Textural Classification

Cohesive Soils

Soil texture, in its most general sense, refers to the surface appearance of soil and is determined by the size of its individual particles. Soil particles are categorized into gravel, sand, silt, and clay based on their sizes, as detailed in textural classification systems like the one developed by the U.S. Department of Agriculture (USDA). This system plays a vital role in identifying soil types and their suitability for various applications.

USDA Soil Particle-Size Categories:

  • Sand: 2.0 to 0.05 mm in diameter
  • Silt: 0.05 to 0.002 mm in diameter
  • Clay: Smaller than 0.002 mm in diameter

Most natural soils are mixtures of particles from these categories. The USDA system combines the percentages of these particle sizes to classify soils into textures like sandy clay, silty clay, and clay loam.

The use of this chart can best be demonstrated by an example. If the particle-size distribution of soil A shows 30% sand, 40% silt, and 30% clay-size particles, its textural
classification can be determined by proceeding in the manner indicated by the arrows in
Figure 1.

This soil falls into the zone of clay loam. Note that this chart is based on only the fraction of soil that passes through the No. 10 sieve. Hence, if the particle-size
distribution of a soil is such that a certain percentage of the soil particles is larger than
2 mm in diameter, a correction will be necessary. For example, if soil B has a particlesize
distribution of 20% gravel, 10% sand, 30% silt, and 40% clay, the modified textural
compositions are

On the basis of the preceding modified percentages, the USDA textural classification is
clay. However, because of the large percentage of gravel, it may be called gravelly clay.
Several other textural classification systems are also used, but they are no longer useful
for civil engineering purposes.