Shrink-swell capacity

The shrink–swell capacity of clay refers to the extent certain clay minerals will expand when wet and retract when dry. Soil with a high shrink–swell capacity is problematic and is known as shrink–swell soil, or expansive soil. The amount of certain clay minerals that are present, such as montmorillonite and smectite, directly affects the shrink-swell capacity of soil. This ability to drastically change volume can cause damage to existing structures, such as cracks in foundations or the walls of swimming pools.

Due to the physical and chemical properties of some clays (such as the Lias Group) large swelling occurs when water is absorbed. Conversely when the water dries up these clays contract (shrink). The presence of these clay minerals is what allows soils to have the capacity to shrink and swell. Some of these clay minerals are: smectite, nontronite, bentonite, chlorite, montmorillonite, beidellite, attapulgite, illite and vermiculite. The amount of these minerals in a particular soil will also determine the severity of the shrink-swell capacity. For instance, soils with a small amount of expansive clay minerals will not expand as much when exposed to moisture as a soil with a large amount of the same clay minerals. If a soil is composed of at least 5 percent of these clay minerals by weight, it could have the ability to shrink and swell.

This property is measured using coefficient of linear extensibility (COLE) values. If a soil has a COLE value greater than 0.06, then it can cause structural damage. A COLE value of 0.06 means that 100 inches of soil will expand by 6 inches when wet. Soils with this shrink-swell capacity fall under the soil order of Vertisols. As these soils dry, deep cracks can form on the surface, which then allows water to penetrate to deeper levels of the soil. This can cause the swelling of these soils to become cyclical, with periods of both shrinking and swelling.

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