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Holes won't hold up expressway

Two sinkholes have appeared in the path of the $332-million Veterans Expressway now under construction in northwest Hillsborough County.

On the surface, the sinkholes, on a portion of the expressway near Bellamy Road, appearedas shallow depressions, no more than a foot deep. But engineers were concerned enough about the ground conditions to add concrete grout and thousands of feet of special netting to the roadbed (graphic, Page 8B).

They expect to complete the 15-mile toll road in October, as scheduled. The road will run from Tampa International Airport to north of the intersection of Van Dyke Road and N Dale Mabry Highway and is supposed to achieve the decades-old goal of helpingtraffic circulation around the west side of Tampa.

Officials call the sinkhole repairs a success _ with as much of a guarantee as you can ever have in Florida. The netting, called geogrid, won't make the road impervious to future sinkholes.

Nothing can. It can only help support the road.

"What it is designed to do, it will do," said Fintan J. Buggy, a geotechnical engineer with Dames & Moore, one of the many firms involved in the expressway project. "It will prevent a sudden, catastrophic collapse, until we can fix the problem."

No one can say if another sinkhole will appear at the same point on the expressway.

Despite the fragile limestone substratum under this part of Florida, the chances of a sinkhole forming in any one place is very small. But the odds worsen as the size of the area increases.

"My own particular prediction, which I've made for years, is that when we have our first death from a sinkhole in Florida, it won't be in a house, but probably a sinkhole will open under a road at 5 a.m., in the dark, and some guy will hit it at 70 miles an hour and go in," said Dr. Barry Beck, former director of the Florida Sinkhole Research Institute.

"By sheer area, there's so much more to a road," said Beck, who now is senior hydrologist and chief of operations for P.E. LaMoreaux & Assoc. in Oak Ridge, Tenn. "If a sinkhole starts to open under a house, it creaks and makes noise. If you're driving, you won't see it until you're on top of it."

Buggy, the Dames & Moore engineer, took exception to Beck's prediction.

"If you wanted to sensationalize it, you could say a sinkhole could open up on Runway 1836 at Tampa International Airport and take out a jumbo jet with 400 lives," Buggy said. "That might make the six o'clock news."

The fact is, no one can say for sure. Sinkholes are Florida's subterranean lottery, developing quietly over hundreds or thousands of years. Rainwater erodes underground limestone, creating hollows that sometimes collapse. Then the sand on the surface collapses, too.

When water tables fall, the water robs limestone ceilings of support. Engineers said vibrations from the Expressway workers probably contributed to the formation of the Bellamy Road sinkholes.

"What you see is the final scene of an act that's been going on for a long time," Buggy said.

Sinkholes not a big factor

When it was conceived about 30 years ago, the later-renamed Northwest Expressway was part of a plan to ease the flow of traffic in and around West Hillsborough as much as possible, given the roads, offices and houses that already existed.

Limited-access toll roads like the Northwest Expressway and the Crosstown Expressway would work in concert with Interstate 275 and Interstate 4 to speed up longer trips across the city. It was thought Tampa would avoid the fate of Pinellas County, which now strains like Gulliver under the vexing band of U.S. 19.

The Tampa-Hillsborough County Expressway Authority chose the route in April 1986, picking a way past subdivisions and wetlands. Florida Turnpike officials took over the job of building the road.

A series of borings they commissioned in July 1988 showed underground soft patches near Bellamy Road _ possible conditions for a sinkhole.

"Although conditions indicate that there is moderate potential for sinkhole development within the proposed expressway alignment, the risk appears to be greatest in the Bellamy Road area," according to a Dames & Moore report.

Expressway officials and independent experts say it's not considered cost-effective to perform detailed borings before choosing a road route.

"What you know when you take a boring is what is inside a 2{-inch hole," said Bob Smithem of HNTB Corp., the general consultant to the Tampa-Hillsborough County Expressway Authority. "You don't know what is three inches away. You hope that if there is something down there big, you hit it."

Even if they had known of possible sinkhole conditions at Bellamy Road before choosing the route, officials might not have acted differently.

"You don't know if (sinkholes) are going to develop," Smithem said. "You don't know where they are going to develop. And they are relatively easy to deal with. Other factors, like wetlands or upsetting homes and families, can be much more important."

Smithem and other officials say they have the engineering techniques to deal with sinkholes as they appear. The combination of polyester geogrid and cement grout is "probably the most effective technique available at this time," said Paul Pilny of the U.S. Soil Conservation Service in Sumter County.

In December, a sinkhole appeared in the path of the southbound lanes, about one foot deep and 15 feet across. Also in December, a second sinkhole was seen south of the first, about 80 feet past the planned embankment wall. It was eight feet deep and 25 feet across. Because the second sinkhole is not on highway property, officials did not repair it.

In January, yet another sinkhole appeared in the path of the southbound lanes. It was one foot deep and seven feet across. It was about 15 feet away from the first sinkhole.

Buggy, the Dames & Moore engineer, said subsequent test borings showed no connection between the two sinkholes in the roadbed and the larger one outside the right of way.

Workers have just completed filling underground crevices around the two sinkholes with about 525 cubic yards of cement grout. They will continue to monitor conditions closely as completion nears.

Life in Florida includes sinkholes

Driving into a sinkhole in darkness was no nightmare for Eddie Warren. It happened.

Before dawn one morning last September, the Hernando High School janitor was driving on State Road 50, 45 miles north of Tampa. He was headed to Publix to pick up food scraps for his pigs.

But his Chevrolet pick-up got no further than the far lip of a hole that had opened in the pavement between 4 a.m. and 5 a.m. Moving at 40 mph, he never saw it.

The truck came to rest across the hole, scraped ground and caught fire. Warren, who hurt his back, was assisted by an onlooker.

He later told a reporter he'd been lucky.

"I could have gone in," he said.

By afternoon, the hole had grown to 30 feet wide and 20 feet deep. It extended to both lanes. Construction crews quickly filled it in that night.

The sinkhole had appeared in two brand-new lanes of State Road 50, part of a $4.2-million widening project. It was suggested the sinkhole was triggered both by the construction activity and recent rains, which can accelerate underground erosion of limestone.

Engineers had done some test boring of the roadway before building. But they didn't expect the results to be able to rule out sinkholes.

"It's very easy to miss them," said Chris Wert, an engineering coordinator for Hernando County.

Veterans Expressway officials said their geogrid installation will prevent any sudden collapse similar to the Hernando County one.

Coping with sinkholes _ or even the possibility of sinkholes _ is a fact of life for anyone in Florida, they said.

"You pretty much have to live with that," said Ray Speer, executive director of the Tampa-Hillsborough County Expressway Authority.

_ Times researcher Barbara Hijek contributed to this report.