On the basis of the grain size of the soil particles.
Gravel: particles larger than 2 mm, but smaller than 63 mm as gravel. The
particles larger than 63 mm are denoted as stones.
Sand: particles smaller than 2 mm, but larger than 0.063 mm.
Silt: particles smaller than 0.063 mm and larger than 0.002 mm
Clay or Luthum: particles smaller than 0.002 mm,
The web-links on this page will lead you to lecture notes from
distinguished faculties, authentic products and service providers in
the areas of Soil Mechanics/Geotechnical Engineering around the world.