As Teton Pass opening nears, WYDOT lays out safety steps

WYDOT has committed to most of what outside experts recommend, but not all

By Billy Arnold, Jackson Hole News&Guide via Wyoming News Exchange
Posted 6/27/24

JACKSON — The Wyoming Department of Transportation plans to reopen Teton Pass by the end of this week, less than three weeks after a June 8 landslide swept away 200 feet of Highway 22.

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As Teton Pass opening nears, WYDOT lays out safety steps

WYDOT has committed to most of what outside experts recommend, but not all


JACKSON — The Wyoming Department of Transportation plans to reopen Teton Pass by the end of this week, less than three weeks after a June 8 landslide swept away 200 feet of Highway 22.

Ahead of the highway’s reopening, a key question has been whether WYDOT would finish studying soils under a temporary bypass that’s next to the area that originally failed.

Preliminary results have indicated the earth is stable, engineers have said. But on Tuesday, Director Darin Westby, geologist James Dahill and Resident Engineer Bob Hammond said for the first time that WYDOT plans to complete the stability analysis before the road opens, which could happen as early as Friday.

“Absolutely,” Westby said. “I want one last double-check to make sure everything went the way it should.”

The News&Guide caught Westby at the tail end of a tour WYDOT organized for media and officials from the east and west side of the pass. As Westby, Hammond, Dahill and a representative from the governor’s office spoke, media formed a tight scrum around the speakers, and legislators, town councilors and commissioners watched, as state officials praised how quickly WYDOT has moved to fix the pass.

In the background, machinery rumbled uphill and down, paving the temporary bypass. In an interview after official speeches, Hammond said WYDOT is aiming to open the temporary road 24/7, with the only major restriction being a reduction in the speed limit so drivers can navigate the 11.2% incline.

A permanent fix is expected this fall.

“We’re hoping to have the new road in place by mid-November. That’s our goal,” Hammond told the News&Guide and other reporters. “But really, we’re in the very initial process.”

In the past two and a half weeks, WYDOT has scrambled to build the 611-foot-long bypass, mustering a team of contractors to work around the clock in the busy summer season and moving some 30,000 cubic feet of dirt to build a new road alignment on the inside of the curve that failed. The department’s speed has raised questions about whether state engineers were sacrificing safety for efficiency as they worked to reopen the critical lifeline connecting Jackson Hole with workers and community members in Teton Valley, Idaho.

In response to questions, WYDOT officials said that they would be completing the soil analysis before opening, and that state engineers plan to install a device in the road that will send real-time safety information to their phones, including information about water saturation and soil movement.

Both are steps that outside landslide experts and geotechnical engineers have recommended.

WYDOT has not, however, done other things those experts have advised, like publish a plan for the bypass. While officials have said they’ll have crews monitoring the road for signs of instability, they haven’t yet committed to putting a human observer on the pass 24/7 before the around-the-clock system is installed.

Still, some specialists say they believe WYDOT is on the right track with the temporary diversion.

“It sure sounds like that to me,” said George Machan, a landslide consultant who advised the town of Jackson on the 2014 Budge Slide as well as WYDOT on past landslide work on Togwotee Pass.

Soils stabilizing

The timing of the stability analysis has been a key question because WYDOT is building the temporary bypass on the inside of the highway curve known as the “Big Fill,” which failed June 8. That has raised questions about why engineers and geologists believe earth next to the area that failed won’t fail again.

Four factors cause a landslide: a substantial “driving force” on top of a slope, a lack of resisting force at the bottom of the slope, a weak layer in or beneath the slope and — critically — water.

The part of the pass that failed on June 8 is known as the “Big Fill,” a 70-foot-high embankment that highway engineers built in the 1960s to make the modern Teton Pass highway a manageable 10% grade for cars.

But, over the years, water got into the fill and, for the last decade or two, caused instability. Engineers put an inclinometer, a device that measures slope movement, in the hill last fall. The fill did not, however, move substantially until a few days before the collapse, officials say. Geologists blame a stout runoff season caused by a combination of hot days and warm nights that pushed groundwater through an underlying layer of natural clay.

The clay also exists a few hundred feet south, where the temporary bypass is being constructed. However, even with that layer present, and without a full stability analysis, Dahill, WYDOT’s geologist, said a number of factors make the area under the bypass more stable than what failed under the Big Fill.

For one, runoff season is ending, there’s less water moving through the mountains, and the landslide that took out the highway hasn’t moved “one iota” since June 8, he said.

The area where the bypass is being built has also not already experienced the same stress as the area under the Big Fill. Until two weeks ago, it was all native soil, without a manmade load on top.

“The soils on this detour are in drier condition, better condition, and have not experienced that loading that the larger bank had for the last 55 years,” Dahill said on top of Teton Pass on Tuesday.

The stability analysis will, nonetheless, confirm whether Dahill’s preliminary assessment is correct.

‘A factor of safety’

A stability analysis will answer some key questions about the area’s underlying geology — but not all. In geology there’s always a margin of error. There will still be outstanding questions about soil and groundwater after the analysis is done, said Machan, the landslide consultant.

“It calculates a factor of safety, but there’s a lot of unknowns,” he said.

A “factor of safety” is a technical figure that approximates how much stronger a system is than it needs to be for its intended load. Said another way, a “factor of safety” estimates how much of a margin engineers have if an oversized truck drives over a road, or an earthquake hits.

The higher the factor, the safer the structure. A factor of safety that’s less than 1 means a structure is not stable. A factor of safety of 1 means a structure is designed to withstand exactly its expected load. A factor of safety of 1.5, meanwhile, means the structure can withstand one and a half times that load.

Dahill has said WYDOT is aiming to build the temporary fix to a factor of safety of 1, if not higher.

A factor of safety of 1 for the bypass may be a bit low, according to 2008 guidelines published in a report from the Transportation Research Board’s National Cooperative Highway Research Program.

Typically, the report says, slopes are deemed safe if the factor of safety is between 1.1 and 1.3.

But states have their own standards, said Ben Leshchinsky, a geotechnical engineer and landslide expert at Oregon State University. Reaching a higher factor of safety can be difficult on certain slopes.

If that’s the case, Leshchinsky said, “other measures need to happen and they always include monitoring.”

Keeping an eye open

After questions from the News&Guide last week, WYDOT officials said Tuesday that they’re investing in tech that will allow them to monitor the bypass 24/7. The department plans to install a $50,000 piece of technology called a “constant array” to take readings on groundwater and soil movement in real time.

“We will be doing constant monitoring that’s 24/7, every minute of the day,” Dahill said.

They also plan to use the device to establish numerical thresholds for movement and water issues. If the bypass moves too much, WYDOT will get an alert and conduct a “thorough evaluation,” Dahill said.

Whether the pass will remain open or closed in the event that threshold is met will be decided in the field.

“When incidents like that occur, we gather the information, we look at the depth and we make an evaluation,” Dahill said. “So yes, it’ll be a concerted effort by many others to open or close.”

Landslide experts said establishing thresholds for movement in advance will help WYDOT make decisions in the field and have guidelines for deciding when they need to increase caution on the bypass.

Some settlement is expected when a new road is built. What’s atypical is if one part of the road moves and another part doesn’t. It’s also atypical to see movement accelerate. Both are signs of a slide.

But measuring how far the fill moves may not be sufficient on its own.

“It’s not only movement. But it’s also the type of movement,” Leshchinsky said. “Does the movement reflect downslope movement or a landslide?”

It’s not clear whether the array WYDOT ordered will arrive before the pass reopens.

Without that technology, experts have said WYDOT can get away with less: Daily readings of a piezometer and inclinometer, devices that measure groundwater and movement, respectively, as long as they’re complemented with human observation. Specialists have said that monitoring should be in “real time.”

Without the 24/7 technology, WYDOT will take readings from the groundwater and movement devices “morning, noon and night,” Dahill said, which is more than officials said last week: Once daily.

Still, Dahill wasn’t able to say whether there would be a human presence on the pass 24/7.

Critics of the department’s initial response have asked why the pass remained open after the crack that became the landslide was first discovered, and why people tasked with monitoring it left overnight.

“I can’t speak for the local crew,” Dahill said Tuesday. “But I’m assuming there will be a maintenance person driving across these paths every day — morning, day and night — as well.”

Hammond, WYDOT’s resident engineer, didn’t say for certain whether watchers would be on 24/7. Any movement, he said, isn’t expected to be instantaneous. Typically, landslides take a few weeks to happen.

But, he said, “we can do whatever we have to to make sure people feel safe up here.”

Mudslide work finishing up

As Wyoming Department of Transportation crews work on the bypass around the Big Fill slide, they have also been working on cleaning up a mudslide further down the Idaho side of a pass, and installing a box culvert to allow water and future mudslides to run under the road.

That work is finishing up.

Bob Hammond, WYDOT’s resident engineer for Jackson, said the road over the culvert will have two lanes open when the upper bypass opens.