An 18-foot-tall block fell from a limestone cliff during blasting work above Arizona 77 between Winkelman and Globe. The work being done is to remove part of a cliff to keep it from falling on the highway.

Road crews blasting rock from a limestone cliff above Arizona 77 between Winkelman and Globe found themselves with a pretty big problem.

The Arizona Department of Transportation said one of the rocks dislodged early Wednesday was larger than expected — and 18-foot-tall block that dropped onto the highway from 150 feet above the road. That forced work crews to keep the road closed longer than scheduled.

The blasting work is a project to remove parts of the limestone cliff to prevent it from falling on Arizona 77.

Crews on the overnight project worked until 6 a.m. to break up the rock and push it off the highway. As a result, a detour using Arizona 77 and U.S. Highway 60 remained the only way around the work area for morning commuters.

The project requires working nights under a full closure of Arizona 77 between mileposts 154 and 161, in the Dripping Springs area. Crews are using explosives to remove potentially hazardous rocks from a cliff over the highway.

Sometimes rocks break at naturally occurring joints beneath the surface that cannot be seen or anticipated, said J.J. Liu, manager of ADOT’s Geotechnical Services division. When the charge is ignited, it opens the crack at that joint and the rock detaches itself, he said.

“The mountain is composed of the Mescal Limestone Formation with large blocks of limestone up to 155 feet above the highway, periodically falling with great energy and destructive potential,” said Brent Conner, a senior geotechnical engineer with ADOT.

The nature of the geology in the Dripping Springs area could mean more unanticipated delays before the project is complete, he added.

“The large block that fell required drilling and blasting at the roadway level to remove it from the travel lanes,” Conner said. “Last night’s work was one of the most difficult for the contractor on this project, but a couple more difficult sections remain.”