Classification systems group soils based on similar engineering behavior. Two main systems: AASHTO (preferred by transportation engineers) and USCS (preferred by geotechnical engineers)
Boussinesq developed stress distribution theory for homogeneous subgrades. Vertical stress depends on radial distance r and depth z. Maximum vertical stress IB equals 0.4775 at r=0. Pressure bulb shows significant stress zone with stress intensity 0.1Q