Glenwood Canyon’s geology: A recipe for rockslides
Joe Elsen still remembers the moment when, drilling to set caissons near the future Hanging Lake Tunnel in the late 1980s, he first encountered the Glenwood Canyon’s “Gray Layer.”
The muddy subsurface was not only an engineering challenge, but its very existence portends an instability that has since resulted in closures and even loss of life. The Gray Layer was formed thousands of years before anyone was around to imagine building a road through the Rockies. A massive rockslide dammed the river, allowing sediment and ash from an eruption near Dotsero to fill the bottom of the resulting lake.
“It all points back to the same thing,” said Elsen, a retired Colorado Department of Transportation engineer. “Every material has its own angle of repose, and it really won’t be happy until it’s at that.”
That means that, generally speaking, straight up and down isn’t terribly stable.
As construction began on Interstate 70 through the canyon, Elsen was among those in charge of removing the most dangerous features.
“There was a fair amount of planning and mitigation,” he said. “I don’t think you can get them all, but I think at the time it was as good as it could get.”
After the I-70 canyon ribbon cutting, Elsen spent some time away before returning to the area as program engineer in 2002. He dealt with major closures caused by rockfall in 2004 and 2010. Although he no longer works for CDOT, he was called in to take a look at the damage after the most recent slide, which caused the longest full closure to date of the stretch of road. The damage, he said, seemed comparable or even less than past events, but harder to route around.
“Some of it’s luck,” he said. “Back then we were able to open traffic under reduced capacity sooner.”
Another significant difference is the location of the slide. Instead of shearing off a cliff face, the initial rock appears to have dislodged from an existing slope of rubble. With rapidly melting snow rendering the pile unstable, it didn’t take much to start a chain reaction.
“Once you get a rock to fall, it has enough energy to knock other rocks loose,” said Cameron Lobato of GeoStabilization International, a private company brought in to help remove unstable rocks after the Feb. 15 slide.
The result was a slope full of 6- to 8-foot diameter boulders that had to be scaled and stabilized by hand.
“CDOT’s internal rockfall and geohazard team is one of the best in North America,” Lobato said. “It’s a lot more stable than when we got there.”
As to what makes Glenwood Canyon so prone to rockfall, Colorado Mountain College associate geology professor Joe Reinig has a theory.
“Where the main rockfalls happen is where you have the most granite exposed,” he said. That granite, along with gneiss and schist (types of metamorphic rock), makes up the bulk of the cliffs through the center of the canyon. It’s truly ancient stone — about a billion years older than the rock on top of it, which itself predates the dinosaurs.
“The fact that it’s been exposed for 65 million years and was old to begin with certainly makes it less durable,” Reinig said.
The top layers, meanwhile, are mostly dolomite and limestone. Products of an ancient shallow sea, they’re quite permeable to rain and snowmelt, which are most common in the shoulder seasons, but are unpredictable. The canyon’s major I-70 closures have come in February, March, June and November.
“When the water filters down and hits that granite, it doesn’t soak up the water at all, so it tends to flow out over the surface,” Reinig said. “It makes that whole slope less stable.”
While Interstate 70 is a priority corridor for CDOT, communications manager Amy Ford stressed that it’s just one of many.
“We have 750 rockfall areas that we monitor statewide,” she said. “Our challenge is that we’re talking about hundreds of miles of canyon and rock face to mitigate.”
The agency’s $9 million geohazards budget is allocated based on risk, with about two-thirds going to risk reduction including permanent rockfall mitigation.
Some of that goes to Glenwood Canyon, but there are plenty of other problem areas like “Million Dollar Highway,” U.S. 550 near Durango, where slides also occurred last week.
“Colorado is one of the leading states when it comes to rockfall. It’s an incredibly expensive endeavor,” Ford said. “We do our very best to meet all of those needs. We don’t have the funds to be able to mitigate everything. The likelihood of that changing anytime would require a vote of the people.”
In the meantime, CDOT is working to get emergency relief from the federal government to help with repairs from the current slide.
“Safety on our roadways is our number one priority,” Ford said.
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