Get Down in Your Dirt

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.