Officials haven’t determined the exact causes of the building collapse in Florida yet. But it brings to mind the problem of ground subsidence, which is a familiar problem in Florida and parts of some other states.
The “Florida Platform”, as geologists call it, sits on old volcanic rock several miles below ocean level. But the sediment which accumulated on top has only provided dry land for some 30 million years. This makes it part of the most recent period of Earth’s history, the Cenozoic. Bedrock underneath the state’s towns and countryside is young limestone, formed from chemical processes and the remains of marine life. It’s not nearly as solid as the massive, ancient limestone in Georgia’s Lookout Mountain region. This makes it vulnerable to chemical and also mechanical stress.
In Florida, the ground water table is typically high or even at the surface. Countless underwater caves pockmark the bedrock below. When such a cave collapses, it forms a sinkhole. This can happen over a period of years. The limestone dissolves slowly in groundwater made aggressive by acids from plants growing at the top. Often, the slow development of a funnel-shaped depression leaves time for evacuation. It gets more dicey when there is a change in the groundwater table. Growth of suburbs always comes with increasing demand for water. More wells are installed, pumping more groundwater to serve the expanding communities. The spiderweb of underground caves, formerly stabilized by the water, now fills with air. Even small disturbances, like vibration from nearby construction or traffic, can make a cave roof collapse suddenly. On the surface, this results in a huge hole swallowing up the structures on top.
Winter Park, Florida is a striking example. On May 8, 1981, this satellite community of Orlando heard a giant rumbling sound when several home and business properties dropped into a sinkhole 300 feet in diameter. Fortunately there were no casualties. Today, the water-filled hole looks just like the multitude of other round lakes in the area. In 2013, a man in Seffner, Florida near Tampa wasn’t so lucky when a sinkhole opened up beneath his house, swallowing his bedroom and sweeping him away. He was never recovered.
The recent building collapse in Surfside may have been for different reasons. But the widespread instability of the ground in the Sunshine State, caused by its geological make-up, necessitates great vigilance about construction and location of buildings.
Rudi Kiefer, Ph.D., is a professor emeritus of physical science at Brenau University His column appears Sundays and at gainesvilletimes.com.