Marion County school District system will map sinkholes

Nov 1, 2011

The following article was published in the Ocala Star Banner on November 1, 2011:

School District will Map Sinkholes

By Joe Callahan

Marion County school officials have ordered specialized mapping of the three largest sinkholes that opened during last month’s rain deluge to determine if they are connected to large underground caverns.

Nine sinkholes, ranging from the size of a shoe print to 15 feet in diameter, opened at five district-owned locations — four schools and the Phoenix Center.

Robert Knight, the district’s supervisor of facilities, said the St. Johns River Water Management District must approve all repair plans and tests. He said the minimum cost to repair all nine sinkholes will be $65,000 total.

Electrical resistivity tests, basically an MRI of the ground, will map out underground caverns in the area of two Howard Middle School sinkholes and another at Ward-Highlands Elementary.

Once those tests are performed, Knight and his staff can more accurately determine the total repair costs. “Depending on what the tests reveal, the cost could go up from there,” Knight said.

Knight said the good news is that none of the nine sinkholes pose any threat to buildings or parking lots.

Knight said that two of the sites — Vanguard High School and the Phoenix Center — each had one sinkhole, which were deemed the easiest to repair. Evergreen had two sinkholes.

The smallest, with an opening the size of a shoe print, was at the Phoenix Center. Vanguard High’s sinkhole was 3 feet in diameter and 6 feet deep, while Evergreen’s largest was about 7 feet in diameter and 5 feet deep.

Knight said those four sinkholes were not linked to an underground chimney, a hollow shaft leading down into a voluminous cavern. The bigger concerns are Howard Middle and Ward-Highlands.

In all, four sinkholes opened up in — or very near — retention ponds on the Howard Middle campus. Officials expect a routine repair of two of those sinkholes, the largest 15 feet in diameter and 15 feet deep.

However, two others may be connected to a chimney, and thus could cause more immediate problems. The largest of those sinkholes is 3 feet in diameter and at least 30 feet deep.

Officials have a similar concern at Ward-Highlands, which had one sinkhole open in its retention area, located in the middle of a circular drive used by buses to drop off and pick up children.

That Ward-Highlands sinkhole is 10 feet in diameter and at least 20 feet deep.

Knight said since the recent three sinkholes of concern are not near buildings, they are hoping to fill the chimney openings with boulders and gravel. Then the district will pack it with dirt and then a layer of clay on top.

The clay will serve as the bottom of the retention pond.

Knight said since the recent sinkholes are not near any buildings, he does not expect the cost to rise to $725,000, like three years ago when sinkholes opened near Hammett Bowen Elementary School and at Booster Stadium.

The sinkholes formed three years ago when Tropical Storm Fay soaked Marion County the first week of school in 2008. A drought left underground caverns dry. Pools of standing water formed and the weight of the water caused the ground to collapse into a cavern.

The final repair cost at Hammett Bowen was about $550,000, mainly because thousands of cubic yards of a concrete slurry had to be poured into a chimney at the bottom of the sinkhole to fill up a large underground cavern.

The cavern had to be filled to protect the building.

The underground mapping at Howard Middle and Ward-Highlands will allow officials to rule out that the caverns are so large that they expand underneath those schools.

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