Expansive Soil and Expansive Clay
The hidden force behind basement and foundation problems
What is an "Expansive Soil"?
Expansive soils contain minerals such as smectite clays that are
capable of absorbing water. When they absorb water they increase in
volume.
The more water they absorb the more their volume increases. Expansions
of ten percent or more are not uncommon. This change in
volume can exert enough force on a building or other structure to
cause damage.
Cracked foundations, floors and basement walls are typical types of damage done by swelling soils. Damage to the upper floors of the building can occur when motion in the structure is significant.
Expansive soils will also shrink when they dry out. This shrinkage can remove support from buildings or other structures and result in damaging subsidence. Fissures in the soil can also develop. These fissures can facilitate the deep penetration of water when moist conditions or runoff occurs. This produces a cycle of shrinkage and swelling that places repetitive stress on structures.
Cracked foundations, floors and basement walls are typical types of damage done by swelling soils. Damage to the upper floors of the building can occur when motion in the structure is significant.
Expansive soils will also shrink when they dry out. This shrinkage can remove support from buildings or other structures and result in damaging subsidence. Fissures in the soil can also develop. These fissures can facilitate the deep penetration of water when moist conditions or runoff occurs. This produces a cycle of shrinkage and swelling that places repetitive stress on structures.
How Many Buildings are at Risk?
Expansive soils are present throughout the world and are known
in every US state. Every year they cause billions of dollars in damage. The
American Society of Civil Engineers estimates that 1/4 of all homes in the United
States have some damage caused by expansive soils. In a typical year in the United States
they cause a greater financial loss to property owners than earthquakes, floods,
hurricanes and tornadoes combined.
Even though expansive soils cause enormous amounts of damage most people have never heard of them. This is because their damage is done slowly and can not be attributed to a specific event. The damage done by expansive soils is then attributed to poor construction practices or a misconception that all buildings experience this type of damage as they age.
Even though expansive soils cause enormous amounts of damage most people have never heard of them. This is because their damage is done slowly and can not be attributed to a specific event. The damage done by expansive soils is then attributed to poor construction practices or a misconception that all buildings experience this type of damage as they age.
Expandable, Shrink-Swell, Heavable Soils?
Expandable soils are referred to by many names. "Expandable soils", "expansive clays",
"shrink-swell soils" and "heavable soils" are some of the many names used for these materials.
Expansive Soils Map
The map below shows the geographic distribution of soils which are known to have
expandable clay minerals which can cause damage to foundations and structures.
It also includes soils that have a clay mineral composition which can potentially cause damage.
How to Interpret the Map
The map above is meant to show general trends in the geographic distribution of
expansive soils. It is not meant to be used as a property evaluation tool.
It is useful for learning areas where expansive soils underlie a significant
portion of the land and where expansive soils might be a localized problem.
All construction projects should include a soil analysis to identify the types of soil present and determine their expansive properties. Local occurrences of expansive soils can be found in all of the soil categories shown on this map.
All construction projects should include a soil analysis to identify the types of soil present and determine their expansive properties. Local occurrences of expansive soils can be found in all of the soil categories shown on this map.
Why Do These Soils Expand?
Soils are composed of a variety of materials, most of which do not expand in the presence of moisture.
However, a number of clay minerals are expansive. These include: smectite, bentonite, montmorillonite,
beidellite, vermiculite, attapulgite, nontronite, illite and chlorite. There are also some sulfate salts
that will expand with changes in temperature.
When a soil contains a large amount of expansive minerals it has the potential of significant expansion.
When the soil contains very little expansive minerals it has little expansive potential.
Changes in Moisture Content Trigger Damage
When expansive soils are present they will generally not cause a problem if their water content
remains constant. The situation where greatest damage occurs is when there are significant or repeated
moisture content changes.
The Bottom Line
It is possible to build successfully and safely on expansive soils
if stable moisture content can be maintained or if the building can be insulated from any soil volume
change that occurs. The procedure for success is as follows:
- Testing to identify any problems
- Design to minimize moisture content changes and insulate from soil volume changes
- Build in a way that will not change the conditions of the soil
- Maintain a constant moisture environment after construction
