As the water content of a clayey soil increases, the behaviour changes from a crumbly solid to a mouldable plastic to a liquid, as can be experienced in everyday observations. The Atterburg limit tests measure the water content that the soil ceases to be a solid (the Plastic Limit) and the water content at which the soil behaves as a liquid (the Liquid Limit). Water content is the ratio of the mass of water to the mass of dry solid, and is given as a percentage.

The difference between these two numbers, which are expressed as percentages, is termed the Plasticity Index. The higher the plasticity index, the greater the shrinkability of the soil and the greater its potential to shrink and heave with changing water content. NHBC Chapter 4.2 categorises soils based on their Modified Plasticity Indices. These categories, along with the proximity of trees, are used by designers to specify foundation depths for new buildings. Soils with the highest Modified Plasticity Indices require the deepest foundations.