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Florida Sinkhole Poses Multiple Challenges

Thu March 14, 2013 - Southeast Edition
Cindy Riley

Officials in Hillsborough County, Fla., continue investigating the sinkhole tragedy that claimed the life of a 37-year-old man and forced construction crews to tear down his Tampa home. Despite an initial search, engineering experts deemed the situation too dangerous to retrieve the victim, Jeff Bush, who was killed when the massive hole opened up underneath his bedroom.

“County and contractor crews delicately demolished the home,” said Willie Puz, division director of public information of Hillsborough County. “Work started on Sunday, March 3, with a county operator and was completed by contractors on Tuesday, March 5. Extra care was taken to try to retrieve any important personal items the family identified. For safety reasons, engineering crews limited the distance the heavy equipment could get to the house.”

Crews used specialized equipment to remove a number of personal items that included bibles, antique firearms, photos, military medals, a china cabinet and a dresser, along with its contents. A trackhoe with an 80-ft. arm and bucket along with a trackhoe with a shorter arm with grapple on the end were both used. Crews also brought in equipment to probe the soil around the home as news broke that a second sinkhole had opened up between two houses — one of them vacant — just three miles from the Bush home.

“The hole is completely filled with gravel, the three homes have been fenced off, and County Code Enforcement officials are evaluating what can be done with the other two houses. What else might take place is still being determined.

“The county’s heavy equipment operator was requested by the family to see them,” said Puz. “This county operator was able to salvage the first items for the family, and ones that meant the most to them. This meeting was very emotional with embraces and lots of tears. It just shows the proficiency and dedication of heavy equipment operators to make a difference in people’s lives in times of crisis.”

Hillsborough is the largest county in the metropolitan area and is the fourth most populous county in Florida. Sinkholes are a common problem throughout the state, but no one was prepared for the latest episode.

“It was shocking news,” said Dr. Shimon Wdowinski, research associate professor of the division of marine geology and geophysics of the University of Miami.

“We are very aware of the sinkhole problem in central western Florida. Until last week, the main issue was property damage, which can be problematic. But still, it’s only property damage. To my knowledge, this recent event was the first sinkhole collapse that claimed life. It is very unusual.”

Located on Florida’s west coast, Hillsborough County is part of an area known as “sinkhole alley.” Wdowinski said acidic rain can eat away the limestone and natural caverns under much of Florida, causing sinkholes.

“We usually worry about sinkhole collapse. If a sinkhole is already open, it still can be dangerous. But we are aware of the danger. The problem is in areas where the sinkholes, which are large cavities in the subsurface, are covered with layers of sediments, soil or other soft material. In these cases, the subsurface cavity can grow by eroding the overriding weak layer into the cavity. As the cavity grows, the upper weak layer get thinner and eventually collapse into the sinkhole. The erosion of the overriding layer depends on changes in groundwater levels. In natural conditions it often happens after heavy rain. In controlled conditions it often happens with the extraction of groundwater during drought years or dry seasons. In central western Florida there are many cases of sinkhole collapse because the geological conditions are suitable.

Wdowinski said moving dirt or soil can cause weakening and lead to a collapse.

“Geophysical instrumentations, as Ground Penetrating Radar (GPR), can detect cavities in the subsurface. In a sinkhole-prone area, it will be wise to run such survey prior to construction.”

The Florida Department of Environmental Protection (DEP) Geological Survey said that sinkholes are a natural and common feature of Florida’s landscape. They are only one of many kinds of karst landforms, which include depressions, caves that are both air and water filled, disappearing streams, springs and underground aquifer systems, all of which occur in Florida. Thousands of naturally occurring sinkholes can be seen throughout the state of Florida including many that connect underground to springs, rivers and lakes.

According to the survey outlined on the DEP’s Web site, sinkholes form in karst terrain from the collapse of surface sediments into underground voids. In this region of the country, solution sinkholes, cover-subsidence sinkholes or cover-collapse sinkholes may be spotted. The first two types will show only slight topographical disturbance to the naked eye, while the third shows an abrupt change in topography and is most associated with the idea of sinkholes.

If a homeowner’s yard is settling, a number of factors can cause holes or depressions. Expansive clay layers in the earth may shrink upon drying, buried organic material, poorly-compacted soil after excavation work, buried trash or logs and broken pipes all may cause depressions to form at the ground surface. If the settling is affecting a dwelling, further testing by a licensed engineer with a licensed geologist on staff or a licensed geology firm may be in order.

Although sinkholes in Florida sometimes occur in sets, most are isolated events. An inspection of land for any sinking or soft areas, however, is generally encouraged. If a sinkhole opens in an area nearby, the hole should be immediately cordoned off and clearly marked to protect traffic. Local law enforcement should be contacted, and city or county road department crews should be called to begin repair work.

Kris Carson, public Information officer of the Florida Department of Transportation (FDOT) said, “When our own staff, or even a citizen, notices a dip in the road that seems unusual, we will get phone calls, and that’s when we will start the investigation process. Our maintenance staff will usually first arrive on scene to take a look at the situation. If we feel the need to do further investigating, we will bring in geotechnical companies. These companies will use ground-penetrating radar, our primary method, then geotechnical drilling is usually performed.

“As all of this is happening, we are developing a repair plan,” said Carson. “In District Seven, which is the Tampa Bay area, we use the companies PSI and Tierra, both based in Florida. Depending on what we find, we will then find a grouting company to perform the repair work.”

Matt Boles, executive director of the Greater Florida Associated General Contractors, said the recent tragedy may prompt some companies to take extra safety precautions.

“I think the biggest thing contractors need to be aware of is soil conditions. They need to know the make up of the soil, be it sand or, for example, clay, which clumps together and can shift. Anytime you’re putting a foundation down, it would be wise to do a study of what’s under the sand or soil.”

Boles added, “Anytime there’s a construction-related accident, contractors tend to pay particular attention to whatever the issue in moving forward. The construction industry is good about keeping up with safety and liability issues, and sinkholes would definitely be one of them. If it becomes a big enough issue, I wouldn’t be surprised if the government enacted legislation that requires you to do a sonogram before you build. But the construction industry tends to stay ahead of these things, so I think they’ll be more in tune to sinkholes in the future.”

Boles admitted, “I was kind of shocked Tampa has experienced this. I think people who are not in sandy soil have to be more aware. It’s something other states need to look out for more than Florida, in my opinion. I would think mountainous areas would be more susceptible to sinkholes than here.”

As for dealing with likely sinkholes, Wdowinski said the most important step, understandably, is preventing human tragedy.

“If we can detect a sinkhole before it collapses, we can save a life, but we may suffer property damage. In some cases, people try to fill a sinkhole with soil or dirt. However, this is a temporary solution, because the processes that erode the upper weak layer into the cavity still occur. So, it’s only a question of time until the fill material will subside into the sinkhole.”

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