Hanging Lake Tunnel lanes set to open
October 2, 2007
GLENWOOD SPRINGS ” After sandwiching a 70-foot crack in the ceiling of the Hanging Lake Tunnel, the Colorado Department of Transportation expects to open its eastbound lanes Wednesday morning.
The project might end up costing around $6 million, said CDOT program engineer Joe Elsen. CDOT increased the scope of work after initial plans and estimates, including things like additional concrete, drilling and shorings for support during construction.
The crack stretched through a 4 1⁄2-foot-thick slab of concrete above the eastbound lanes. The slab weighs about 3 million pounds. CDOT closed the tunnel on March 30 and began diverting traffic through the westbound lanes after becoming increasingly concerned about the crack in February.
Elsen said about 25 million pounds of dirt had to be excavated to get 30 feet down to the roof of the tunnel. With a system of supports in place, workers laid down concrete about a foot thick above the damaged slab, and a 6-inch-thick layer of “shotcrete” ” or concrete from a gun below ” to the bottom. Steel bars now connect the layers through more than 800 holes drilled through the original slab. Another 200 or so holes were drilled and bars placed for additional support, connecting with things such as the sides below the slab, Elsen said.
CDOT contracted with Concrete Works of Colorado for the repairs.
Work continues on filling in the excavated dirt that had been piled about 30 feet high above the cracked tunnel. Large foam blocks and layers of “geotextile grid” are being put down instead of only replacing dirt and rock. This will reduce the weight on top of the tunnel and should also help absorb impact from any future rockfall. The geotextile grid is a thin rubber web that helps reinforce soil and spread energy from potential rockfall over a larger surface. That part of the project might be complete within another week or two.
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Pete Mertes, a CDOT resident engineer, said the crack was discovered in July 2006.
“We believe a rockfall caused the sheer cracking to occur” in January 2002, he said.
The rockfall came from an outcropping that towers hundreds of feet above the eastern side of the five-story control center that sits between and above the tunnels.
The Federal Highway Administration directed CDOT to conduct an inspection after a tunnel in Boston had a falling ceiling panel that caused a fatality, but CDOT conducts random inspections and may have found it anyway, Mertes said.
“The great thing is that we found it when we did and were able to get in and repair it,” he said.
One of the biggest challenges with construction was the difficulty in getting heavy machinery to the area above the damaged tunnel. There were also other variables in the complex project that forced some engineering work to be done on the fly, Elsen said.
“It’s a unique place,” he said. “It’s extremely difficult to access.”
CDOT plans to keep an eye on the tall rock outcroppings to monitor and hopefully predict possible rockfall. This would probably involve monitoring “targets” on the rock with a survey gun, Elsen said.