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What is expansive clay soil
03/06/2020

What Is Expansive Soil?

Here at Dalinghaus Construction Inc., we talk a lot about foundation settlement and foundation heave. Expansive soils are responsible for both these phenomena.

What is Expansive Soil?

Expansive soil is a soil/clay (such as montmorillonite or bentonite) that is prone to expansion or shrinkage due directly to variation in water volume. Expansive soils swell when exposed to large amounts of water and shrink when the water evaporates. This continuous cycle of wet to dry soil keeps the soil in perpetual motion causing structures built on this soil to sink or rise unevenly, often requiring foundation repair. Expansive soils are comprised primarily of minerals (incredibly fine particles) with little to no organic material and are thus incredibly viscous, proving difficult to drain.  

So, let’s unpack this a bit. Expansive soil is generally a clay that is inherently susceptible to swelling and shrinking due to its chemical composition. This swelling and shrinking is directly related to changes in the water table.

Wet = Expand

Dry = Shrink

Now, expansive soils are referred to by names, including – expanding soil, expansive clays, shrink-swell soils, and heavable soils. Turns out, dirt has a heckuva lot of sobriquets.

Check out our exhaustive Ultimate Guide To Foundation Repair

Expansive soils contain minerals, such as smectite clays, that are prone to absorb copious amounts of moisture. When they take on water, they increase in volume. The more water they absorb, the more their volume increases.

*Note – Expansive soils are known to expand to 10% or more (and this is not uncommon). Then, in colder areas of the great USA, frost can expand water by a whopping 9%!    

This change in volume can exert enough force and enough pressure on a home or other structure to cause serious cosmetic and structural damage. This pressure can be as great as 5,000 pounds per foot.

Expansive soils also shrink, compress when they dry out. This shrinkage can remove structural support from a home or structure and result in damaging subsidence. Large cracks and fissures can also develop in the soil  

This repetitious cycle of swelling and shrinkage puts serious stress on your home’s foundation causing either foundation settlement or foundation heave – both will get worse over time.

Although expansive soils can be located in almost every state, the problems related to expansive soils are the most severe and widespread in California, Nevada, Arizona, Colorado and other western/southern states.  

Fun Facts with Brian – Each year in the United States alone, expansive soils cause $2.3 billion dollars in damages to houses, roads, pipelines, and other structures. This is more than twice the damage from floods, hurricanes, tornadoes, and earthquakes combined.  

A list of expansive soils include:

  • Smectite
  • Bentonite
  • Montmorillonite
  • Beidellite
  • Vermiculite
  • Attapulgite
  • Nontronite
  • Chlorite
  • Pedialyte (for after those hard nights when the other expansive soils soak up liqour like a sponge)  

Currently, builders are required by law to have a Geotechnical Engineer (G.E.) prepare a soils report to identify the expansive soils and give the builder recommendations to build a home that will withstand expansive soil.

The engineer may recommend a post tension slab for a new home for example or push/helical piers to push through the active zone of soil to hit bedrock for an existing project.

In addition, these recommendations often include removal of the expansive soils, importation of non-expansive soils, soil chemical treatments, a post-tensioned or structural floor foundation, gutters, and downspouts.  

*Note – Proper drainage is a must for expansive soil treatment.   

Not sure you have expansive soil?

The majority of the time, the expansive soil is going to put undue stress on your foundation, whether it be slab on grade, raised, basement, post-tension, or pier & beam. (Check out our  5 Types of Residential Foundations Popular in America: A Brief Foundational Overview.  

This stress/pressure results in signs and symptoms that can be traced directly back to foundation issues instigated by expansive soil. Read the following articles to get a better understanding of the signs and symptoms:

If you think that you might have expansive soils and are concerned it might be impacting your home, health, or peace of mind, click on our link below for a FREE foundation inspection if you live in Central AZ or Southern California -

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