An expansive clay is a clay capable of producing large changes in volume, in direct relation to changes in soil moisture.

Mitigating the effects of an expansive clay on structures made in areas with expansive clay is a challenge in geotechnical engineering. Clays expand with moisture and contract when dry, forming deep cracks. This process favors the mixing of materials from deeper horizons, since when the cracks are filled with external material, when the clay rehydrates, it expels part of the deeper material due to the lack of space. This process repeated for years ends up generating vertisol type soils.