Expansive clay is a clay that is prone to large volume changes that are directly related to changes in water content. Mitigation of the effects of expansive clay on structures built in areas with expansive clays is a major challenge in geotechnical engineering. Cities built on top of clay compact the clay beneath as groundwater is drawn down and water is wringed out of the clay. Rebound caused by heavy rain can destroy or crack buildings, and permanent land subsidence may occur in large cities with significant skylines.
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