Venus sinkhole may have stabilized

What caused a large sinkhole to collapse beneath the feet of Luis Hernandez last week?

Watering in the nearby groves may have contributed, but most sinkholes have been eroding for hundreds of thousands of years, a South Florida Water Management District expert said Wednesday.

Orpha Hernandez, who runs a 10-acre nursery near U.S. 27 along with her husband, said the hole in their backyard has stopped expanding. The hole opened on Thursday afternoon where Luis Hernandez was standing.

"It's still the same size," Hernandez said Wednesday. "It has a lot more water in it now."

One day after the collapse, the hole had grown to 140 feet wide, and it had filled with 3 feet of water. On Wednesday, the water was probably about 10 feet deep.

Earlier, her husband had seen watering at the orange grove next door.

Extensive watering can cause sinkholes, said Tony Gilboy, well construction program manager, for Southwest Florida Water Management District, who has been investigating sinkholes for 30 years.

During the January cold snap, a rash of sinkholes occurred around Dover and Plant City as growers used copious amounts of well water for 11 days to protect fragile strawberries and vegetables, Gilboy said.

However, most citrus growers use micro irrigation, Gilboy said. They pump mist into the air, which consumes smaller amounts of water.

Geo-history

Florida is built on a layer of sand, a layer of clay, and a layer of water-soluble limestone rock. That carbonate platform develops voids and cavities, Gilboy said.

"That is the makeup of the aquifers, where we get water from," he said.

Over hundreds of thousands of those years, slightly acidic water flowing through the limestone erodes the bedrock structure, which becomes overburdened with clay and sand. First, the clay layer bends, then it collapses, rattling down into the limestone cavity.

There are over 2,000 reported self-reported incidents in the Florida Geological Survey database. Viewed from the air, the state is dotted with major and minor sinkholes, some of which have developed into lakes, Gilboy said.

The Hernandezes were told that if the sinkhole within a few hundred feet of their mobile home expands, they may have to move it.

Gilboy said sinkholes usually stabilize within a week, so it appears the critical period has passed.

Sinkhole Q&A

Why do sinkholes form? Acidic groundwater dissolves into underground cavities and caves in the limestone, which collapse.

My yard is settling. Do I have a sinkhole? Maybe, but there are other factors: buried organic material, buried trash or logs, broken pipes, poorly-compacted soil after excavation, and expansive clay layers that shrank upon drying.

I have a sinkhole. What should I do? Mark the hole and keep away children and pets. If walls are sinking, sagging, or cracking, stay out of the building. If the hole is under or near a building or swimming pool, property owner's insurance may cover assessment and repair. Call the property insurance adjuster.

How long does a sinkhole grow? A circular hole typically grows for minutes or hours. Slumping sediments along the sides may stop after a day.

How should a sinkhole be filled? Anything buried in the earth potentially affects groundwater. Use native earth materials, clean sand or concrete. Broken limestone rip-rap or concrete plugs the bottom of the sinkhole and creates a stable foundation. Add clay sand to form a barrier and prevent water from enlarging the hole. Add sand and top soil.

A sinkhole opened in my next door neighbor's yard. Should I be concerned? Florida sinkholes sometimes occur in sets, but most are isolated events.

Who should inspect my sinkhole? There is no agency with responsibility. Florida Geological Survey receives calls from homeowners, but does not have sufficient staff to visit all new sinkholes. FGS's database of reported subsidence incidences is available at www.dep.state.fl.us/geology

Do I need a permit to fill a sinkhole? No, if it is dry. Those containing groundwater may require an Environmental Resources Permit, available through the regional water management district.

More info: (850) 488-9380.

Source: Florida Division of Emergency Management