The foundation of the spire in the Skydome of the National Corvette Museum was undercut by approximately 30 percent in the collapse, but has not moved since engineers began monitoring it soon after the collapse. © National Corvette Museum
A team of engineers is working diligently to secure a large sinkhole at the National Corvette Museum and raise the final three cars from the bottom.
March 18, 2014—Early in the morning of February 12, emergency personnel in Bowling Green, Kentucky, responded to a security alarm at the National Corvette Museum. A burglar alarm had triggered on one of the doors at the facility’s Skydome, where exceptionally rare Chevrolet Corvettes are displayed beneath a dramatic steel frustum. Museum staff arriving at the scene discovered that eight cars were indeed missing from the display. None of them had exited through the door.
A massive karst formation had opened an oblong sinkhole 45 ft wide, 60 ft long, and 27 ft deep beneath floor of the facility, near the bright red steel central spire in the frustum. Security video footage captured the initial collapse and the first two cars to fall into the hole before the video feed to the security system was cut.
A team of engineers and construction experts quickly assembled on-site to assess the situation and determine the immediate steps to stabilize the structure. Local firm Scott, Murphy & Daniel, LLC, was among those called and is serving as the project manager and coordinator of the team. Geotechnical consultant Hayward Baker, Inc., headquartered in Hanover, Maryland, was also brought in.
“The immediate concerns, certainly from my point of view, were: what status is it in? What’s the likelihood of it growing? What other areas might it impact?” says Michael Marasa, P.E., M.ASCE, a senior engineer for Hayward Baker. “The first challenge was trying to get some understanding of the magnitude and expanse of the feature.”
Also on the team from the early hours was Dennis Smith, P.E., L.S., M.ASCE, a principal of DDS ENGINEERING, PLLC, in Bowling Green. Smith previously served on the museum’s board of directors and his firm is providing surveying, civil, and geotechnical engineering on the Motorsports Park that the museum is developing nearby.
The sinkhole is approximately 45 ft wide, 60 ft long, and 27 ft deep.
Three cars were immediately visible and were the first retrieved.
Three cars remain in the sinkhole. © National Corvette Museum
“Once I had a chance to evaluate what the inside situation looked like and was able to see that the center spire had a great deal of material missing below it, I decided the best thing for us to do was intercept one of our surveying crews and establish a baseline to start monitoring that structure for movement,” Smith recalls.
An initial assessment indicated that the steel frame of the frustum was structurally sound, although the foundation beneath the large central spire was undercut by about 30 percent. Forensic surveying, which included the use of reflectorless target surveys, among other techniques, also revealed numerous cracks in the concrete slab around the hole.
“We started scanning the hole below the floor, as well, to get an idea of how big a hole we were dealing with—where the ‘safe zone’ was to be around the hole,” Smith says. “We saw very quickly there wasn’t a real clear-cut safe zone, per se.” Early in the process, personnel who entered the Skydome were secured in safety harnesses attached to the structure by retractable lanyards.
Western Kentucky University’s Department of Engineering volunteered a team to survey the sinkhole, employing a miniature unmanned aerial vehicle (UAV). The school’s Department of Geography and Geology has also helped with the effort. The images the UAV’s cameras relayed to the team confirmed what the reflectorless target surveys had indicated.
“It was clear we were dealing with more than just what you could see on the surface,” Smith says. “We could see other passages leading out of the hole. The collapse had apparently started a long time ago. How long ago, we don’t know. We believe the slab was probably not being supported by soil for some hours or maybe some days before the collapse.”
The team reached this conclusion in part by studying the security camera video, which reveals that as the slab collapsed into the sinkhole, very little dust rose out of the hole. The consensus by the team was that had soil collapsed at the same time as the slab, a large dust cloud would have emerged from the hole.
With a better understanding of the sinkhole, the team began to bolster the frustum’s foundation. The team devised a system of micropiles for both the exterior ring foundation and the central spire. “We started drilling around the outside to both get our feet wet with regard to optimal technique and work out of everybody’s way,” Marasa says. Ultimately 23 micropiles were installed.
The geotechnical and structural engineers agreed on a plan to either anchor the micropiles in 8 ft of solid rock or grout them into 25 ft of lightly fractured rock, depending on conditions at each location. The piles have varied from 75 to 220 ft in depth, Marasa says.
“Every time we drill a new micropile location for the building, we come up with some different and concerning subsurface conditions,” Marasa says. “If you look closely at the sidewall of the sinkhole where the spire is, you see sizeable pieces of rock fairly close to the ground surface.” On the opposite side, however, “there are not only no signs of competent rock, you start to see the mouth of a pretty good sized cave,” he says.
The 2009 Prototype ZR1 Corvette was the first raised from the
sinkhole on March 3, 2014. The car started with the aid of a
battery jumper and was driven out of the Skydome as the team
cheered. © National Corvette Museum
The top priority of the team once the structure had been stabilized was to develop a plan to retrieve the Corvettes, some of which are exceptionally rare and valuable. The rarest, the one millionth Corvette to roll off an assembly line, is valued at more than $750,000, according to insurance values obtained by Car and Driver Magazine.
“Monetary value is just a piece of the puzzle,” Marasa explains. “Every vehicle in the museum has a story. And the story is why it’s in the museum. That’s what’s driving [the effort] to recover them all.”
Retrieving the Corvettes presented a formidable engineering challenge, Smith says, even with some of them resting close to the surface. The team performed numerous calculations on various scenarios for siting the required cranes near the sinkhole to lift the cars.
“There just wasn’t a good answer,” Smith says. “The only good answer was to set the crane outside the building. And that wasn’t an option.” The team decided on March 1 that the best workable plan was to bring a crane into the building and locate it at the optimal site available, as determined after several hours of computer modeling.
The uppermost car in the sinkhole, a 2009 Prototype ZR1, was connected to the crane and raised 1 ft and held there for 30 minutes. The team then lowered the car, conducted more evaluations, and left the crane in place for two days. When there were no changes to the sinkhole or the structure observed, the ZR1 was raised out of the hole on March 3. The car was started with a battery jumper and driven out of the Skydome.
“There was a big cheer that went up through the whole team,” Smith recalls.
Five of the eight cars have since been retrieved from the sinkhole. Some were lightly damaged, others heavily. General Motors has volunteered to restore the cars, but for now they remain on display in other parts of the museum.
Two of the remaining cars haven’t been seen since the collapse. The goal is to retrieve them all, but the team can’t rule out the possibility that some have fallen into deep, dangerous underground caves. The team has already experienced good fortune in that regard: the one millionth Corvette was perched on a deep crevice.
“If they hadn’t grabbed it the way they did, when they did, it very likely could have slid off to the north into another significant cavity, which would have made it extremely difficult to get,” Marasa says. “I’ve reviewed those security camera videos six ways to Sunday, and I have an idea where I think the seventh one is, but I have no idea where the eighth one is. These are the pearls we are going after. You can’t do things such as drive sheet piles in the excavation because you don’t want to cut one of the cars in half. There are boulders and rock fragments all over the place, so you don’t know what you are hitting, either.”
The final plan to address the sinkhole won’t be formulated until all of the cars are retrieved. That will involve some work to shore up the sides of the sinkhole to make it safer for personnel to enter it. The best engineering solution to that challenge is still being resolved. That will be followed by light excavation work that makes use of soil vacuums.
Once the cars are retrieved, two primary options for repairing the museum are currently under discussion, Marasa says. “We can either isolate ourselves from the features or we can remediate the features.”
The isolation approach would involve a series of micropiles capped with a structural slab. The drawback is that some of those piles will likely need to be as deep as 220 ft and there would be no way of knowing at the onset how deep they would need to go, given the variable conditions. The remedial approach would involve filling the sinkhole and the surrounding caves with low-mobility grout (LMG).
“The reason you would use LMG is that it’s a very stiff, low-slung material,” Marasa explains. “You’re not going to fill up the county with fluid grout. It’s a much more surgical application.” The drawback with LMG is that the location and volume of the subsurface cavities in unknown.
“Ultimately that final plan won’t be prepared until we get the other cars,” Smith says. “We need to see what the material that is in there now looks like. We’ve dealt with sinkholes in buildings before. But never to this extent and certainly not with Corvettes in the bottom of them.”