New wall on Vail Pass is an example of the technological advancements being made in mountain development
By holding back the infamous Miller Creek slide, crews were able to realign the road in a way that could not be done during its original construction in the 1970s
CDOT/Courtesy image
A massive new retaining wall cuts into the mountainside on Vail Pass, allowing more room for a curve on the interstate to be made wider and safer.
But the 40-foot tall, 1,100-foot-long wall doesn’t just allow for the interstate alignment to be modified, it also holds back a historical landslide path and solves a problem geotechnical engineers first encountered five decades ago in charting Interstate-70s course through Colorado.
At the time, the idea to put an interstate over Vail Pass was a history-in-the-making concept due to the fact that it would be the first project in Colorado subjected to the National Environmental Policy Act review process. Colorado prepared its first Environmental Impact Statement for the Vail Pass project in 1972.
“This was the first time that the state of Colorado looked at trying to make a road lay lightly on the landscape,” said John Kronholm, a resident engineer with CDOT.
Doing so would require years of study. From 1971 to 1977, CDOT spent approximately $2 million on geotechnical studies, hiring geologists and soil engineers to help construction engineers select the optimum location for the roadway. Approximately 30 person-years of geotechnical expertise and 10 drill-crew years were expended in conducting the geotechnical investigations.
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Upon completion in 1978, the National Academy of Sciences published a report titled “Engineering Solutions to Environmental Constraints: I-70 Over Vail Pass,” which provided an overview of the final geotechnical investigations on the 14-mile Vail Pass I-70 alignment.
Forty years later, geotechnical engineering consultant Paul Macklin found himself reviewing that report. CDOT was looking to revamp Vail Pass, identifying safety improvements that could be made, and a modification of the curve in the Miller Creek landslide area near mile marker 167 was included in the suggested improvements.
Macklin cited the 1979 report in saying the most difficult and challenging problem for the original team of geotechnicians was the Miller Creek landslide area.
“In fact, it scared them so bad they crossed Black Gore Creek and moved the alignment onto the southern side of the creek,” Macklin said.
But before they did, engineers determined that lowering the water table in the area could help contain the potential for landslides in Miller Creek area north of Black Gore Creek.
“Water is usually the culprit in a landslide,” Macklin said. “So if you can do something about access to water in these soils, then you have a good opportunity for stabilizing.”
CDOT, in the 1970s, determined that the water in the Miller Creek area soils could be reduced by lowering the water table via a series of holes drilled horizontally to allow the water to drain. In the spring of 1974, an attempt was made to bulldoze an access road into the Miller Creek Slide area so drilling equipment could be brought in, but “because the area was saturated and not frozen, the dozer became helplessly mired,” according to the report. “A second road was successfully built about 6 m (20 ft) higher.”
In the end, the 1970s-era geotechnical engineers were able to successfully lower the water table in the area with the horizontal drainage holes and build a retaining wall on the slope, but it wouldn’t accommodate a wide-turn alignment of the highway, so the alignment was moved farther south and a narrower, more dangerous turn was used. The curve wasn’t ideal, but the geotechnical engineers at the time didn’t think the Miller Slide would allow an alignment that was.
“They didn’t think it would work,” Macklin said.
Fast forward 40 years to the late 2010s: CDOT was getting serious about revamping portions of Vail Pass, and an environmental assessment and conceptual design was undertaken on an idea known as the I-70 West Vail Pass Auxiliary Lanes Project. A design for $200 million in safety improvements on West Vail Pass was presented, with a new lane being the most significant change, as the project title suggests. But curve modifications were also suggested, including a new alignment of the highway in the Miller Slide area.
The project is being managed by Kronholm, who hired Macklin as a geotechnical engineering consultant. Macklin said he immediately noticed that the new alignment in the Miller Slide area resembled the original alignment that was planned and abandoned in the 1970s.
Macklin was confident that CDOT, using new technology, could build something close to the original alignment suggested in the 1970s, expanding on the work that was already done to lower the water table in the area. He revisited the 1979 report, but wasn’t able to find the information he was looking for.
“We don’t know anything about the horizontal drains, unfortunately,” Macklin said. “Whatever records they had back then, they probably exist somewhere but they’re lost, they’re not readily available. I haven’t been able to find any record of how many there are, how wide they are, or where they’re located.”
But Macklin knew those original drains had worked, lowering the water table 35-50 feet in the Miller Creek area.
“They were pretty effective,” he said. “So we kind of piggybacked on that idea.”
Concerned that the new work could interfere with the existing drains, Macklin reached out to Bob Barrett, the author of the 1979 report, for more information.
“He’s still around in the Grand Junction area,” Macklin said. “So I called him and asked him what happened to all the designs for the horizontal drains.”
Macklin didn’t exactly get what he was looking for from Barrett, but eventually he was able to locate some of the outlets for the drains, and recommended the installation of 29 new drains to lower the water table another 30 feet or so.
The soil was monitored through the use of ShapeArray instruments that detect movement in the earth and send information to a cloud-based database. Access to modern tools like ShapeArrays is one of the reasons why Macklin says, with a level of confidence his predecessors in the 1970s could not attain, that the geotechnical engineers of today will be successful in stabilizing the Miller Creek landslide area.
“CDOT used inclinometers on Vail Pass back in the day — it had to have been the first use of that technology in Colorado, because they just weren’t available before then — but you had to go measure them manually,” he said. “They might measure them every hour, but now it’s electronic, and we can measure every minute if we want to. That technology just wasn’t available back then.”
Water-level monitoring has advanced considerably, as well.
“Water-level measuring has been around for a while, but once again, it was measured manually back in the day, whereas now it’s electronic,” Macklin said. “So they’re able to pick up a surge in the spring, during a melt event, and we’re able to see what’s going on throughout the year, when it drops off.”
With water table lowered and the monitoring in place, Macklin could confidently say the threat of landslide had been reduced to the point where a large retaining wall could be constructed in the Miller Slide area. It was completed during the fall of 2023.
Kronholm said engineers used both science and art in constructing the wall, along with a dose of modern technology.
“It’s a hand-sculpted, soil nail, shotcrete wall — colored concrete sprayed onto a wire netting, and the wire netting is attached to 30 to 50 foot long nails that are drilled into the side of the hill,” Kronholm said. “The wall itself is around 14 inches of this sprayed on shotcrete; we only needed around 10 inches of the shotcrete to hold the wall up, and then this extra four inches is the part of the facia that’s hand sculpted and crafted to look like a natural rock outcrop.”
The Vail Pass project was a test project for this new type of wall, with the technique first used in the project’s very first safety improvement, a runaway truck ramp.
“It came out pretty good using the same construction technique,” Kronholm said.
The design is now catching on. In addition to the Vail Pass wall, CDOT is using the same wall construction technique on the I-70 Floyd Hill project between Evergreen and Idaho Springs, said Project Director Kurt Kionka.
“Several options were presented to I-70 Floyd Hill stakeholders during the I-70 Mountain Corridor Context Sensitive Solutions process that meet the I-70 Mountain Corridor Aesthetic guidelines,” Kionka said. “This wall has been implemented in several locations along I-70 including Vail Pass. The sculpted shotcrete was chosen not only due to the aesthetic look, but also because it provides a cost savings from a typical concrete wall.”
