There are two soil classification systems in common use for engineering purposes. The Unified Soil Classification System [ASTM D 2487-93] is used for virtually all geotechnical engineering work except highway and road construction, where the AASHTO classification system [AASHTO M 145-87] is used.
Both systems use the results of grain-size analysis and determinations of Atterberg limits to determine a soils classification. Soil components may be described as gravel, sand, silt, or clay. A soil comprising one or more of these components is given a descriptive name and a designation consisting of letters or letters and numbers which depend on the relative proportions of the components and the plasticity characteristics of the soil.
Grain-Size Characteristics of Soils
Atterberg Limits and Plasticity
The Unified Soil Classification System (USCS)
The AASHTO Classification System
AASHTO Classification System Examples