"Cohesive" is generally used to describe things or ideas that stick together, and that is what it means when applied to soil as well. When engineers analyze soil composition, they look at the differences in texture, strength, and consistency that distinguish cohesive soils from non-cohesive soil environments. Cohesiveness of soil is important when a building or road will be built on it or when workers are excavating an area.
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All Soils Are Not Alike
Many people think of soil as little more than dirt, the stuff that gets on your fingers while preparing a garden bed and dirties the knees of your pants. But all soils are not alike, and that becomes important when you are building something on it.
In fact, soil is far from being a uniform substance, and it varies widely. While gardeners think in terms of well-drained versus poorly drained soil or acidic versus alkaline soil, the important difference when one is building is cohesive versus non-cohesive soil.
Cohesive vs. Non-Cohesive Soil
Cohesive soils are fine-grained soils, soils with particles that clump or stick together. Think of how some soil forms a ball when you squeeze a handful of it. These soils are characteristically soft with a high moisture content. When they dry, they can become almost as hard as cement. Their structural strength depends on moisture content.
Non-cohesive soil is just the opposite. No matter how hard you squeeze a handful of sand or gravel, you can't make a ball of soil. Non-cohesive soils do not clump together; rather, their grains remain separate and apart from each other, or "free-running" kinds of soil that may be prone to liquefaction in earthquakes.
Differences in Soil Texture
It doesn't take a rocket scientist to know that soils have different textures. It is obvious to anyone working with soil that different types of soil feel different. Compare potting soil to gravel or sand stone, for example. These differences in texture are a result of the types of rock that formed the soil in a particular area.
Remember that what you see as soil today used to be rock. Over decades or centuries, weather and water erosion work together to grind down existing rocks into soil particles. You can spot texture differences in the shapes, sizes, and arrangement of particles that make up the soil.
Clay is generally made up of fine particles that bind the soil together in cohesive soil. Since non-cohesive soil environments contain little to no clay or fine particles, they do not produce soil that binds together.
Why This Matters
A soil's ability to become compact and keep this consistency under pressure determines whether it will provide a suitable foundation for building. Obviously, this determination must be made early in the planning process for construction projects. You don't want to put a freeway on non-cohesive soil.
Differences between cohesive and non-cohesive soil play a significant role in determining whether a particular area will work with a plan for landscaping or building.