The road is closed from U.S. Highway 191, near Helper in the east, to U.S. Highway 89, near Thistle Junction in the west.
UHP spokesman Jeff Nigbur said the highway might not be open until 11 p.m.
The slide occurred between 2:30 and 3 p.m., leaving mud, rock and other debris in the road, Nigbur said.
"It came down and pushed through the concrete barriers like dominoes," he said.
The Utah Department of Transportation has been doing road construction, creating a 20-foot dirt embankment along the highway. When a deluge of rain hit the area, part of the embankment slid onto the highway.
The slide is affecting only two lanes of traffic, but officials had to close the entire section of the highway in order to maneuver loaders in and out of the area and clean up the road.
Daniel Boone of West Jordan was headed toward Scofield to a Boy Scout camp when he was stopped by the UHP roadblock near Thistle. At 4:30 p.m., he had been waiting for about 2 hours, and he said he would continue to wait until the road opened.
Effie Openshaw, of Hooper, also ran into the Thistle road block. She was heading to Price for a wedding, but was determined to find an alternate way through Fairview.
Motorists are being advised to use alternate routes, including Interstate 70, which is about 80 miles south, and U.S. Highway 40, which is about 50 miles to the north.
One driver put his faith in his four-wheel drive, maneuvering his pickup truck alongside the Union Pacific railroad tracks to drive past the slide. He did well for about a half mile, then the truck got stuck.
"We decided to do what we could to get through," said the driver, who refused to give his name. "We probably have been digging out for about two hours."
Ultimately, a tow truck and a Union Pacific maintenance vehicle were needed to pull the truck off the tracks.
The Utah Highway Patrol also reported a smaller mudslide on State Road 191 about 4:15 p.m.
"It didn't seem like it was real big," said UHP spokesman Cameron Roden.
The debris was removed by 6 p.m. and traffic lanes were reopen.
- Tribune reporter Jason Bergreen contributed to this story.
