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Florida Geological Survey - Hazards - Problem Soils
Page heading: Problem Soils

Shrinking, Swelling, and Subsiding Soils

The soil beneath homes or buildings may contain naturally occurring earth materials that can cause those structures to become damaged. Soil beneath a home or other building may shrink or swell, damaging the structure. The water table naturally moves up and down (fluctuates) in response to the amount of rain an area receives. If large amounts of rain cause the water table to be higher than usual some types of subsurface clay may swell with resulting foundation damage. Drought may result in lowering of the water table causing some types of clays to shrink. This may also result in foundation damage such as cracking.

The Hawthorn Group is a geological layer that lies beneath much of Florida. Smectite, a type of clay that may shrink during times of drought and swell if it becomes wet, is common throughout the Hawthorn Group. The chemistry of the water that comes in contact with smectite may contribute to the shrinking or swelling effect. In some areas swelling clays are referred to as “pipe clay.” Contractors may recommend soil tests for various types of problem soils including pipe clay before foundation design is finalized.

Organic matter is extremely common in subsurface sediments of Florida. The organic matter is formed from the remains of wetland land plants that were deposited over hundreds of years. These deposits can be covered by other types of soils so they can’t be seen at the surface. When organic matter is exposed to air it may become oxidized and slowly destroyed http://sofia.usgs.gov/sfrsf/rooms/sustain/sugarcane/conserving.html. It may also lose volume or shrink due to compaction. Slow sinking or subsidence results from these processes and it has the potential to cause foundation and structural damage such as cracking.

Damage such as cracking of foundations and other structural problems can be caused by sinkholes and subsurface voids as well as problem soils. When this type of damage occurs homeowners are advised to contact their insurance companies. Various tests are usually performed to determine the cause of the damage.

Last updated: November 10, 2014

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