What are active and dormant areas of slab-on-grade foundations?
When a slab-on-ground foundation is placed in the ground, the presence of the slab inhibits the ground under the foundation from wetting up or drying out due to wet and dry weather respectively. This is most pronounced in the central area of the foundation. Since this area is protected from seasonal weather related changes in the soil moisture, the ground supporting the central area of the foundation does not shrink and swell very much compared to the area of the foundation near the perimeter or edge of the foundation. Most of the soil shrinkage and swelling occurs in the area of the foundation within around 8 to 10-feet of the foundation edge. This area near the edge of the foundation is called the active area since this where most of the soil distortion due seasonal weather changes occurs. The following sketch shows the relative locations of the active and dormant areas of a rectangular slab-on-ground foundation.
