WYDOT announced that Wyoming State Highway 22, Teton Pass will reopen this Friday with an interim detour around the Big Fill landslide at milepost 12.8.
JACKSON - As the Wyoming Department of Transportation aims to open the temporary Teton Pass detour next week, only three weeks after a landslide took out part of the pass, geotechnical engineers say monitoring is key.
WYDOT is planning to monitor the detour with instruments. But officials did not say by press time whether they plan to staff observers on the road around the clock, a key piece of advice from outside engineers.
"The once-daily reading, combined with the visual monitoring, would be a satisfactory way to get this detour up and running," Darren Beckstrand told the Jackson Hole Daily. Beckstrand is an engineering geologist with Landslide Technology, an Oregon-based firm that specializes in remediating large landslides.
James Dahill, WYDOT's geologist, said Thursday that the department will be installing inclinometers, which detect slope instability, and piezometers, which measure groundwater, to monitor conditions. Data from those devices will be used in the landslide model WYDOT has developed and the ongoing slope stability analysis. Dahill said WYDOT will monitor the devices daily.
In response to questions from the Daily, underpinned by recommendations from engineers, Dahill also said the department will calculate a key safety metric for the road before the public drives it.
He further explained why the department believes the earth underneath the detour is stable and said WYDOT is preparing a report outlining the detour's engineering and reviewing it internally.
In doing so, Dahill answered a number of unanswered questions about the detour.
The goal of monitoring is to understand whether the ground is moving and whether water could compromise it, said Ben Leshchinsky, a geotechnical engineer at Oregon State University who specializes in landslides.
Monitoring movement and water in real time is generally ideal, Leshchinsky said.
There are sensors that can detect earth movement and water issues as they happen, and beam that information to a smartphone. But those sensors can be expensive, and take weeks, if not months, to build.
In their stead, inclinometers and piezometers that are read daily are a good solution in emergencies, as long as they're paired with people watching the road, engineers told the Jackson Hole Daily.
The safety metric in question is called a "factor of safety," a standard calculation used to express how much stronger a system is than it needs to be for an intended load. WYDOT had not determined the road's factor of safety by Tuesday afternoon, but Dahill said officials will before people drive it.
In general, the higher the factor of safety, the safer the structure. A factor of safety of one means that a structure is designed to withstand its expected load. An FOS of 1.5 means it can withstand one-and-a-half times its expected load.
It's not simple to say what factor of safety the detour is being built to, Dahill said.
"But the FOS will certainly be over 1.0 and based on concurrently occurring activities and changing (improving) conditions could be even higher," he wrote in an email to the Jackson Hole Daily.
Stability analyses are also underway as the department classifies and tests the soil for strength, Dahill said. But there are a number of factors that give the department confidence in the underlying geology: warmer temperatures, reduced snowmelt in the mountains and the fact that the original landslide hasn't grown.
"In other words the original failure outline is isolated to a very limited area," Dahill said in his email. That indicates there are "no additional instabilities in the area," he added.
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 water. Reduced snowmelt reduces the risk of more water getting into the soil, and WYDOT's decision to remove fill from the top of the area that collapsed has reduced the driving force. Redistributing it and using it for the temporary road has also increased resisting force," according to Dahill and Bob Hammond, WYDOT's Jackson resident engineer.
George Machan is a landslide expert who consulted for the town of Jackson on the 2014 Budge Slide. He's a consultant with Landslide Technology and has worked with WYDOT on other slides in the Teton area.
Though Machan has said WYDOT should be careful about placing too much fill under the detour, which is directly adjacent to the part of the road that collapsed, he said Thursday that the department's removal of some "driving force" and decision to investigate geology below the slide and below the road is sound.
"It seems, honestly, that they're approaching it correctly," Machan said.
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