Unified Soils Classification


Unified Soil Classification is a structure that explains the texture and grain size of a soil in engineering and geology. This classification system is needed to keep track of the laboratory's results for each soil. The classification system divides soil into two primary groups, coarse-grained and fine-grained. These groups are further divided on the basis of different soil attributes.


Soil is tested in laboratories to discover the plasticity index. The liquid limit and particle-size distinctiveness help to classify the soil into categories based on those results.


USC is the first step to laboratory explorations for geotechnical engineering functions. The classification name, symbol and explanatory information is utilized to provide information for the assessment of the soil's important assets for engineering use. The groups in USC are linked with the engineering performances of the soils.

Scope and Class Nomenclature

Scope is restricted to organically occurring soils, and categorizes mineral and organo-mineral soils derived from laboratory outcomes on particle-size individuality, plasticity index and liquid limit for engineering uses. It is used when accurate categorization is necessary and when quantitative information is needed to establish functioning distinctions between soil types. In most cases, soil classification results in a group symbol and name. However, in some cases a dual symbol is used.

Coarse-Grained Soils

There are several subdivisions within the coarse-grained soils classification. Coarse-grained soils are split into two groups: gravels and sands. Gravel is further broken down into two groups: cleans gravels (symbols GW and GP) and gravels with fines (symbols GM and GC). GW consists of gravel-sand combinations along with well-graded gravels that contain little or no fines. GP contains inadequate graded gravels along with gravel-sand combinations that consist of little to no fines. GM are silt gravels together with gravel-sand-silt blends, while GC are gravel-sand-clay blends along with clay gravels. Sands are divided into clean sands (symbols SW and SP) and sands with fines (symbols SM and SC). SW contains well-graded sands and gravelly sands that have little to no fines. SP consists of inadequate graded sands and gravelly sands that contain little or no fines. SM is sand-silt combination together with silt sands, while SC is clay sands along with sand-clay blends.

Fine-Grained Soils

There are six types of fine-grained soils that are broken down into different groups. First the fine-grained soils are divided into silts and clays that have a liquid limit of less than 50 percent, and another group of silts and clays that have a liquid limit of more than 50 percent. Silts and clays with less than 50 percent liquid include ML, CL and OL. ML is for inorganic silts, silt or clay fine sands, very fine sands and rock. CL consists of inorganic clays with low to medium plasticity, along with gravel-sand-silt-lean clays. OL consists of organic silt clays with low plasticity and organic silts. Silts and clays with more than 50 percent liquid include MH, CH and OH. MH consists of micaceous or diatomaceous fine sands or silts, inorganic silts and elastic silts. CH consists of fat clays and inorganic clays of high plasticity. Finally, OH consists of organic clays of medium to high plasticity.

Keywords: unified soil classification, soil classification methods, standard soil classification

About this Author

Elaine Pratt started her freelance writing career in 2000 and since has gained extensive experience writing on real estate, home and garden, and business-related topics. Elaine writes for personal blogs and private clients including eHow and Garden Guides. Pratt holds a Bachelor of Business Administration from the University of Illinois.