Massive boulder on State Route 77 caused morning delays between Winkelman and Globe

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Massive boulder on State Route 77 caused morning delays between Winkelman and Globe

State Route 77 was closed between Winkelman and Globe longer than scheduled Wednesday morning for rock blasting operations after an 18-foot tall boulder dropped onto the highway from about 150 feet above.

Crews worked until nearly 6 a.m. – four hours after the planned reopening of the highway – to break the boulder into smaller rocks that could be pushed off the highway. As a result, a detour using State Route 177 and US 60 remained the only way around the work area as area residents began their morning commute.

Crews are working nights under a full closure of SR 77 between mileposts 154 and 161, using explosives to remove potentially hazardous rocks from a cliff that hangs over the highway.

ADOT strives to provide timely information to assist the motoring public in planning for restrictions, closures and detours, but sometimes the unexpected happens and announced schedules are not met.

This is particularly true with geotechnical operations such as the work on SR 77 in the Dripping Springs area.

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 it detaches itself,” he said.

SR 77 Boulder

Having an unanticipated boulder break free increases the time needed to remove the rock and repair damage to the highway, particularly with some extremely dense types of rock that are common in Arizona.

“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.

Removal of these large blocks of limestone before they come down on their own is the reason for the blasting work, he said. 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.”