USCS stands for Unified Soil Classification System [ASTM D2487]. It is the most common soil classification system used by geotechnical engineers. It simply uses sieve analysis data for coarse grain soil and liquid limit (LL) of fine grain soils to classify the soil into standard groups.
The first letter specifies the soil types, like G= Gravel, S= Sand, M= Silt, C= Clay, and O= Organic. The specific properties of soil are as suffices like,
For coarse grain soils:
W= well-graded, P = poorly-graded, M = silty, C = clayey,
For fine grain soils:
L= Low plasticity
H= High plasticity
Ref.
DP Coduto, Foundation Design: Principles and Practices, 2nd Edition, Prentice Hall, 2001