Major Soil Types

The major engineering categories of soil are gravel, sand, silt, and clay. There is not unanimous agreement on the exact division between each of these major soil types, but gravel and sand are universally considered coarse-grained soil, for the individual particles are large enough to be distinguished without magnification. Silts and clays are considered fine-grained soil because of their small particles—too small, for the most part, to be seen unaided.

The most commonly used divisions for classifying soils for engineering and construction purposes are shown in the table. On a comparative basis, the division sizes between gravel and sand (4.76 mm or 2.00 mm) and between sand and silt–clay (0.074 mm or 0.05 mm) are actually quite close. As a result, lack of agreement on these division sizes normally does not cause serious problems.

Particles larger than gravel are commonly referred to as cobbles or boulders. Again, no unanimous agreement exists on the range of sizes. When gravel extends up to the 200 mm (8 in.) size, anything larger would be termed a boulder. Where the 80 mm (3 in.) size, or thereabouts, is taken as the upper size for gravel, the sizes between 80 mm and 200 mm may be designated as cobbles, and anything larger than 200 mm (8 in.) as boulders. However, 150 mm or 300 mm (6 in. or 12 in.) may also be taken as the division between cobbles and boulders. As for sands and gravels, these discrepancies usually do not cause serious problems. Conventionally, when a construction project requires a particular material, it has become standard practice to indicate the soil or aggregate requirements on the basis of size instead of, or in addition to, classification.

In conclusion, particle size serves as the basis for classification of sands, gravels, cobbles, and boulders.

The classification of a fine-grained soil as either a silt or a clay is not done on the basis of particle size but, rather, is based on the plasticity or nonplasticity of the material. Clay soil is plastic over a range of water content; that is, the soil can be remolded or deformed without causing cracking, breaking, or change in volume, and will retain the remolded shape. The clays are frequently “sticky.” When dried, a clay soil possesses very high strength (resistance to crushing). A silt soil possesses little or no plasticity and, when dried, has little strength. If a small sample of moist silt is shaken easily but rapidly in the palm of the hand, water will appear on the surface of the sample but disappear when the shaking stops. This is referred to as dilatancy. When a sample of moist clay is similarly shaken, the surface will not become wetted.

The reason for the difference in behavior between clay and silt relates to the difference in mineralogical composition of the soil types and particle shape. Silt soils are very small particles of disintegrated rock, as are sands and gravels, and possess the same general shape and mineralogical composition as sands and gravels (which are nonplastic). The clay minerals, however, represent chemical changes that have resulted from decomposition and alteration of the original rock minerals. The effect is that their size and shape are significantly different from those of other types of soil particles. This is discussed further in a following section.

Naturally occurring soil deposits most generally include more than one soil type. When they are classified, all the soil types actually present should be indicated, but the major constituent soil type should dominate the description, while the soils of lesser percentage are used as modifying terms; for example, a material that is mostly sand but includes silt would be classified as a silty sand, whereas a silt–clay mixture with mostly clay would be termed a silty clay.

Although a soil may be predominantly coarse-grained, the presence of silt or clay can have a significant effect on the properties of the mixture. Where the amount of finegrained material exceeds about one-third of the total soil, the mixture behaves more like a fine-grained soil than a coarse-grained soil.

The condition also exists where small fragments of decomposed vegetation are mixed with the soil, particularly fine-grained soils. Organic material mixed with the nonorganic soil can have striking detrimental effects on the strength and compressibility properties of the material. The presence of organic material should be carefully considered. A foul odor is characteristically though not always associated with such soils, as is a blackish or dark gray color. Soils in this category are designated as organic (e.g., organic silt or organic clay) in comparison to a nonorganic designation for soil free of decomposed vegetation.