Tunnel ready for impact
Glenwood Springs Correspondent
Vail, CO Colorado
LENWOOD CANYON, Colorado ” A rockslide that occurred in January 2002 is suspected to be the main cause of the crack in the Hanging Lake Tunnel, which resulted in a closure of the eastbound bore in March of 2007.
“We didn’t know that it was cracked,” said Glenwood resident engineer for Colorado Department of Transportation Pete Mertes. “There was an inspection but nothing was found at the time. The rock slide damaged it and the crack propagated over time.”
Over the next four years, a 70-foot crack in the 4.5-foot-thick concrete slab above the tunnel developed. CDOT crews monitored the slab of concrete and discovered the crack in July of 2006 but didn’t really become concerned with it until February of 2007. From there, the repairs lasted only six months and were completed in November of 2007.
Today, the tunnel is structurally secure and crews have implemented other means of hopefully preventing things like this from happening again.
“It was finished last fall,” said program engineer for CDOT Joe Elsen. “It was kind of an interesting project.”
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After the crack was repaired, Elsen and Mertes headed up operations on how to reestablish the surface above the tunnel to replicate its natural surroundings, Elsen said. The most obvious would be to put back the millions of pounds of dirt that was removed from the area, however, Elsen and Mertes had other ideas.
“That was kind of interesting,” Elsen said. “We replaced the dirt with some incorporated structural-grade foam blocks to back fill the area instead of dirt.”
According to Mertes, dirt backfill weighs in around 120-pounds-per-cubic-foot, that adds up when you consider the thousands of cubic feet of dirt that rest on top of the tunnels.
“By taking the load off you give the structure the ability to have more weight carrying capacity in the future,” Elsen said.
Debris will continue to trickle down the canyon walls but alleviating several million pounds of weight by replacing the dirt with the foam blocks essentially makes the concrete slabs able to withstand more of what comes down.
“The foam blocks weigh about 2.4-pounds-per-cubic-foot, so it’s about 50 times lighter than the soil backfill,” Mertes said.
Mertes said that the damage wasn’t caused by the dead weight on top of the slab but was more likely a result of the rock fall. However, according to Elsen, the 16X4X2.5-foot foam blocks will also absorb the impact of another rock slide better than dirt ever could.
“The blocks are light and tough enough to absorb and deflect the energy created by another rockfall,” Elsen said.
CDOT thought about rock slides a different way, however, and installed two types of monitoring systems that measure movement of rock outcroppings above the tunnel and a system in place to record “slab deflection” above the tunnel as well. The information is monitored by a central station in Denver and relayed to the Hanging Lake station.
“We wanted to have a safe place to work,” Mertes said. “If any type of movement or future rock fall were going to happen, we would have some sort of early warning.”
The total cost for the repairs came in at $5.7 million and took just six months to complete.