Expansive soil is defined by Chapter 18 of the International Buidling Code and many types of clay, sandy clay and even clayey sand meet this definition.  When the moisture content of expansive soil increases, the soil volume can dramatically increase.  When the moisture content of expansive soil decreases, the soil volume can dramatically decrease.  As an example, the potential vertical movement up and down is estimated to be 6 inches in some areas.  Variations in soil moisture content due to wet and dry seasons or other sources can cause the soil volume to shrink and/or swell non-uniformly.  

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Solutions for expansive soil can be successfully applied to other cases of volumetric soil changes such as settlement, frost heave and collapsible soil.

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The subgrade under slab-on-ground foundations are typically modified to reduce the potential vertical movement and the slab will generally rise and fall if the subgrade swells and shrinks.  However, under isolated slabs (slabs over an under-floor space to separate the slab from expansive soil) even if the subgrade is modified to reduce the potential vertical movement, the slab is essentially at a fixed elevation.  Non-isolated plumbing under isolated slabs has led to a great deal of damage, and consequently major litigation, over the years.

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Learn more here about how current codes require protection of plumbing from expansive soil, how the only effective way to protect plumbing under isolated slabs is to isolate the plumbing, and how a change in the 2024 International Buildiong Code will explicitly require isolation of plumbing under isolated slabs so that buried plumbing and non-isolated void systems are prohibited under isolated foundations.