Clays generally undergo changes in volume with corresponding changes in their water contents. When they are dried, shrinkage and cracking occur, and if they are rewetted after drying, swelling occurs. The amount of swelling after rewetting in the presence of available free water depends primarily on the type of clay minerals in the clay. Clays containing montmorillonite show an almost reversible swelling and shrinking on rewetting and redrying, whereas clays containing kaolinite or illite show an initial large volume decrease on drying with only limited swelling on rewetting. Table 4.1 shows the average particle size and swelling capability of these minerals. The former (high volume change) clays are often referred to as high swelling clays and the latter (limited swelling on rewetting) are generally identified as low swelling clays.