Latest weapon against sinkholes: Super foam

Workers resorted to an unusual brew of concrete and expandable foam when plugging sinkholes in Plant City and North Tampa on Bearss Road last month.

That same goopy mix, a kind of test recipe that could get wider use around the state in coming years, is almost sure to be called upon again as Florida enters the sinkhole season.

This is the time of year, as dry May gives way to rainy June, when engineers and geologists get uneasy knowing sinkholes can appear under roads and buildings at any moment.

They're almost impossible to detect until a depression forms, which by that time might be too late.

Ground-penetrating radar, which can detect the holes, works only when engineers know where to start searching.

"This is when we're extra vigilant, when we start making contact with those emergency contractors to make sure they're ready," says Christa Deason, spokeswoman for Florida's Turnpike Enterprise.

Hillsborough County says it's ready with its unusual grout mix.

Unlike most agencies' concrete-only formula, the county combines expandable foam polymer with concrete, or grout, to fill crevices and trim sinkhole plugging costs, which can run into the tens of thousands of dollars and require roads to be closed for several days.

So far, Hillsborough likes what it sees: a 30-percent savings compared to concrete-only jobs, and the foam-concrete mix holds up over time. That's important because 2009 looks to be especially active when it comes to sinkholes.

Already, the county has received reports of eight sinkholes, mostly in the Plant City area, due to farmers pumping ground water to irrigate crops and protect strawberries in cold weather. Hillsborough had that same number in all of last year.

Florida's ongoing drought is the main culprit.

Droughts lead to sinkholes because the decline in groundwater reduces the hydrostatic pressure in limestone where water builds up. As that pressure diminishes, soil and clay above the limestone can collapse, causing sinkholes to form.

Heavy rain can accelerate the holes because surface soils get saturated and heavy, causing further collapse, said geologist Tony Gilboy of the Southwest Florida Water Management District.

The county uses a variety of strategies to fight sinkholes depending on their size, location and whether they continue to grow.

Not every hole gets plugged with grout. Some are backfilled with dirt and monitored. So far this year, three needed to be plugged, the largest last month on Bearss Road where workers injected 380 cubic yards of the foam-grout mix.

The formula gets mixed on site in a cement truck and looks like toothpaste when it comes out of the hose. The hoses are pushed into holes drilled into the ground.

"There's no limit to the size it can fill that I know of," says Frank Deese, a general manager in the county's Public Works Department. "It's stronger than the surrounding soil but you don't need a jackhammer to get it out."

The county started experimenting with its grout-foam mix about five years ago after officials attend a conference in 2003.

Deese says the formula is used by public works agencies in California and a few other states, but so far Hillsborough is the only Florida agency using it.

"We're the test case," he said. "It's cheaper because you're using 30 to 40 percent less grout."