<P> 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 . </P> <P> 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 . </P>

Expansive soils expand and contract due to changes in what
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