GLENWOOD SPRINGS, Colo. (AP) The Colorado Department of Transportation has come up with a plan to repair the 70-ft.-long (21.3 m) crack in the concrete above the Hanging Lake Tunnel: the $2 million sandwich.
Planning is ongoing and many details have yet to be worked out.
The crack is thought to have been started by rockfall in 2002 from a rock outcropping that towers hundreds of feet above the eastern side of the five-story command center that sits between and above the tunnels. CDOT has monitored the crack and became increasingly concerned in February.
Around that time, engineers said, “It’s gotten worse, and it’s starting to run a little water,” said Joe Elsen, program engineer for CDOT.
On March 30, CDOT closed the eastbound lanes of the tunnel and began diverting traffic through the westbound lanes out of concern about the crack.
It stretches through a 4.5-ft.-thick (1.37 m) slab of reinforced concrete above the eastbound lanes. The slab weighs approximately 3 million lbs. (1,390,777 kg), Elsen said.
But a person looking upwards from the eastbound lanes wouldn’t see that slab or the crack. They see the floor of the “plenum,” which is a long, low-ceiling, tunnel-like cavity above the lanes that can be used for emergency ventilation powered by giant fans inside the complex.
Elsen said a crucial part of the repair process is monitoring the crack and the high rock wall above it for any further movement. He said he and a CDOT geologist inspected the wall via helicopter on April 5. They determined that the best option was not to scale or bring down any rock or debris, but instead to keep tabs on the wall by placing electric sensors to monitor any movement.
“We’re going to instrument it so we can get an idea if it’s going to move or not,” Elsen said.
The plan is to then remove the floor of the plenum and begin bracing the crack with a series of steel struts. From there, excavation could begin on the 8,000 cu. yds. (6,100 cu m) of dirt and rock piled approximately 30 ft. (9.1 m) high on top of the cracked slab.
The damaged slab will be sandwiched between two additional slabs of concrete approximately 105 by 40 ft. (32 by 12 m) long and 1 or 2 ft. (.3 or .6 m) thick. The two slabs above and below will be connected by steel bars through 1,000 or more holes drilled through the cracked slab.
The lower slab might be constructed using “shotcrete” or concrete sprayed out of a gun. Instead of replacing the dirt once repairs are complete, Elsen said, large lightweight foam blocks might be used with a lesser layer of dirt on top.
CDOT has contracted with Concrete Works of Colorado to do the work, and is consulting with the Parsons Transportation Group.
Elsen said that Concrete Works completed the Grand Avenue paving project and is scheduled to start a $4 million Interstate 70 paving project near the tunnels on May 7.
“They’re experienced in moving dirt and doing structural concrete as well,” he said. “They thoroughly plan what they’re going to do and they execute it.”
Elsen said that in March the Transportation Commission appropriated $2 million to address the problem. He said approximately $200,000 is going into design and $1.8 million will go toward construction. He wasn’t sure if that amount would be enough as there are many variables in the plan.
He wouldn’t give an estimate of how long it might take to complete repairs, but at the time the tunnel bore was closed it was reported that CDOT hoped to finish repairs by the time the first snow flies in the fall or, ideally, sooner.
Elsen said he was “very appreciative of the responsiveness of the Union Pacific Railroad” in helping transport equipment to the tunnel area.