Manatee Home Inspection Services Blog: Everything You Need to Know About SINKHOLES

By Jenni Boucher

During the weekend of August 10-11, 2013; the Summer Bay Resort near Disney lost an entire building to a sinkhole, raising awareness of this very important issue. Florida, especially the central part of the state, is known for having a large number of sinkholes compared to other states in the U.S.; however sinkholes occur all over the country and around the world. As fellow Floridians, we feel that you need to be prepared in the event that you ever experience one for yourself.

A sinkhole opening under a FL home

What is a sinkhole? This has been a question posed by many homeowners and their insurance companies. According to Wikipedia, a sinkhole is a natural depression or hole in the Earth’s surface which may have various causes. In most cases, sinkholes occur in areas where the ground is very moist, but they can also occur in even the driest of climates. They can vary in size from 3.3 feet to over 2,000 feet wide, and several hundred feet deep. Hernando, Pasco and Hillsborough counties are now being known as “sinkhole alley” since 2/3 of all sinkhole cases reported in 2006 to 2010 in Florida came from these counties. How many sinkholes occur in Florida? In 2006, more than 2,300 claims were reported. In 2010, that figure jumped to almost 6,700! The total cost of damage covered by insurers during this period was $1.4 billion.

A sinkhole opening up in a FL back yard

Sinkholes are usually caused by water movement underground. This could be because of an underground stream or river, a dramatic change in the water table due to drought or heavy rain, heavy ground water pumping to preserve strawberries and citrus in cold months, a water or sewer main break, or in coastal areas due to a failing/cracked seawall. Many times in northern states, it can be caused by the collapse of an underground cave or mine. The rock underground is eroded by the movement of the water, a process called suffusion. Carbonate rocks such as limestone or salt beds are the most susceptible to suffusion when circulating ground water removes the carbonate cement that bonds these types of rocks together.

A sinkhole caused by rainwater leaking through the pavement and moving soil into a ruptured sewer line below.

Do you think you may have a sinkhole? Sinkholes are different from other holes that may occur in the ground from such things as burrowing rodents or other animals. With a sinkhole, the soil will have collapsed into the void under the surface, not be dug out such as with an animal hole. They often appear overnight when the ground is moister, and expand rapidly. If the sinkhole is occurring under the structure, the first visible signs (and sometimes audible!) will include cracks in the walls, ceiling and/or slab, movement in floor tiles, a sudden decrease in water quality (broken pipe) or visible gaps between the walls and ceiling. Most of these things can occur over time with settling in the foundation, but with a sinkhole present, these changes will occur over a period of hours to weeks. It is important to monitor changes in these types of cracks. The easiest way to do that is to draw a straight line with a pencil perpendicular (straight across) the crack. Monitor any changes to how both sides of the line come together. With proper monitoring, this will give you some idea as to how fast the crack is expanding.

If you suspect that you have a sinkhole on your property, it is imperative that you seek out the services of a geotechnical engineer to identify the hole and advise you how to proceed. It would be advisable to vacate the premises immediately and take any non-replaceable valuables with you, at least until the engineer has evaluated the hole.

Once a sinkhole has been identified, and depending on its severity, there are ways to save your home. In some cases the home is a total loss, in which case it will need to be demolished and re-built. In most cases the home can be saved if the sinkhole is caught early.

The most common method of filling in a sinkhole is pumping expandable concrete grout under the home to fill the void. However, in the opinion of many structural engineers, this is only part of the overall remediation process. Most structural engineers recommend the installation of metal rods/beams and/or concrete pilings/beams under the home to support the weight of the home on the stable bedrock below, followed by concrete grouting. There are many types of companies that can do these types of repairs, including general contractors, foundation contractors, building relocation companies, and even sinkhole specialists. Whichever type of company you choose, make sure that the company has experience working with sinkholes, holds some type of construction license, has adequate insurance in case an issue occurs, and is working in conjunction with a structural engineer. Too many times there have been recurring issues with sinkholes that have been improperly repaired, only to cause issues for the future owners of the home.

Diagram showing typical under-pinning and filling of the sinkhole with concrete grout.

Another consideration for all Floridians is sinkhole insurance. Many property insurance policies do not cover sinkholes, leaving homeowners with no place to live and no money to repair/replace their home if they are not covered. Make sure that your property insurance includes coverage for sinkholes, since they can happen to any Floridian, at any time.

Sinkholes can be catastrophic if not caught early, and repaired somewhat easily if they are. Since we are in Florida’s summer rainy season, it is very likely that more sinkholes will appear. If you suspect a sinkhole on your property, make sure to call a professional immediately for the safety of your home and your family.