Utah Geological Survey April, 2000Public Information Series 69

Rock falls are a natural process of cliff andhillside erosion. They consist of large rockfragments from a cliff, or boulders from aslope that bounce, roll, and slide down ahillside and come to rest in a “runout” zone ator near its base.

Many different processes cause rocks tobecome unstable and fall including gradualweathering and erosion, tree-root growth, andweakening of supporting rock by saturationfrom ground water. Excavation for a road cutor building may weaken bedrock support.Rock falls are commonly triggered byearthquake ground shaking, rapid snowmelt,wide diurnal temperature changes, andintense storms.

Although many rock falls occur in Utah eachyear, few are recorded or cause damage.However, from a history of 50 rock fallsbetween December 10, 1887 and October 10,1999, we can make the followingobservations.>> All Utah earthquakes of magnitude 5 or

greater (nine total) triggered rock falls.>> Of the 39 rock falls that were not

triggered by an earthquake and themonth of occurrence is known, mostoccurred during the spring (11) andsummer (12) months. Presumably thisis because of spring thaw, summercloudbursts, and diurnal temperaturechanges. July and March had thehighest (10) and second-highest (5)occurrences respectively, per month.

>> Personal injuries from rock falls andslides were rare. However, four peoplewere killed and 11 people were injured,two seriously, in 50 events.

>> Five rock falls damaged homes, 21impacted highways, three damaged railroad tracks, and two breached water conduits.

STATE OF UTAHDepartment of Natural ResourcesUtah Geological Survey

Rock-Fall HazardsWilliam F. Case, Utah Geological Survey

Ron Drake, Moab Times-Independent.

“Big Bite”

Castle Valley Rock Fall

At approximately 7a.m. on July 8,1985, 48,000 cubic yards of rock fell from Porcupine Rim, GrandCounty, barely missing an unoccupied house (circled) in the

community of CastleValley. Dust enteredopen windows andcovered the interior to adepth of an inch. Thelargest rock fall boulder inthe runout zone was 50 X15 X 15 feet. Thetriggering mechanism forthe rock fall is unknown;it may have occurredwhen the night-cold,east-facing cliff was

warmed by the intense morning sun. Other recent rock falls show aslight-colored scars elsewhere on the face of Porcupine Rim and asrelatively vegetation-free, light-colored rock-pile slopes (talus) belowthe scars. Geologically older rock-fall scars and their deposits aredarker and have more vegetation relative to the amount of timesince the event. The scar at the “big bite” was produced by a rockfall witnessed in October 1978. Buildings constructed below the rimin the runout zone are in harm’s way.

Rock fall on Potash Road (SR279), D&RG rail tracks, and theColorado River, Grand County.Craig Morgan, UGS.

Rock-fall damage to OlmsteadAqueduct, Provo River Canyon, UtahCounty. Robert Robison.

Future rock fall, and runout zone of past rockfalls, Emery County. Michael D. Laine.

Homes (circled) in rock-fall runout zone,Bloomington, Washington County

Dust produced by simultaneous rock falls triggered bya magnitude 5.3 earthquake in Emery County. TerryA. Humphrey, Bureau of Land Management.

Rock falls and slides are a threat to property and life within the runout zone. The runout zone extends from the slope below the source downward to atleast the maximum distance from the cliff that rocks have fallen and rolled.

Utility and transportation corridors like power lines, pipelines, tracks, andhighways aredamaged by rockfalls more oftenthan buildings

because theycommonly extendalong the base ofslopes.

The Dead HorsePoint State Parkrock fall in 1984was recorded onseismographs up to30 miles away.Seismographsrecorded events upto a magnitude 2.5due to large rockfalls in YosemiteNational Park; thefalls produced anairblast that toppledtrees and strippedbranches. Rock-fallwitnesses often

report the sound of a sonic boom.

The Utah Geological Survey documents and evaluates geologic hazardssuch as rock falls to protect citizens of Utah. If you wish to report a rockfall or other landslide, or want more information, please contact the UtahGeological Survey at:

Utah Geological Survey, PO Box 146100, Salt Lake City, UT 84114-6100, phone 801-537-3300. The office is located at 1594 West NorthTemple, Salt Lake City. The web site is www.ugs.state.ut.us.

Chain-link fence protection.

Ditch, berm, and chain-link fence for rock-fallprotection, Bluff Street, St. George.

Alarm fence (dashed area) along railroadtrack, Weber Canyon, Weber County. RichGiraud, UGS.

Rock-Fall Risk Reduction

In addition to avoiding the runout zone,rock-fall risk is reduced by (1) preventingrocks from falling, (2) removing them, or(3) providing protection when they fall.

Rock bolts, buttresses, wire mesh, orshotcrete (concrete sprayed on a roadcut) will help prevent rocks from falling.

Good drainage will relieve pressure onrocks perched on a slope and reduceweathering and erosion.

Chain-link fencesoffer protection fromlocal, small rocks.Gabions or Jerseybarriers also provide protection from rollingrocks.

Rolling boulders canbe trapped in a ditchwith a berm toprevent them frombouncing out.

Railroad tracks canbe monitored forlandslide, including rock-fall, damage with wiresensors that sound analarm when they are hit.

Pipelines are oftenburied for protection fromfalling rock.

Roofs over highways andrailroad tracks shieldthem from falling rock.