Expansive clay is a clay that is prone to large volume changes that are directly related to changes in water content.
The mineral make-up of this type of soil is responsible for the moisture retaining capabilities. Soils with smectite clay minerals, including montmorillonite, have the most dramatic swelling properties. Expansive soils typically contain one or more of these clay minerals: montmorillonite, smectite, bentonite and illite.
Mitigation of the effects of expansive clay on structures built in areas with expansive clays is a major challenge in geotechnical engineering.