Monday, December 18, 2017
Public safety

Experts study massive Pasco sinkhole for clues to avoid the next one

LAND O'LAKES — When contractors began priming Lake Saxon's rugged shoreline for new homes in the 1960s, they did what was then common practice.

They dredged heaps of soil from the water's depths and used it to extend the lakefront of what is now Lake Padgett Estates, historical aerial photos show.

Now, more than 50 years later, geologists are studying the dredging, along with many other possible factors, to see what caused a massive sinkhole to open up last month that destroyed two homes and rendered five others uninhabitable.

BACK STORY: Sinkhole swallows home, forces evacuations in Land O'Lakes neighborhood

Experts admit that no one might ever know exactly what caused Pasco County's largest sinkhole in decades, but a team of University of South Florida researchers have descended upon Lake Padgett to find out as much as they can about what happened.

They want to know if anything might have predicted the devastation, including the way the neighborhood was developed.

"If we can say where these things are likely to occur," said Lori Collins, a USF archeologist leading the team, "smarter decisions and regulations may emerge from that."

•••

Fred Wilsky was living in North Florida working for a paper company in the early 1960s when he decided to drive to Land O'Lakes to visit his parents at their lakefront home. He was startled to find the landscape of his childhood transformed.

Across Lake Saxon, just to the east of Lake Padgett, the shoreline had been extended dozens of feet.

(NOTE: Click on the image and slide left-to-right for a before-and-after view of the area)


Wilsky, now 90 and living in the family home, said he was not entirely surprised to see the sinkhole form.

"They're a fact of life here, like fleas," he said. "Dogs get fleas. We get sinkholes."

Indeed, Lake Padgett Estates sits in the heart of so-called sinkhole alley, a swath of central Florida extending from upper Pinellas and Hillsborough counties north and west to Marion and Volusia counties. Like much of the state, the land sits atop porous limestone known as karst that groundwater flows through and over time can erode.

In Land O'Lakes alone, 336 instances of sinkhole activity have been reported since 2003, the year the county started keeping track.

"I don't want to sound alarming, but a lot of people who move here from up north may not be aware of this type of terrain," said Dave DeWitt, a chief geologist with the Southwest Florida Water Management District, commonly known as Swiftmud. "And in this part of the state, you just don't know what's underneath you."

But even by Pasco standards, the July 14 implosion was remarkable. What started as a depression beneath a trailered boat became a 230-foot-wide, 50-foot-deep mudpit that devoured two homes within hours and left an entire community unsettled.

ANXIETY: After huge sinkhole opens, residents weigh future with unease

•••

Collins, who is leading the team of 13 USF researchers mapping the sinkhole's formation, said many Land O' Lakes residents have contacted her, hungry for any information beyond the daily news reports that might help them understand what happened on Lake Saxon.

"There's a lot of anxiety, as you can examine," said Collins, co-director of the University of South Florida Library's Digital Heritage and Humanities Collections.

The USF team has been studying the area's history, from its decades as an expansive orange grove to its conversion into a neighborhood. It grew from just 20 homes in 1967 to 300 in 1974, according to the U.S. Geological Survey, and building continued there until the 1990s.

While they are far from drawing conclusions about any relationship between the reshaping of the land and the sinkhole's formation, it is something the researchers are looking at closely, Collins said.

Three instances of sinkhole activity were recorded by the Florida Geological Survey in the 1980s along Lake Saxon's northern shore, not far from the sinkhole. Both homes it destroyed previously had been fortified after experiencing settling.

MORE: Is sinkhole damage sinking Tampa Bay property values?

"I don't know if any of these thing are of impact," Collins said of the development at Lake Padgett Estates. "But even if we don't draw conclusions, we want people to be able to see things and make their own decisions. Like, 'Do I want a house where a lake used to be?'"

•••

Sinkholes occur naturally in Florida. But they also can be provoked by human activity, including intensive groundwater pumping for irrigation or residential taps, or by work above ground that destabilizes the sometimes crumbly limestone below.

Geologists are calling the Land O'Lakes crater a "cover-collapse" sinkhole, which occurs when soil above a void in the limestone gives way. They tend to occur abruptly.

Sinkholes caused by the actions of people are most often the cover-collapse kind, according to a 1999 report from the U.S. Geological Survey.

"The increasing incidence of induced sinkholes is expected to continue as our demand for groundwater and land resources increases," physical scientist Ann Tihansky wrote in the paper.

Lake Padgett Estates sits between two large groundwater well fields. The day the sinkhole developed, groundwater levels recorded at the closest well to the neighborhood were within a normal range, according to Swiftmud data. So it is unlikely that pumping spurred the sinkhole, at least in the immediate sense, the agency said.

But that doesn't mean water wasn't a factor.

Florida had just emerged from a drought, and then welcomed consistent rainfall at the end of June. When water levels drop and then heavy rains suddenly pound the dry ground, the crust above a sinkhole is more likely to collapse, said DeWitt, the Swiftmud geologist.

When rain falls onto undeveloped parts of Florida, it filters widely through sand and dirt on its way to the Florida aquifer. But when roads and houses are added to the terrain, much of that water is routed to retention ponds and lakes, Tihansky said in an interview. Changing the natural flow can concentrate water pressure below ground in new ways, which can also contribute to sinkholes.

"We don't really understand why exactly one sinkhole area gives and one doesn't," Tihansky said. "But being aware of them, and engineering for them, can and should be done."

•••

With its history of sinkholes, Pasco County passed an ordinance in 2008 that requires engineers to test for potential geological hazards before new neighborhoods are built. If they detect a sinkhole risk, they make recommendations to avoid a problem.

That's a more robust approach than in many Florida counties, said Jennifer Motsinger, executive vice president for the Tampa Bay Builders Association.

"If you live in a master-planned community in Pasco, you can feel good about the fact that the developer took the time and money to make sure the land is properly prepared and can sustain houses," she said. "If the home is 30 to 50 years old, you don't have that reassurance."

Standard practice is to test the land 20 to 30 feet below its surface, according to geotechnical engineers. That's "better than nothing," said Jim Flynn, marketing manager of L.R.E. Ground Services, one of the most active sinkhole repair companies in Central Florida. But he said most of the work his company does addresses voids that run deeper than that.

But deeper ground testing can cost anywhere from $5,000 to $15,000, Flynn said. Many developers forego additional testing, which is why even newer houses in Florida are frequently fortified after they're built.

Most of the time, remediation is prompted by minor settling issues. In all, 6,647 Pasco homes have been stabilized due to sinkhole activity since 2003, records show.

Pasco officials plan to revisit the ordinance after the Lake Padgett Estates sinkhole is emptied and stabilized, said Kevin Guthrie, assistant county administrator for public safety. The county reviews all relevant policies and ordinances following a disaster, he said.

For now, he can only speak to what the county must do in the immediate future: Clean up the hole, and figure out what to do with prime real estate marked by a yawning chasm.

Contact Laura Newberry at [email protected] Follow @LauraMNewberry.

   
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