Unified Soil Classification System (USCS) is a widely used engineering classification system for soils based on particle size distribution (gradation) and plasticity characteristics (Atterberg limits). The purpose of USCS is to classify soils (primarily mineral and organo-mineral soils) into categories for engineering purposes using results from prescribed laboratory tests: sieve analysis for grain size and Atterberg limits for plasticity. It assigns a two-letter symbol (and often a group name) to describe the soil's texture, grain size, gradation, and plasticity behavior. It provides a standardized, consistent way to communicate soil properties relevant to engineering behavior (e.g., strength, permeability, compressibility, workability). This facilitates design, construction, and analysis in geotechnical engineering by grouping soils with similar expected performance.
Standards
- ASTM D2487 - Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System).
Primary and Secondary Letters
USCS uses a two-letter symbol:
First letter (primary) - Indicates the major soil constituent or group.
G - Gravel (coarse-grained, >50% of coarse fraction retained on No. 4 sieve)
S - Sand (coarse-grained, >50% of coarse fraction passes No. 4 sieve)
M - Silt (fine-grained)
C - Clay (fine-grained)
O - Organic (organic silt or clay)
PT - Peat (highly organic)
Second letter (secondary/descriptive): Refines the classification based on gradation (for coarse-grained) or plasticity (for fine-grained).
For coarse-grained (G or S)
W - Well-graded (wide range of particle sizes, good distribution)
P - Poorly graded (uniform particle sizes or gap-graded)
M - Silty fines
C - Clayey fines
For fine-grained (M or C)
L - Low plasticity (lean, liquid limit < 50%)
H - High plasticity (fat, liquid limit ≥ 50%)
Common Applications and Limitations
Applications
Geotechnical engineering for foundations, embankments, pavements, earth dams, and site characterization.
Civil engineering projects involving soil as construction material or foundation support.
Used alongside field descriptions and other tests for soil logging and reporting (e.g., in borings/logs).
Limitations
Primarily laboratory-based (requires sieve analysis and Atterberg limits); not ideal for all field conditions or highly variable soils without testing.
Focuses on engineering behavior but may need supplementation for specific applications (e.g., environmental, agricultural, or detailed mineralogy).
Does not address all safety or site-specific factors; professional judgment is required.
Organic soils and highly variable deposits may need special consideration or additional classification systems.
