Rockfall specialist Ty Ortiz slips his state highway vehicle off westbound Interstate 70 and onto the shoulder shortly after starting up the steep grade of Georgetown Hill.
A rocky hillside rises more than 1,000 feet above the roadway. About 50 feet up the slope, rockfall netting is strung between steel posts on foundations drilled into the rock.
“This fence has taken a lot of rockfall impacts,” said Ortiz, 41, lead engineer for the Colorado Department of Transportation’s rockfall control program.
About 30,000 motorists drive I-70’s Georgetown Hill daily and many are unaware of the protective barriers that are designed to greatly reduce the number of rocks reaching the road.
In 2003, a motorist was killed on Georgetown Hill when a boulder came off the mountain and smashed through his windshield.
Three years ago, CDOT began a rockfall control program for the 2-mile stretch of Georgetown Hill that is spending up to $1.5 million a year through 2013 for fences and other mitigation measures.
“What makes Georgetown unique is the potential for rock slides to occur high on the slope above the highway,” Ortiz said. “With a long rollout of up to 1,000 feet or more, rocks can gain large amounts of energy and momentum.”
CDOT has conducted controlled releases of large rocks from the top of the slope to see how high they will bounce. It aids in determining how high fences must be constructed.
Colorado has hundreds of locations where rockfalls pose a danger to roads and motorists. Over the past several decades, more than a dozen people have been killed by rocks and boulders that struck their vehicles.
The state is applying more money and more science to assessing rockfall dangers and mitigating the threat. One new technique is to use radar scanning of rock faces to look for excessive geological movement that could indicate an impending rockfall.
CDOT spends a total of about $3 million a year in rockfall control, with special attention to such high- risk and high-traffic areas as I-70’s Glenwood Canyon and Georgetown Hill, and U.S. 6 through Clear Creek Canyon, west of Golden.
The agency has a rating system to determine where the threat of injury or death from rockfalls is greatest. To decide which areas get priority for such mitigation measures as fencing or netting, CDOT looks not only at the area’s geology and slope geometry, but also daily traffic volume.
Georgetown Hill is a high-priority area.
There, Ortiz points to “braking elements” on the rockfall fence — looped wire ropes attached to the netting.
When a boulder hits the fence, the braking rings stretch, allowing the netting to expand and dissipate the rock’s energy.
Along U.S. 6 in Clear Creek Canyon, the threat is more from rocks breaking off the sheer rocky cuts that rise just above winding roadway.
The mitigation technique in the canyon more typically is to drape netting over the face of the cut to help direct falling rocks into the ditch beside the road.
Recently, rock scalers from an Arvada drilling and blasting company worked to remove a boulder jutting from a steep cut above the highway near its junction with Colorado 119.
They were scaling the cut before applying the netting.
The workers, from Yenter Companies, had driven steel anchors into the slope above the 20-ton rock.
Roped to the anchors, they worked with bars and other tools to try to free it. Finally, they slipped an airbag behind the rock and using a hose tethered to a compressor on the ground, pumped air into the device. It dislodged the boulder, which fell onto protective mats on the road to the cheers of the workers.
About 90 percent of CDOT’s rockfall mitigation relies on draped netting and roadside ditches.
Back at Georgetown Hill, Ortiz monitored another rockfall control technique under installation.
As an alternative to fences, which are designed to stop rocks that tumble from high on the mountain toward the highway, Yenter employees in one location are constructing rockfall “attenuators” — netting strung on posts in series up the slope.
Attenuators progressively slow falling rocks by allowing them to slip below the netting and fall to the next fence.
“The idea of attenuators is to control the descent of the rocks,” Ortiz said, so that by the time they get to the bottom of the mountain, they’ve lost so much energy that they fall harmlessly into the ditch beside the highway.
Jeffrey Leib: 303-954-1645 or jleib@denverpost.com
A year-round threat
Some vehicle-related rock-slide and rockfall fatalities in Colorado:
Feb. 18, 2007: A brick-sized rock kills an 11-month-old strapped into his car seat after smashing through the windshield of a pickup on Colorado 133.
Sept. 16, 2006: A 53-year-old Grand Junction woman driving a semi is killed instantly when more than 300 tons of rock loosened by two days of rain smash into her cab on Interstate 70 east of Grand Junction.
Sept. 25, 2003: A Silver Plume man dies when a boulder topples from Georgetown Hill next to I-70 and smashes through his windshield.
Aug. 16, 2003: A 37-year-old Avon man is killed on I-70 outside of Georgetown when a boulder crashes through the windshield of his Honda Civic.
Jan. 14, 2002: A wheelbarrow-sized rock hurtles off a 200-foot cliff and fatally crushes a 7-year-old boy riding in a pickup truck on I-70 near Glenwood Springs.
Dec. 9, 1999: A 55-year-old Pennsylvania tourist dies when a boulder crashes through the window of a shuttle van and hits him in the head. The van was traveling on I-70 near Georgetown.
May 16, 1999: A rock slide with boulders as large as office desks slams into a recreational vehicle, killing a 30-year-old Avon woman and injuring her husband. The slide occurred 1 mile west of Georgetown.
Feb. 26, 1995: Kathleen Daily, 47, and her sons, Tanner, 10, and Shea, 6, die when a one-ton boulder crashes down the wall of Glenwood Canyon and smashes into their Chevy Suburban.
May 17, 1993: Rain triggers a rock slide that sends a 250-pound rock into the side of a Chevy Blazer on Colorado 119 near Central City, killing the driver and forcing the Blazer into Clear Creek.
Aug. 10, 1987: Nine people ultimately die and more than a dozen others sustain injuries when a Colorado highway department road crew dislodges a car-sized boulder that hurtles down Berthoud Pass on U.S. 40 and smashes into a tour bus.
Sources: The Associated Press and Denver Post archives. Compiled by Barry Osborne, The Denver Post
