There are different types of soil used under foundations. Each has its own special properties and negative qualities. The kind of soil that is used is chosen based on the kind of property that is being built, what the weather is like in the area, and the site manager’s personal preference.    Soil that has 10% or more clay content has the most shrinkage capacity. Organic matter can be mixed into clay soils to make them less malleable, and although this works in agriculture, in construction, mixing different types of clay increases the risk of an uneven foundation. Mixing is also very time-consuming, which is why engineers tend to stick with one soil type when it comes to construction projects.    Clay Soils    Clay soils are those that contain traces of clay in them. There’s the traditional expansive soil and loam. Expansive soil has the highest chance of shrinking and compacting because they are highly absorbent. Expansive soil is still used in construction, however, because it holds together extremely well, which saves time when compacting the soil. Its pliability makes it easy to manipulate, so it’s still used despite its swell-shrink capacity.    Loam is considered to be the best kind of soil for under foundations. It’s a mix of silt, clay, and sand. It’s pliable enough to compact well, but it doesn’t have as much clay, so its swell-shrink capacity isn’t as significant as clay-rich soils. However, loam has clay content in it, so it’s still able to shrink.    Sand and Gravel
Sand contains very little clay content. Because of this, it doesn’t hold moisture very well and it doesn’t shrink when dried out, since compaction is very minimal. However, because it doesn’t compact well, it’s loose and has a higher chance of shifting.    The same thing happens if the foundation is made of rock or gravel. The particles are too big, so water flows right through. If the water dries up, the gravel will still stay in place. However, the air between each individual rock leaves room for shifting. This is why sand and gravel are rarely used in places where earthquakes are frequent.