Florida Geological Survey - Hazards - 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
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
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.