Beware of Fat Clay

When building a home, you should have the soil on the construction site tested for plasticity to determine how it will change under different moisture conditions. This is important because some soil is so “plastic” that (if the condition is not corrected) it can actually break concrete floors, crack the walls and destroy the foundation of your home. Highly plastic soil is sometimes referred to as “fat clay” that swells excessively and loses stability when it becomes wet. Fine-grained soils that contain a high percentage of clay are greatly affected by water, turning to mud when it rains and undergoing large volume changes-sometimes up to 40 or 50 percent, as the soil dries. Not only do clays swell and lose stability when they become wet, but they also retard the drainage of water. If plastic soil is found on your building site, the ground must be over-excavated and replaced with clean gravel. Homebuilders often choose not to include a basement, or to protect the basement walls from excess moisture by surrounding them with a layer of gravel. A fine-grained clay-filled soil with a high plasticity index may require considerable treatment, especially if used in a moist location.