Sinkhole: Disney World Visitors Walking on Holey Ground?
Fuller-Wright, Liz, The Christian Science Monitor
Sinkholes have never struck Walt Disney World directly, though the terrain suggests that it could be vulnerable to them. On Sunday, a sinkhole opened up in a resort compound less than five miles from the theme park.
Sinkholes start with water and limestone, two things Florida has in abundance. Limestone dissolves in water, and the more acidic the water, the faster the limestone gets eaten away. What starts as a small hole, deep underground, can grow bigger and bigger until it's a cave you could walk upright in. Flowing ground water keeps dissolving away the top, sides, and bottom of a limestone cave, enlarging it in all directions. For people living on the surface above these caverns, there's no obvious sign that the ground beneath their feet is being eaten away from below.
Geophysical surveys can peer beneath the surface, but to the naked eye, everything looks fine. There's no sign that you're essentially walking on the surface of a hollowed-out eggshell.
Until a tipping point is reached, and the roof of the cave collapses - caves in, literally - along with everything that had been precariously perched above.
That's what geologists call "cover collapse." It happens quickly, though in the case of the sinkhole that opened in Florida late Sunday night, it took a couple hours, giving residents time to evacuate from the surrounding buildings before the catastrophic collapse that lasted only a few seconds. Thanks to the rapid evacuation, even though the 24-unit building has been written off as a total loss, no one was hurt. In addition, officials say that the sinkhole is unlikely to grow any further. That's the one good thing about cover collapse sinkholes: Once the cavern below has collapsed, like an underground bubble bursting, it's over.
In addition to immediate damage caused by the cover collapse, there's a secondary effect caused by the fractured limestone, warns Boo Hyun Nam, an engineer at the University of Central Florida. "Gasoline, heavy metals, organic compounds - whatever soluble toxic contaminants you had at the surface can, if there's a sinkhole, slip through cracks and reach the aquifer and damage drinking water quality."
Dr. Nam, who is looking for a way to predict sinkholes, describes cover collapse sinkholes this way: "We have a lot of voids (holes) in limestone. Then, on top of the limestone, we have a clay layer or a sand layer. If we have a clay layer, good. Clay has some cohesion," which gives it some strength to hold against the weight of overlying soil, roads, buildings, etc. "It will hold it, hold it, and then, all of the sudden, it cannot hold it, and BOOM, it collapses. That's a cover collapse."
The other type of sinkhole, more common in eastern Florida, is caused by "cover subsidence," and is common where sand sits atop the limestone. …