DENVER — Spring means more warmth, more sunshine, and more non-frozen precipitation. It also means more thawing. And across Colorado, that results in more rockfall.
Each year on average, the Colorado Department of Transportation's Geohazards Team responds to 40 to 50 rockfalls that require additional investigation or response.
CDOT monitors more than 750 locations recognized as having chronic rockfall hazards.
"Rockfall can happen anytime on roadways around the state," explained Elise Thatcher with CDOT. "We have literally hundreds of locations where rockfall has been identified to be a potential risk at times. The spring is an especially common time for rockfall. This is because of what's called the freeze-thaw cycle."
This cycle's name speaks for itself — it happens at times of year when temperatures warm during the day, freeze overnight, and then warm the following day. Moisture that made its way into the cracks in rocks above the roadway expands and contracts with the changing temperatures, which can eventually break rocks from the face, Thatcher said.
The freeze-thaw pattern is most often seen in the spring and fall.
Winters with heavy snow totals, especially in places with a lot more rock faces, mean the ground loosens up even more than normal and the moisture in the rock's cracks creates more pressure toward separation.
Thatcher said CDOT is closely monitoring the roadways for these kinds of incidents as spring kicks off.
"We have maintenance employees that are literally dedicated to different stretches of highway around the state to make sure we have eyes on what's going on, whether it's traffic impacts, whether it's potholes, whether it's rockfall," she said. "So, we're always eyes on the roadways."
At the same time, drivers can keep themselves safe by staying extra alert on the roads.
"Especially in the spring, keep an eye out for falling rocks or rocks that are on the roadway," Thatcher said. "If you see rockfall, slow down. Make sure you're not speeding already."
She called it "not wise" to try to drive over any rockfall in the roadway, even if it's minor. Instead, carefully change lanes, she said.
CDOT often places road signs in places where rockfall is most likely to happen.
Thatcher explained that some of the most common areas for rockfall in Colorado are De Beque Canyon near Grand Junction, Glenwood Canyon near Glenwood Springs, and Dowd Canyon (or Dowd Junction) near Vail Pass.
The most extensive rockfall damage CDOT has seen was during the floods and mudslides of 2021 in Glenwood Canyon, she said, adding that those incidents included not only rock but moisture. This led to significant damage to Interstate 70 that resulted in temporary and then long-term repairs.
While this was an extreme case, even more minor rockfall incidents can cause damage to the roads. CDOT reported that in a typical year, it will spend about $6 million on rockfall-related activities, which includes not only emergency responses affecting transportation but also proactive mitigation efforts.
CDOT has several rockfall protections in place to prevent any injuries to life, property or the road.
Some are obvious to any onlooker, like fencing or netting. These methods are typically used when CDOT crews cannot keep rock attached to the rock face, but expect pieces to fall in the future. When it does happen, the fences or nets catch the debris and crews regularly clear it out.
In other instances, CDOT can drill into the rock to physically keep it together.
"Now, we can only do this in so many locations, as you can imagine, because it takes a lot of equipment. It takes a lot of manpower," Thatcher said. "And we can do that only so often. But in places where we've got identified funding and it's the right kind of work for that rockface — we do pursue that."
In addition to these prevention measures, CDOT also forcefully breaks rocks that could potentially pose a hazard at an unknown time. This often require full closures and explosives.
In September 2020, CDOT shared the below video sharing what this work looks like from a bird's eye view.
CDOT's maintenance crews also regularly report rockfall so its geohazards team can learn where it is becoming more common and track that area's history.
"Due to the incredibly mountainous nature of Colorado, we can't cover every rock face," Thatcher continued. "But in areas where we've identified that it's more likely for that to come down and where geographically it makes sense for us to be able to attach those materials that I talked about — rockfall fencing and netting or even drilling into the rock face — we do pursue those options."
Here's a look back at some recent rockfall events in Colorado:
- In March 2019, rockfall at Dowd Junction forced the closure of Interstate 70. A rock pierced the hood of one car. Luckily, there were no injuries.
- In September 2019, Highway 119 in Boulder Canyon was closed after 8,000 cubic yards of rock fell onto the road during normal blasting for a flood control project. Normally, these blasts bring down about 400 cubic yards of rock (for reference, a cubic yard is about the size of a washing machine).
- In February 2020, both lanes of Highway 6 through Clear Creek Canyon were closed after rockfall damaged fencing along the highway.
It's also important to remember that rockfall happens in open spaces and the backcountry too:
- Two women helping a climbing group were injured in a rockfall in Clear Creek Canyon in June 2021.
- Two people were rescued and one man died after a rockall and avalanche in Rocky Mountain National Park in May 2022.
- Four climbers had to flee for their lives from a rockslide in Rocky Mountain National Park in late June 2022. Nobody was injured.
