CDOT studies rock shed for Glenwood Canyon slides
The Aspen Times
Colorado officials are studying if a fortified rock shed is needed to protect a portion of Interstate 70 from a potentially catastrophic rockslide in Glenwood Canyon.
The Colorado Department of Transportation hired a consultant to look at the technical and financial feasibility of the rock shed at mile marker 125, just west of Hanging Valley Tunnel and 1 mile east of where a rockslide closed the interstate for several days last month.
Massive rocks fell several hundred feet from the canyon walls in 2004 and 2010 and punched holes through concrete bridge decking at mile marker 125. Interstate 70 was closed briefly after both incidents for repairs.
CDOT got a grant from the Federal Highway Administration in 2010 to start the study looking at a rock shed — a tunnel of sorts to keep rocks off the road — and other possible steps, such as a warning system. CDOT decided in January to commission its consultant, Parsons Transportation Group, to move ahead with further study of the rock shed, according to Ty Ortiz, CDOT’s geohazards program manager.
The Feb. 15 rockslide didn’t weigh into the decision to look at a rock shed at a different, susceptible location, he said. However, the February event demonstrated how vulnerable the closure of the canyon is to the economy of the Roaring Fork Valley.
A large rockslide at mile marker 125 could cause even greater problems, officials said.
“There is a potential for a rock slide that would take the bridge out completely,” said Ralph Trapani, program manager on the rock shed study for Parsons and a former CDOT engineer who helped build I-70 through Glenwood Canyon.
The interstate designs at mile marker 125 and 124 are different, so the damage from slides in the two spots could be much different, Trapani said. Both lanes of interstate are elevated on bridges for several hundred feet west of the Hanging Lake Tunnel. The lanes are offset but on a terraced alignment on solid ground at mile marker 124.
The damage from rockslides at mile marker 124 can gouge the concrete and take out guardrails, but it can’t cause major structural damage, according to Trapani. “In a case like that, it’s difficult to justify a high-cost option like a rock shed,” he said.
But the bridges at mile marker 125 could be damaged or even destroyed by the size of rocks that have historically fallen in that area, he said.
Ortiz said boulders 40 to 50 tons fell in 2004 and 2010 at the site closer to Hanging Lake Tunnel. They were “at least double the size” of the rocks that fell last month 1 mile to the west.
“They’re kind of different beasts,” he said of the slide areas.
At mile marker 125, the pathways of the 2004 and 2010 slides were within 100 feet of one another. The earlier event deposited about 2,000 cubic yards of material. The latter deposited about 1,500 cubic yards.
Trapani said if a bridge or both bridges were destroyed near Hanging Lake, it could take a year or more to complete repairs and millions of dollars.
Even without a catastrophic slide, it’s nearly certain rocks will regularly fall that will punch holes through the decking, Ortiz said. “Every time it’s going to do this, it’s going to cost us millions of dollars,” he said.
Ortiz is more concerned about several large rocks falling at once rather than one extra large rock plummeting. “What we haven’t seen is a bunch of 15-foot rocks hitting the bridge,” he said. “If five blocks fell, what would happen?”
Trapani and his crew are running computer models of rockslides and using evidence from past events to test a rock shed. The preliminary data indicates the length would have to be about 400 feet, starting slightly west of the west end of the Hanging Lake Tunnel. More study is under way of how far west it would have to extend.
The structure must be capable of handling impact loads of rocks the weight of semis tumbling 1,000 feet. In other words, Trapani said, the structure must be able to handle a “mega-rockfall.” No amount of safety netting on the canyon walls will prevent rocks those size from falling.
Trapani noted that civil engineers who work in the mountains learn an important lesson early on: “Mother Nature bats last.”
The top of the rock shed would be covered with 20 to 30 feet of sand and polystyrene foam to create a cushion to absorb most of the impact. The steel structure would have flexibility to further absorb the force of the blow.
The rock shed wouldn’t have a curved roof like snow sheds that slough off debris. The rocks cannot be sent flying off toward the Colorado River and the railroad across the canyon floor.
“We’re not attempting to get the rocks to go off a ski jump,” Trapani said. The rocks would be removed after incidents.
The cost of a rock shed would be in the “tens of millions” of dollars, Trapani said. Parsons’ studies will likely be finished by the end of the year.
Ortiz said CDOT would assess the project on several fronts — safety of motorists, protection of the bridges, economic loss of road closures from rockslides and cost of the shed. The appearance will also be assessed.
“Aesthetics are important,” Ortiz said. “People drive through the canyon for a reason.”
The rock shed would be massive, because it must cover four traffic lanes, shoulders, the bike path and tie into the slope. Ortiz said rock sheds are regularly built by railroads, but he knows of only one on a roadway in the U.S. — the Rain Rocks Rock Shed along Highway 1 on California’s Big Sur Coast. That 245-foot-long shed opened in January 2014 after a $39 million construction project. More information on that project is at http://www.discover-central-california.com/rain-rocks-rock-shed.html.
Any decision on a Glenwood Canyon rock shed would be made in 2017 at the earliest.
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