A rockfall slowed traffic Friday 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 to insert dynamite and blast the boulder into smaller fragments for removal.
Traffic control personnel were at the site Friday east of County Road 40 to provide traffic guidance with one alternating lane of traffic, according to a news release from CDOT.
Travelers were urged to slow down and watch for workers and equipment. Traffic delays may be possible through the weekend. No oversized loads are being permitted.
Todd Jones with CDOT said crews are drilling 25 holes with a rock hammer where dynamite sticks will be placed. Twenty of the holes are 6 feet deep, and five are 4 feet deep.
“It is going well. The rock is really hard, but our drill is getting through,” he said. “It is harder than Memorial Rock.”
An estimated 25 pounds of dynamite will be used to blow the boulder apart. Then the rubble will be hauled away.
The highway will be closed in both directions while the boulder is blown apart and cleared, said Lisa Schwantes, CDOT communications manager. Traffic will be stopped a safe distance away.
Officials said they hoped to clear the boulder by Friday, and dynamite was on the way from Ridgway. If not, then a traffic signal will be installed at the site to allow safe alternating traffic around the boulder. Work will resume over the weekend if necessary.
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, Schwantes said.
Damage to the highway will not be known until the boulder is broken apart. CDOT officials said they have asphalt ready to patch 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 contributed to the section of sandstone cliff band to break loose and slide onto the highway. The boulder broke off from an overhanging sandstone cliff band 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 15 people from CDOT were at the scene, plus engineers from a geohazard consultant firm.
No injuries were reported.
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 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.