A rockfall has slowed traffic on Colorado Highway 145 between Dolores and Rico, east of the small community of Stoner.
The boulder landed in the southbound lane at mile marker 27, not far from the site of a massive boulder slide in 2019.
Colorado Department of Transportation crews were drilling holes in the boulder all day Friday to insert dynamite and blast the boulder into fragments for removal.
It was blown apart about 5 p.m. Traffic was stopped in both directions for about 20 minutes for blasting and clearing. The highway was then reopened for travel.
Traffic control personnel were on site Friday east of County Road 40 to provide traffic guidance with one alternating lane of traffic, according to CDOT. A portable traffic signal has been installed at the site to guide traffic through the weekend.
“We wanted to get most of the blasting done before the weekend ski traffic,” said Lisa Schwantes, CDOT Region 5 communications manager. “Drivers are asked to give themselves some extra time.”
There will be minor traffic delays at the rockfall site through the weekend. No oversized loads are being permitted.
Travelers were urged to slow down and watch for workers and equipment.
Todd Jones, of CDOT, explained they drilled 25 holes with a rock drill where dynamite sticks would be placed. Twenty of the holes were 6 feet deep, and five were 4 feet deep.
“It is going well. The rock is really hard, but our drill is getting through,” he said. “It’s harder than Memorial Rock.”
Sixteen pounds of dynamite was used to blow the boulder apart. The blast sent a plume of red dust high into the air, and scattered debris across the highway.
A large piece of the boulder remains, and crews will return Monday to blast it apart. The rubble will be hauled away.
When the remaining piece is blown up Monday, traffic will be stopped a safe distance away.
Larger pieces may need to broken up by a “boulder buster,” Jones said. The portable unit is placed over a drill hole filled with water. A small charge is set off, and the pressure of the charge and water breaks apart the rock without debris flying.
CDOT geohazard crews will survey the area to determine whether rockfall mitigation work is needed, said Schwantes.
Damage to the highway will not be known until the rest of the boulder broken apart and debri removed. CDOT officials said they have asphalt ready to repair the road.
CDOT program engineer Kevin Curry said an initial survey of the hillside above the rockfall did not reveal other rocks or boulders that were in imminent danger of falling.
He said the combination of erosion, moisture and freeze-thaw conditions likely caused the section of a sandstone cliff to break loose and slide onto the highway. The boulder broke off from an overhanging section about 30 feet above the highway. It did not tumble.
Warm daytime weather melted snowpack in the area, and nighttime temperatures have been below freezing. The melting and freezing of water in the rock expands fractures, which triggered a piece of the cliff to calve off.
The sandstone boulder is estimated to weigh 200 tons, CDOT officials said. It is 11 feet tall and 30-by-16-feet wide.
The rockfall was reported by the Colorado State Patrol at 6:30 a.m. Friday. About 20 people from CDOT were on the scene, plus engineers from a geohazard consultant firm.
No injuries were reported.
In 2019, CDOT conducted mitigation work on a stretch of the highway from mile marker 14 to mile marker 29 in an effort to lower the risk of rockfalls.
The construction work came after two large boulders fell onto the highway, which runs along the Dolores River, in May 2019.
On May 24, 2019, two house-size boulders fell 1,000 feet onto the highway at milepost 21.
The highway was closed for a few days at mile marker 21 while crews blasted apart the 2.5-million-pound boulder that cratered directly onto the road. Nine weeks later, the highway had been repaired, at a cost of $1.12 million.
The second boulder, weighing in at 8.3 million pounds, plowed through the asphalt, creating an 8-foot-deep trench and stopping on the highway shoulder.
It was left in place, and Colorado Gov. Jared Polis christened it Memorial Rock in honor of military service. The incident occurred on the Friday of Memorial Day weekend.