What are Fat Clays?
​Fat clays are cohesive and compressible clays of high plasticity, containing a high proportion of minerals that make it greasy to the feel. Difficult to work when damp, but strong when dry.

High plasticity clays have microscopic mineral grains that are especially attractive to water. While the clay generally can take on a tremendous amount of water and still maintain its strength, its volume will expand. It only takes one bad event like a broken water line or a large spill to wet the subgrade and begin the swelling process. Once the swelling begins, it can continue for years.

Concrete floors and foundations can lift or sink from either the pressures developed by the swell or from when the clay dries and begins to shrink. Depending on the changes in moisture content, the amount of shrinking and swelling can be from a ½ inch to 3 inches during the lifetime of a home.
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The process of slope failures in high-plasticity clays involves formation of surface cracks, moisture infiltration through the cracks into the soil mass, a reduction in suction and hence shearing resistance of the soil, and ultimately slope failure when the driving stresses exceed the shearing resistance of the soil. Similar processes can impact other earth structures such as retaining walls and pavements.​