One of the most common types of foundation designs for use on expansive soil uses widely spaced, deep in-ground concrete beams, connected by a thin concrete slab that is usually 4” thick. The foundation beams and slab are all in full contact with the underlying expansive soil. When the soil gets wet and expands with a force that exceeds the total gravity load of the house and the foundation, the foundation and house structure move upward. The expanding soil has nowhere to go.
The above drawing shows how the forces of expanding soil push up on the underside surface of the foundation. It also shows the uplifting friction forces, caused by the expanding soil, on the vertical sides of the foundation beams.
For low to medium expansion soil, the typical Wafflemat foundation will usually be built on a flat site, as shown in the following drawing:
For high and very high expansion soil, the Wafflemat system is used with deepened beams spaced at intervals determined by the foundation design engineer. Wafflemat can also be used to enhance other foundation systems, such as the Ribbed Slab design, by having the Waffleboxes located between the deep beams of the Ribbed Slab. The Waffleboxes absorb the volume change of the expansive soil.
The above drawing shows a Wafflemat design for high to very high expansion soil, which uses some deeper beams for greater rigidity. Note that a hybrid design using both the Wafflemat and the Ribbed Slab foundation systems can also be used as shown above for high to very high expansion soil. When the soil at your site is highly expansive, Wafflemat allows you to safely build on the site.