Residents living within high rock-fall-hazard zones in Rockville, Utah, face the possible consequences of a large rock fall similar to the fatal event that occurred last December.

That is the principle finding of a geologic investigation into the rock fall that killed two people on December 12, 2013. That afternoon, a huge, joint-controlled rock mass, with an estimated volume of almost 1,400 cubic yards and weighing about 2,700 tons, detached from the cliff face at the top of the Rockville Bench, near Zion National Park.

The rock mass fell onto the steep slope below the cliff, and shattered into numerous fragments. The rock fall debris then moved rapidly downslope before striking and destroying a house, detached garage and a car. The largest boulder to strike the house weighed an estimated 520 tons.

Following the event, the Utah Geological Survey (UGS) completed a geologic investigation to determine what caused the incident and what risk still remained for the area. The purpose of the recently-released 20-page report was to document the characteristics of the fatal rock fall; evaluate future rock fall hazards at and near the site; and provide recommendations for homeowners, the Town of Rockville, and other officials to consider in managing future rock fall risk.

Results of the investigation show a second, large, joint-controlled rock mass is partially detached from the cliff face above the site, and could fall at any time. Additionally, the slope below the cliff is littered with boulders related to both the December 12 and earlier rock falls, all of unknown stability. Elsewhere along the bench top, cliff-face-parallel joints are present, which can serve as detachment surfaces for subsequent rock falls. UGS concluded the rock-fall hazard at, and adjacent to this site, remains very high. 

“The December 12 rock fall, and the frequency and distribution of past rock falls in Rockville, amply demonstrate that residents living anywhere within high rock-fall-hazard zones in Rockville face the possible consequences of a large rock fall similar to the one that occurred in December,” said Bill Lund, senior scientist for the UGS geologic hazards program.

Although this was the town’s first fatal event, this rock fall is at least the sixth large rock fall within Rockville in the past 13 years. Three of those struck and damaged buildings at the base of the Rockville Bench. In 2011, the UGS prepared a rock fall-hazard map of Rockville and identified numerous houses and other structures at the base of the Rockville Bench that are in a high rock fall-hazard zone, as was the house where the fatalities occurred. Statewide, at least 20 deaths are known to have resulted from rock falls since 1850.

An evaluation of the December 2013 rock fall site by Utah Department of Transportation (UDOT) geotechnical engineers (included as an appendix in RI 273), concluded that rock-fall mitigation technologies, such as rock fall fences or catchment ditches, would be ineffective due to the large size and high energy of the typical boulders involved, and that the cost of prevention strategies such as rock bolting the cliff areas would greatly exceed the value of the endangered properties and the effectiveness would be questionable.

Results of the UDOT analysis leave residents living within high rock-fall-hazard areas of western Rockville with two options: (1) accept the hazard and continue to live in the high-hazard area knowing the consequences of that decision could result in significant property damage and may prove fatal, or; (2) relocate from high-hazard areas.

It is the responsibility of the Town of Rockville to ensure current and prospective future residents and land owners within high rock-fall-hazard areas are made fully aware of the hazard, so they can make an informed decision regarding their future course of action regarding rock-fall hazard. The UGS recommends Rockville investigate the possibility of acquiring the properties in high rock fall-hazard areas, so occupied structures can either be retired (torn down), or moved to safe locations outside of those areas.

Investigation of the December 12, 2013, Fatal Rock Fall at 368 West Main Street, Rockville, Utah (UGS Report of Investigation 273) is available on CD (20 pages) for $14.95, also available as print on demand. The report can be viewed on the UGS website at http://geology.utah.gov/online/ri/ri-273.pdf.

For further information, contact William Lund at 435-865-9034.