Expansive soil, also known by such names as expansive clays, expandable soils, shrink-swell soils, and heavable soils, can produce significant forces on slabs, foundations, retaining walls, underground pipes and other structures resulting in property damage. These soils contain some proportion of clay minerals such as smectite, bentonite, montmorillonite, or others that have a propensity to adsorb water. The chemistry of these materials is such that they can adsorb significant quantities of water thus swelling or expanding the volume of the soil mass. Likewise, if the soil mass were to lose this water, the soil would collapse or shrink and experience a notable decrease in volume. As is evident from this discussion, the proportion or percentage of the soil mass that is composed of clay minerals, as well as the type of clay minerals, will determine the potential for more or less volume change.
One of the primary means by which expansive soils can influence a structure is through this expansion process due to an increase in moisture content. Oftentimes this influence is called “heave” as it induces an upward force on any structure supported by the soil. Additionally, it can create increased lateral earth pressure forces on retaining walls, basement walls, and foundations.
When an expansive soil is dried out it will shrink, pull away from, and remove support from the overlying structure. This subsidence can result in settlement of the structure.
This shows how significant variation in the moisture content of the soil is to these issues. It is not so much a high moisture content or a low moisture content as it is the moisture content fluctuation or differential over time and/or over an area. Two types of fluctuation should be considered. First is a uniform global fluctuation. This is often due to seasonal changes in rain patterns like a drought or a rainy season. These changes will affect the structure, site, and soil mass as a whole. Second is localized fluctuations. These are due to things like topography and site drainage, structure rainwater runoff patterns, a water leak, or irrigation systems. These issues can affect a particular area of the supporting soil mass while not affecting another area depending on the situation. These issues should be considered in both the analysis of damage patterns as well as designs for remediation. 
Another useful fact in the analysis of potential expansive soil influence on a structure is confinement. Using the basic principles of mechanics, the downward pressure provided by a structure will tend to confine the upward heave of an expansive soil mass. Since the downward pressure at every point in a structure is not equivalent, the areas with greater load should heave upward less than the areas with less load applied. Put another way, “Soil movement will be minimized where confining pressures are the largest while movement will be greatest where the magnitude of the confining pressure is the smallest.” 
Identification / Classification
In order to identify and classify a soil with regard to its expansion potential, a soils map, field observations, and/or laboratory tests must be considered. Geology.com provides a general U.S. soils map showing the geographic distribution of the swelling potential of area soils. However, as discussed on the site, this is very general information and is not meant to provide detailed information for individual properties. With regard to field observations, a good indicator of an expansive soil is visible desiccation cracks. These result from and show the dimensional shrinkage in the soil mass. Also, the damage patterns observed and their agreement with the previously discussed mechanics can indicate an expansive soil condition. Numerous lab tests and correlations with soil properties can be performed to determine the categorization of soils with regard to swelling potential. In general, a liquid limit in excess of 40 and a plasticity index in excess of 15 can be considered an expansive clay.  Also, a simple mold test can give an idea of swelling potential. This is done by placing a moist sample in a mold of known volume, drying the sample, and measuring the volume of the dried sample which has shrunk away from the mold. The change in volume divided by the dry volume will give the percent of expansion. Soils having a percent expansion in excess of 10% are at least somewhat expansive.
In a soil mass, there is an upper layer where the moisture content tends to vary over time. Deeper into the soil column the variation diminishes until at a certain depth there is minimal and uniform variability. This upper layer is known as the active zone. The depth of this zone can be determined through testing. Within this region, an expansive soil will tend to expand and shrink. Below this region, the volume will be relatively stable. The depth of the active zone is an important piece of information as it is used in determining the amount of heave and is the depth beyond which drilled shaft isolated footings need to be founded in order to function properly outside the influence of the expansive soil volume changes.
Magnitude of Swell
The amount of heave or swell (vertical distance the free surface will rise) can be determined through two common soil tests. These are the unrestrained swell test and the swelling pressure test. The details of these tests are beyond the scope of this introduction.
Dealing with Expansive Soils
Several different measures can be taken to address placing structures in areas with expansive soils. First would be to remove the expansive soils to a certain depth and replace them with non-expansive soil which is then properly compacted. This is relatively straightforward but can be costly. Second would be to change the soil by such means as compaction, prewetting, installing some means of controlling the variation in soil moisture content, or the application of chemical treatments or stabilizers to the soil. Third would be to design the structure such that it is strong enough and/or flexible enough to compensate for the projected heave/shrinkage or circumvent the expansive soil by installing a deep foundation beyond the depth of the active zone. Any combination of these techniques could also be utilized.
 “Expansive Soil Problems and Solutions.” Your Foundation Repair Guide-An Underground World Revealed. N.p., n.d. Web. 29 June 2017
”Damage To Foundations From Expansive Soils.” ResearchGate (1985): 1-7. Web.
Das, Braja M. Principles of foundation engineering. Boston, MA: Cengage, 2016. Print.