JERRY McBRIDE/Durango Herald
RED MOUNTAIN PASS – Two workers tethered to ropes used pry bars and their hands to pull rocks away from a steep mountainside Friday on Red Mountain Pass, which has been closed for six days because of a massive rockslide.
The pass will remain closed through the weekend. No estimate for reopening the pass has been given.
“We do know our work5, and the closure will continue into next week,” said Nancy Shanks, spokeswoman for the Colorado Department of Transportation.
A shower of rocks came tumbling off the mountainside about every 20 minutes Friday as workers sent boulders crashing down from 900 feet above U.S. Highway 550. Each cascade sounded like a hard rainstorm or hailstorm that lasted 30 to 60 seconds.
The slide occurred Sunday night when a giant slab – about the size of a football field, 25 feet thick – came loose and crumbled into bits during the long fall. It covered about 200 feet of the highway, up to 10 feet deep in some spots, said Paul Wilson, maintenance area supervisor with CDOT.
Road crews cleared about 1,000 cubic yards of debris Sunday night and reopened the pass. But when the morning sun hit the south-facing slope, the rocks started coming down again.
Two drivers reported broken windshields and at least one had a flat tire. Rock fall has not caused any injuries.
The slab likely came loose as a result of the freeze-thaw process, in which water freezes between the rocks at night and thaws in the morning. When the water freezes, it expands, slowly eroding the mountainside.
In geologic terms, it is a rather insignificant event, Wilson said. Red Mountain Pass closes briefly at least once a year, he said. What makes this one different is the amount of material that came down and that it broke into so many pieces, he said.
The debris flow is clinging to an 85-degree to 90-degree slope, he said.
“Normally, it will run, and we’ll clean it up, and we’ll be good until the next episode,” Wilson said. “It just keeps trickling down, and, of course, now it’s more because those guys are scaling on it.”
CDOT is aware of 760 rockfall sites across the state. Each one is evaluated and rated based on slope, moisture trends, temperature fluctuations and fracturing potential, said Ty Ortiz, a CDOT rockfall specialist.
They can be mitigated with metal nets, rock bolts, fences, and, in some cases, “rock gluing” – spraying a mixture on the loose rock to cement it into place, he said.
But none of those mitigation measures are likely on the Red Mountain slide, because it covers a huge area, and the road is too narrow for fences.
Blasting is another method of mitigating a rockslide, but CDOT is not blasting at the site because it could trigger new slides. Blasting also would not jar loose all of the rocks resting precariously above the highway.
That is why workers are prying loose rocks away by hand. CDOT may install displacement instruments along the rockslide to monitor if it move in the future, Ortiz said.
While the rock scalers can pull down material that poses an imminent threat, they can’t come close to mimicking what the next major rainfall will do in the way of loosening more debris, he said.
A helicopter is being used to ferry workers to and from the site, and to drop off tools.
It is unusual for rockslides to close a highway for this long, he said. The last one he could recall was in 2010 when huge boulders smashed through a bridge on Interstate 70 near Glenwood Springs, which closed the highway for about four days, he said.
It also is unusual for rockslides to occur during the middle of the winter. But the days have been unseasonably warm along Red Mountain Pass, which has contributed to the freeze-thaw cycle.
A CDOT official said Wednesday’s high temperature reached 56 degrees.
The highway is closed between mile markers 87 and 92, or near Ironton Park, and just south of Ouray. The slide occurred just north of the turnoff to Engineer Mountain Trail in an area known as Ruby Walls, a curvy section of road just south of Bear Creek Falls, two miles south of Ouray.
Drivers going from Durango to Ridgway (10 miles north of Ouray) must go over Lizard Head Pass via Mancos, Dolores, Rico, Placerville and Ridgway.
An average of 2,200 vehicles travel the pass daily. The pass likely would have experienced higher-than-average traffic volume this weekend because it is a three-day weekend for some, with the Martin Luther King Jr. holiday.
CDOT has installed signs throughout the region warning drivers of the closure.
But at least one driver disregarded the closure signs Thursday.
A man driving a Ford Ranger with a camper shell and Utah license plates broke a padlock and lifted a closure gate at the southern end, Wilson said. He reached the rockfall site where CDOT workers turned him around.
He acted like he didn’t understand the road was closed, Wilson said. A deputy with the San Juan County Sheriff’s Office stopped him, and a trooper with the Colorado State Patrol issued him a ticket, Wilson said.
“It’s been quite a battle with the traveling public,” he said.
An earlier version of this story misstated the name of Ruby Walls.