8 August 8, 2005 ROCKY MOUNTAIN CONSTRUCTION acppubs.com Rockfall Mitigation On I-70 By Carol Carder A Eliminating the hazard of falling rocks on mountainous highways is impossible, but the risk can be substantially reduced A friend who grew up in Georgetown, Colo., reminisces how her bedroom win- dow faced the steep slope above town and how she liked to listen to the rattle of rocks falling at night from the heights onto U.S. Highway 6 below. Today the 2.2-mile stretch of Interstate 70 between Georgetown and Silver Plume, about an hour west of Denver, has earned the dis- tinction as Colorado’s most recognized rockfall hazard area. “The area is high profile because there’s so much traffic,” says Ty Ortiz, engineering geologist, Colorado Depart- ment of Transportation materials lab. “The annual average daily traffic count of 30,000 vehicles means 21 vehicles pass through the identified rockfall area each minute, so rocks impacting traffic is greater than in lower traffic areas.” CDOT’s historical data for George- town Hill show falling rocks have caused approximately 100 accidents with 17 injuries and three fatalities in the past 24 years. In April 2004 a rockfall took out the 12-foot-high fence installed in 2000 on the slope above the Georgetown Loop Over- look. Last fall Yenter Companies Inc. of Arvada replaced this fence and also installed a second attenuator fence approx- imately 300 feet higher up the slope to slow down rocks and to reduce the chance they will bounce over or plow through the bottom fence. This $475,000 project is the first phase to protect the highway. The next phase in fiscal 2005-06 will install other fences higher up the slope. CDOT’s rockfall hazard rating sys- tem, first published in 1994, identifies and rates over 750 cut slopes throughout the state using a combination of slope meas- urements, traffic data and geology. In this system Georgetown Hill is divided into 15 distinct cut slopes. However, the rockfall incidents that severely affect traffic come from far above the cut area. CDOT prepared a rockfall potential study of Georgetown Hill mapping bedrock outcrops and rockfall chutes. In this area rocks can start from up any- where on the 40-percent slope up to 1,800 feet above the highway and attain enough velocity to take out almost any fence installed at the bottom, according to Ortiz. The Colorado Rockfall Simu- lation Program (CRISP) models the speed, energy and bounce height of a rock as it rolls down a slope. With this study, CDOT is able to select specified locations for rockfall mitigation based on solid engineering analysis. “In areas where you can narrow down the rockfall area to a few hundred feet, you can build a traditional rockfall bar- rier fence and not worry about rock bouncing over it or plowing through it, but in areas with these long rollouts, rock can gain enough velocity to bounce over or take out a fence,” Ortiz explains. Coiled wire mesh sections rests on the side of I-70 awaiting helicopter hoist to construction side on Georgetown Hill. Georgetown Hill photos by John Gross, CDOT Crews prepare the metal spreader beam that will be attached to the wire mesh top to keep it from crumpling during transport.
Transcript
8 August 8, 2005 ROCKY MOUNTAIN CONSTRUCTION acppubs.com
Rockfall MitigationOn I-70
By Carol Carder
AEliminating the hazard of falling rocks onmountainous highways is impossible, butthe risk can be substantially reduced
A friend who grew up in Georgetown,Colo., reminisces how her bedroom win-dow faced the steep slope above town andhow she liked to listen to the rattle ofrocks falling at night from the heightsonto U.S. Highway 6 below. Today the2.2-mile stretch of Interstate 70 betweenGeorgetown and Silver Plume, about anhour west of Denver, has earned the dis-tinction as Colorado’s most recognizedrockfall hazard area.
“The area is high profile becausethere’s so much traffic,” says Ty Ortiz,engineering geologist, Colorado Depart-ment of Transportation materials lab.“The annual average daily traffic countof 30,000 vehicles means 21 vehiclespass through the identified rockfall areaeach minute, so rocks impacting trafficis greater than in lower traffic areas.”
CDOT’s historical data for George-town Hill show falling rocks have causedapproximately 100 accidents with 17
injuries and three fatalities in the past 24years. In April 2004 a rockfall took out the12-foot-high fence installed in 2000 on theslope above the Georgetown Loop Over-look. Last fall Yenter Companies Inc. ofArvada replaced this fence and alsoinstalled a second attenuator fence approx-imately 300 feet higher up the slope toslow down rocks and to reduce the chancethey will bounce over or plow throughthe bottom fence. This $475,000 projectis the first phase to protect the highway.The next phase in fiscal 2005-06 willinstall other fences higher up the slope.
CDOT’s rockfall hazard rating sys-tem, first published in 1994, identifies andrates over 750 cut slopes throughout thestate using a combination of slope meas-urements, traffic data and geology. Inthis system Georgetown Hill is dividedinto 15 distinct cut slopes. However, therockfall incidents that severely affecttraffic come from far above the cut area.
CDOT prepared a rockfall potentialstudy of Georgetown Hill mappingbedrock outcrops and rockfall chutes.In this area rocks can start from up any-where on the 40-percent slope up to1,800 feet above the highway and attainenough velocity to take out almost anyfence installed at the bottom, accordingto Ortiz. The Colorado Rockfall Simu-lation Program (CRISP) models thespeed, energy and bounce height of arock as it rolls down a slope. With thisstudy, CDOT is able to select specifiedlocations for rockfall mitigation based onsolid engineering analysis.
“In areas where you can narrow downthe rockfall area to a few hundred feet,you can build a traditional rockfall bar-rier fence and not worry about rockbouncing over it or plowing through it,but in areas with these long rollouts,rock can gain enough velocity to bounceover or take out a fence,” Ortiz explains.
Coiled wire mesh sections rests on the side of I-70 awaitinghelicopter hoist to construction side on Georgetown Hill.Georgetown Hill photos by John Gross, CDOT
Crews prepare the metal spreader beam that will be attached to the wiremesh top to keep it from crumpling during transport.
The strategy on Georgetown Hill isto install a series of strategically placedattenuator fences. These fences drapeover sections of the rockfall chutes andare installed without bottom supportropes so falling rocks slow down butdon’t take out the fence. Metal sleevesinstalled over the wire ropes making upthe anchors will prevent shearing of thefence support anchors, which is causedby angular rocks moving with high rota-tional velocities. When hit by rock, thesleeves will spin rather than permittingthe rock to slice the ropes, as happenedto the barrier fence in 2004.
Installation a Tall OrderYenter Companies faced some tall chal-
lenges installing the fences. The com-pany brought in two helicopters, inDecember a Bell 206 B-3 to hoist thecolumns, then in February a jet-poweredBell 206 L-4 Long Ranger from SaltLake City to lift metal netting sections.
“We had to haul everything to set thecolumns 275 feet to 300 feet up the 40-percent grade, so we built a pulley systemto take up the equipment and materials,”Mick Muller, P.E., Yenter project manager,explains. To control quality, Yenter crewsbatched the concrete at the bottom andsent it up in 5-gallon buckets.
Because CDOT wanted the new fencesup before the 2005 spring run-off dis-lodged rocks, the contract period was25 working days. However, with themany bad weather days, the workspanned from November to mid-March.During the installations the crews used a
temporary rockfall fenceon the road shoulder andspotters above to warnthe workers on themountainside of anyfalling rock.
As the first step, theworkers drilled theanchors 6 feet deep forthe columns. Then thefirst helicopter hoistedin the 750-pound, 15-foot-high steel columns,and the crew anchoredthem into the holes withconcrete. The next stepwas attaching the 1,200-pound sections of metalnetting. This extra strongmesh is similar to chainlink mesh used under-water by the military toseize submarines. Work-ers attached the top ofthe 15-foot-wide by 30-foot long chain link to aspreader beam to pre-vent it from crumblingduring transit. Thenworkers kept the meshspread out as the heli-copter picked it up. Dueto the weight of the chainlink sections and the highaltitude, the pilot was theonly one on board thehelicopter. Also, the helicopter carried just10 gallons of fuel and refueled betweeneach lift.
At the installation site, the crews on theground grabbed a tagline to maneuverthe mesh into place and attach it to the
acppubs.com ROCKY MOUNTAIN CONSTRUCTION August 8, 2005 9
The mesh resembles a large chain link fence with sections 15 feetwide by 30 feet long and weighing 1,200 pounds.
A worker reaches for the spreader beam that will be used to hoist the meshsection. Keeping the mesh spread out as it’s picked up by helicopter is tricky.
Above: The pilot carefully maneuvers a jet-powered Bell 206 L-4Long Ranger over the chain link section where the ground crewattaches it for hoisting. The pilot is the only person on board tokeep weight low at this high altitude. Inset: The chopper carefullylifts the heavy mesh before heading across I-70 to the rock fallmitigation area. High winds, common in mountainous areas, canchallenge aerial deliveries even for experienced pilots. Below: Thechopper pilot deftly lowers the mesh over the steel framework.
Once the mesh is securelyfastened to the frame, it isdisconnected from the helicopter.
steel framework. Once the net was secure-ly attached, it was disconnected from thehelicopter, and workers finished the instal-lation while the pilot headed back formore fuel and another section of mesh.
During the helicopter-assisted instal-lation of the support poles and netting,CDOT notified drivers of the up to 20-minute delays on I-70 with its Intelli-gent Transportation System VariableMessaging Signs (VMS). The VMS sys-tem started on the east side at RooneyRoad, which is 35 miles from the proj-ect, and 50 miles away on the west sideat Vail Pass.
The 12-foot-high bottom fence is2,400 square feet, and the 15-foot-highfence on the mountainside is 1,100square feet. The composition of the net-ting is 5/16-inch-diameter 6-inch diag-onal weave cable net with wire meshconnected on the uphill side. One-half-inch wire support ropes are connected tothe netting in a diagonal pattern to addadditional support and weight. The net-ting is suspended from the 1-inch wiresupport ropes and is draped for approx-imately 15 feet on the ground.
The idea, according to Ortiz, is to haveenough netting draped on the ground toslow rock as it hits the system but not somuch as to accumulate fallen rock anddebris which would add unwanted stresson the attenuator. Upon completion,CDOT tested the attenuators by rollingrocks into them and observing the reac-tion at impact. The attenuators worked asdesigned. The 3-foot to 4-foot rocksrolled from a distance between 300 feetand 500 feet impacted the attenuatorswith approximately 100 foot-tons ofenergy. As planned, the rockfall barrierat the shoulder easily stopped the rocksthat made it over or through the attenu-ators. Observations from these rockrolling exercises will help CDOT engi-neers determine the location of the nextset of attenuators on Georgetown Hill. Avideo of the rock rolling tests and aslideshow of the fencing installation is inCDOT’s Spring 2005 edition of CoTripQuarterly at www.cotrip.org. ■
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Above: Crews secure the 1,200-pound piece of mesh into a huge chain link fence. This extra-strong mesh is similar to chain link mesh used underwater by the military to seize submarines.
Above: From their precarious perch on the mountainside, ground crew members dangle as theysecure the mesh.
Above: Between airlifts of mesh, approximately 15 minutes apart, aviation crew refuels thehelicopter with only 10 gallons of jet fuel to keep gross weight down.
In Clear Creek Canyon
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“Geologic hazards are inherent alongthe mountain highways in Colorado.Landslides – debris flows – avalanches –and rockfall,” begins the audio for thevideo Colorado Department of Trans-portation prepared on the I-70 George-town Hill mitigation project. “Some forcesof nature are unstoppable and unpre-dictable,” CDOT geologist Ty Ortizremarked. Mother Nature illustrated thetruth of that statement on Tuesday, June21, at 11 a.m. when 1,500 tons of rock slidoff a 150-foot-high outcropping on a cor-ner of US-6 just a mile and a half east ofthe SH-119 junction. A known troublespot, the slope was draped with wiremesh net last year, but the rock slid rightunder it leaving the top anchors intact.
CDOT awarded a $760,000 contract toAmes Construction of Aurora to removerock and repair the road damage in 25days. It’s ironic that just last SeptemberEdward Kraemer Construction fast-tracked replacement of three bridgedecks in 12 days on this heavily traveledhighway stretch. (“Kraemer ReplacesBridge Decks in Record Time,” RMC,Dec. 13, 2004.) US-6 handles 12,000vehicles a day on the canyon route toSH-119 that connects to popular moun-tain gaming communities Black Hawkand Central City.
The transportation department movedrapidly to address the emergency. A teamof CDOT geologists led by Ortiz con-verged on-site that afternoon. Tony DeVi-to, P.E., Region One foothills residentengineer, recounts how CDOT immedi-ately called in four experienced con-tractors to survey the damage. By 11p.m., the team led by DeVito and Ortizhad issued generic specifications to thecontractors for a bid. Additional infor-mation, such as protecting the creek dur-ing slope excavation with a berm offallen rock, followed by noon on Wednes-day. By 3 p.m., CDOT had received bids
from Ames, Lawrence Construction andKiewit Western.
“We were able to get a contractor on-site immediately because we have anemergency contracting procedure,” DeVi-to explains. “A couple of years ago, justafter the Vail sinkhole incident, Craig Sir-acusa, our chief engineer, instituted theprocedure with the state controller.”CDOT used the new emergency proce-dure for the first time in responding toa damaged bridge pier near Trinidad.
Later, the procedure enabled CDOT torespond immediately to the rockfall inci-dent in Glenwood Canyon on Thanks-giving Day 2004.
Gary Self, CDOT contracts and mar-ket analysis branch manager, says,“Essentially, the procedure allows us toget a qualified contractor on-site doingthe work, then follow up with the required
CDOT staff looks up at US-6 rockfall chutein Clear Creek Canyon. All photos by GreggGargan, CDOT
Rockfall
Drilling crew working from cranebasket prepares high rock face forblasting.
paperwork and contract.” According tothe state’s operational manual, “An emer-gency is a situation that creates an imme-diate threat to public health, welfare orsafety…” and “there is insufficient timeto obtain a written waiver of the require-ments for issuance of a commitmentvoucher pursuant to this fiscal rule beforerequiring goods or services to respond tothe emergency.”
Unlike the bridge construction a yearago, safety mandates will prevent Amesfrom using creative scheduling and plan-ning to expedite the work. It’s illegal toblast at night.
Tom Flick, Ames Construction projectmanager, expected to move to two 10-hour shifts the week of July 10 “as soonas the canyon is opened up a bit.” Amesis handling the blasting with in-houseexpertise to remove the bedrock judgedunstable by CDOT geologists. Subcon-tractor Hark Drilling of Phoenix is sta-bilizing the rock face with rock boltingand wire mesh netting.
Complications arose quickly, howev-er, and on July 15, CDOT had to an-nounce, “Despite working 24 hours aday, seven days a week, the completionof the project has been delayed untilmid-September to accommodate furthermitigation.” CDOT’s Ortiz explained,“Through our rockfall mitigation effortsover the last several weeks, we havedetermined that in order to make thisslope safe in the long run, the most appro-priate course of action is to excavate alarge amount of potentially unstablematerial from the mountain.” Thus, Amesis to remove another 35,000 cubic yardsof rock, and total cost of the work, includ-ing CDOT and Ames activity, is expect-ed to reach $3 million. ■
View of rocks on road. Note that wire stabilizing mesh installed last year still hangs from its top anchors.
Above and below: The mangled gravel hauling trucks that were caught in the slide.
The blast – cloud of dust with lots of rocks in air.
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