The Utah Geological Survey (UGS) was notified of a large rock fall in Rockville, Utah last Wednesday morning that damaged several buildings.  Geologists say it could have been much worse and are worried about the possibility of second boulder breaking loose.  Around 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 the home Tamara Burton was renting.

Tyler Knudsen and Bob Blackett, UGS geologists, were asked by Rockville Mayor Alan Brown to investigate what happened.  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, fell over a ledge and began to roll,” said Knudsen. 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.  But the debris severely damaged or destroyed many of the nearby outbuildings, including a chicken coop and a barn-type structure. A single two-foot by four-foot rock shard traveled through the entryway of Burton’s home, damaging the door and breaking a hole in the wall.  

Luckily, there were no reported injuries to residents or animals. 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,” said Knudsen.

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. It is already detached from its source ledge and sitting very precariously on a 45-degree slope of weak, easily erodible siltstone. 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 to assess the hazard and provide mitigation options at this site, as well as other locations in town below the rock-fall producing cliff.

The Utah Geological Survey provides timely scientific information about Utah’s geologic environment, resources and hazards.