the curve that collapsed in the Big Fill Slide over the weekend.
Doing so is common practice when a highway collapses, said George Machan, an engineer and expert on landslides who consulted for the Town of Jackson when a part of East Gros Ventre Butte collapsed at the Walgreens store on Broadway in 2014. But the plan is not without risks, Machan said.
It just depends how it’s executed.
The primary risk is if WYDOT puts too much material on the inside of the curve, which could decrease the stability of the outer slopes. More fill material would help level the bypass with the existing highway. But filling the interior curve with too much material “might be pushing it,” Machan said.
“I would try to hug the terrain as best as possible, to try to not invite another failure,” he said. “Trying to build a detour on the inside — that is a common approach.
“It’s just got to be engineered right,” Machan added.
On Tuesday, WYDOT announced that it plans to have the two-lane detour built on the inside of the curve in the next two weeks.
To do so, it plans to use local fill material to level the highway.
If successful, that plan would restore connectivity between Wilson and Victor, Idaho, and resurrect a vital economic lifeline connecting both sides of Teton Pass.
To get the road open quickly, Bob Hammond, resident engineer for WYDOT’s Jackson district, designed the bypass Sunday with other engineers. It’s not a formal plan, but engineers are using their best judgment to devise a solution, Hammond said.
The slide that took out the highway early Saturday happened in an area known as the “Big Fill,” a man-made embankment created in the 1960s when Teton Pass was rebuilt in its current location. The idea was to raise the slope of the mountainside to make it uniform, and make driving the pass easier. All told, crews dropped about 70 feet of material on Teton Pass’ native soils to make that happen.
WYDOT says the failure happened in the base layer underneath the manmade embankment. The area has been a known problem for decades, with a noticeable bump in the highway. But officials said a strong burst of snowmelt and runoff caused additional instability underneath and the eventual collapse of the fill.
Building the detour on the inside of the curve should avoid the instability in the base layer, Hammond said.
“We’re not going to be at risk of that same material, and hopefully in the next couple of months that subgrade will stabilize even further,” he said. “The temporary road is off on a very stable piece of ground.”
That, at least, is the hypothesis.
WYDOT geologists are now in the field testing to confirm that the inner curve is stable.
In general, Teton Pass and other mountainous areas in Teton County are highly prone to landslides, which occur when water seeps into the soil and creates a weak layer. When enough water gets into the soil on steep slopes it can reduce friction enough for the upper layer of rock to break free and fall downhill.
Teton Pass is particularly prone to slides because it has a plethora of weak rocks — limestones, carbonates, shales and sandstones — next to each other, said James Mauch, hazards geologist for the Wyoming Geological Survey. Where those sedimentary layers contact each other, water gets in the cracks.
“That’s a discontinuity and potential weakness to be exploited,” Mauch said.
That process can happen over decades, which is what Bob Smith, a distinguished professor of geology and geophysics at the University of Utah, thinks happened at the Big Fill Slide on Teton Pass.
There is no evidence of an earthquake in the area that would have precipitated either that slide, which caused the manmade fill to crash down off the mountainside, or the mudslide at the weigh station closer to Idaho. Both slides were caused by similar, natural patterns, Smith and Mauch said: excess runoff from hot days and warm nights.
But the mudslide that covered the lower portion of the pass happened in natural material. There, water ran on top of the earth, entrained sediment and traveled toward the highway. The upper crack, meanwhile, appears to have first formed farther down in the soil. The soil that collapsed is man-made fill.
“It’s weak material in the first place,” Smith said.
That calls attention to WYDOT’s plans for building the temporary interior curve on more fill material, and its plans for a longer-term fix — rebuilding the “Big Fill” and the highway over it.
There’s “nothing wrong with rebuilding the embankment,” said Machan, the landslide expert. “But they’ve got to think about how to make that embankment stable.”
In addition, WYDOT will have to figure out how to better drain water from the outer curve and whether engineers need to replace some of the softer fill material with stronger base layers, such as boulders.
“These are the kind of questions they have to look at to best rebuild the foundation,” Machan said.
Hammond said WYDOT is already contemplating some sort of “stabilization effort” in the native material underneath the “Big Fill.” It’s not yet clear what that will look like.