What is the Unified Soil Classification System (USCS)?

USCS classifies soils based on grain size distribution and plasticity characteristics using a two-letter symbol. Coarse-grained soils are classified by gradation (W = well-graded, P = poorly-graded) with prefix G (gravel) or S (sand). Fine-grained soils are classified by plasticity (L = low, H = high) with prefix M (silt) or C (clay). Examples: SW = well-graded sand, CL = clay of low plasticity, ML = silt of low plasticity. Organic soils use O prefix.