The time to look at your soil is before you build. Architects and engineers are great resources, but there's a lot you can learn on your own, if you're willing to get your hands dirty.
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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.
Testing Basics
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.
| Quick Tips |
| 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. |