Active & Dormant Areas

These two concepts are keys to understanding how slab-on-ground foundations work.      

What are active and dormant areas of slab-on-grade foundations?

When a slab-on-ground foundation is placed, 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.

At first, the entire slab is active

When the slab is cast, the entire slab area is active. Moisture in the ground becomes trapped underneath the slab foundation. The moisture causes the clay soils to swell. The ground under the foundation swells into a mound shape. The slab stiffening beams or footings settle into the supporting soil. The settlement is especially pronounced at the perimeter grade beam.

The middle area of a slab on ground foundation reacts to the growing mound by lifting. This process continues until the soil under the middle area of the slab has absorbed all the moisture it can. The entire slab will be active until the soil under the middle area has absorbed all the moisture it can take.

Until every area of the foundation is active: the perimeter rises when it rains, and falls when dry weather returns. The middle area lifts as the ground under the slab swells due to moisture being trapped underneath the slab.

As the slab ages, the middle slab area develops an active area and a dormant area

Typically, after around seven years, the mound underneath the middle area of the foundation can grow no more. The ground has absorbed all the moisture it can. Since the soil underneath the middle area of the slab is not significantly affected by seasonal weather changes, it becomes relatively dormant.

The perimeter, on the other hand, is subject to significant changes in soil moisture for the entire life of the foundation. Thus, the perimeter area never becomes dormant. It remains active as long as there are seasonal weather changes.

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