Will super fences be able to stop 7-ton boulders on I-70?
During the Wednesday and Thursday closure of Interstate 70 through Glenwood Canyon, eastbound motorists are advised to take Colorado 13 north from Rifle to U.S. 40 at Craig and east over Rabbit Ears Pass or down Colorado 131 back to I-70.
Garfield County residents traveling east on I-70 on the closure days are encouraged to do so before 8 a.m. or after the canyon reopens those evenings.
The Colorado Department of Transportation advises that Cottonwood Pass is not a viable route for large vehicles, and Garfield and Eagle counties have made plans to keep large trucks off the road.
Colorado 82 over Independence Pass is not an alternate route for large vehicles because it has a 35-foot vehicle length limit.
During the July 27 closure of Glenwood Canyon two semis were stalled on Cottonwood Pass when one jackknifed, and 20 oversized vehicles were stopped trying to go over Independence Pass.
Four rockfall mitigation fences are being installed during round two of Glenwood Canyon closures Wednesday and Thursday at the site of a major rockslide in February.
Altogether, the canyon already has about 40 rockfall fences, but this new infrastructure is utilizing the latest rockfall barrier technology, said Kathleen Wanatowicz, the project’s public information manager.
Wednesday’s work will focus on a helicopter flying the barriers’ 13 steel poles up to the highest locations, the top point being about 300 feet up the northern canyon wall. Then crews will work on installing the fence materials on Thursday.
Interstate 70 will be closed to traffic from 9 a.m. to 3:30 p.m. both days, forcing either long waits or long detours.
This project was initially planned for late July, but strong downdraft winds proved challenging for the helicopter, and the project had to be delayed.
Wanatowicz said the project will require one more canyon closure. That closure has not yet been scheduled, but Wanatowicz estimated it would be before September.
Three fences rated to absorb 5,000 kilojoules of energy are being installed. That’s equivalent to withstanding about 3.6 million foot-pounds of force, and project manager Jim Stepisnik said these fences — produced by the Swiss company Geobrugg — are capable of stopping a 6- to 7-foot boulder.
The project is installing a fourth 1,000-kilojoule fence.
“The technology is a widely known rockfall barrier system,” said Wanatowicz. “The manufacturing of this system dates back over 30 years with its origin in Europe.”
The fences’ steel nets are designed to absorb high-energy, concentrated loads. During a “maximum impact event,” the fences absorb the blow using high tensile steel wire system; the flexible steel nets have an extremely high energy absorption capacity, said Wanatowicz.
Geobrugg’s 8,000-kilojoule fence, for comparison, can absorb the impact of a 20-metric-ton boulder traveling 62 mph, according to the company. Geobrugg markets these fences as a cost-effective alternative to earthen barriers, cutting down on material and effort of installation.
The fences are supported by two to four catch ropes, which run diagonally through a ring net.
Each fence is made with ring nets with high-tensile steel wire. Each ring is 30 to 35 centimeters in diameter and made of seven to 19 strands of wire. A second mesh fence is overlaid to catch smaller rocks.
Each section has u-brakes at the sides and near the ground that reduce strain on the support ropes, according to Geobrugg.
“The braking system on the netting allows the net to be extended on impact and flex more than it would under normal conditions. The flex capacity keeps the rocks from breaking the fence,” said Wanatowicz.
“The fences are installed on a foundation and attached to a tension post system,” she said. “The contractor has been working on the foundations for the fences, which involves excavation of the area and forming of the concrete foundation slabs that are designed to accept the slope of the hill. The fences will be attached to the steel posts with an anchor support cable system.”
These barrier fences are tested under the toughest vertical drop conditions in accordance with European and Swiss guidelines, according to Geobrugg.
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