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Rock fall damages several buildings in Rockville

ROCKVILLE, Washington County — A large rock fall occurred in Rockville Wednesday, damaging several buildings, the Utah Geological Survey reported.

There were no injuries in the rock fall, which geologists say could have been much worse. Concern also centers on the possibility of a second boulder breaking loose and causing similar damage to the small community located at the entrance of Zion National Park.

About 7:30 a.m., a boulder estimated to be 35 feet by 30 feet by 30 feet broke loose and rolled down a hill toward a home rented by Tamara Burton.

Tyler Knudsen and Bob Blackett, UGS geologists, were asked by Rockville Mayor Alan Brown to investigate. According to air photos, the boulder had been there for at least four years after detaching from a ledge and sliding about 20 feet and coming to a rest on a 45-degree slope above Burton's home.

"Yesterday, following several days of rain, the boulder appeared to have slid about 10 feet and then fell over a ledge and began to roll," Knudsen said.

The boulder rolled down a shallow ravine and struck another large, partially embedded rock-fall boulder at the slope's base. The impact shattered the falling boulder into smaller boulders, shards and dust.

The debris severely damaged or destroyed many of the nearby outbuildings, including a chicken coop and a barn-type structure. A single 2-foot-by-4-foot rock shard slammed through the entryway of Burton's home, damaging the door and leaving a hole in the wall.

Debris from the impact also damaged two cars. The farthest boulder found from the impact area was about 180 feet away.

"If there had not been a large embedded boulder at the slope's base to break up the falling rock, the outcome surely would have been much worse," Knudsen said.

Concern now focuses on a second similarly sized and similarly positioned boulder, which lies about 80 feet away from where the rock fall originated, and it appears to be analogous to the boulder that failed in every way, the survey said. It is already detached from its source ledge and sitting very precariously on a 45-degree slope of weak, easily erodible siltstone, according to the survey.

Inspection of the base shows where surface water has eroded out a void around the boulder, posing a very high hazard to structures below.

UGS provided a preliminary summary to Rockville, with a recommendation that the city retain a qualified geotechnical firm with expertise in rock-fall hazard assessment to perform a detailed investigation.

e-mail: amyjoi@desnews.com