Foundations rest on soil, soil pushes against their sides, and wet soil pushes water and humidity against them, so it's hard to plan for a foundation without a basic understanding of soils. The average person thinks of soil as dirt. For engineers, soil is a complex material worthy of a lot of study. In fact, there are thousands of soil varieties, but the main categories are gravel, sand, silt, and clay. What separates them is basically the size of the particles. Gravel is made of big chunks; sand consists of grains as small as the width of a human hair; silt is made of still smaller particles that are nearly microscopic in size; clay has particles too small to see. Most soils are blends of these main types, with names like “clayey sand” or “sandy silt.” Soil also has air and water mixed into it, so compacting the soil with rollers, pounding or vibrating equipment densifies and strengthens it.
Getting Down to the Dirt
To be absolutely sure of your soil, you have to send a sample to a soils lab. If they find more than 12 percent clay, the clay will be analyzed for its behavior when wet. This is because clay can turn to liquid, reduce the soil's bearing strength, and cause the soil to exert pressure on the foundation. On a large commercial project, soil “borings” are taken vertically in two-foot increments. On a residential project, builders often rely on instinct and rule of thumb, because some building departments don't insist on a soils report. Unfortunately, it can be hard to identify a soil by eye, or to predict its behavior by guesswork. A soil that seems to have a lot of gravel or sand in it could still contain 20 to 30 percent clay. If it does, it's going to act like clay, which can give your project poor drainage and plenty of problems.
So, do some creative detective work on your site. First, walk on the soil. If you leave a boot mark, try driving a stake into the soil. Since it usually takes six or seven whacks to drive a stake into the ground, a stake that goes in with one or two solid drives probably indicates soil that lacks strength and needs to be compacted.
Next, if your site is already under excavation, take a handful of damp soil from the bottom of the excavation and ball it up in your hands. If it crumbles apart when you release it, it is a granular soil (with lots of sand or gravel). If it holds together, it's a silt. If it stays in a ball when you drop it from two feet, it's probably a clay. To be sure, you might also try rolling the ball of soil into a noodle or worm shape. If you can roll it into a pencil shape without having it crumble, consider it clay, and make sure your next call is to a soils engineer. If ever you suspect clay in your soil, a full workup will be in order. It's always worth investing $1,000 or so in engineering work before you invest your life savings in a home site.
The Bottom Line On Soils
For home sites, the bottom line is pretty simple: You want soil that has good bearing capacity, exerts relatively low lateral pressure, and drains well, so that you can have a stable, dry foundation. The best natural soils for these purposes are sands and gravels. Silts and clays are fair, but the softest ones are poor. Then there are soils such as peat, expansive clay, and improperly deposited fill, which are so bad that they must usually be removed and replaced - often at considerable cost to you.