Concepts:

According to USCS, the coarse grained soils are classified on the basis of grain size distribution and fine-grained soils are classified based on plasticity.

Coarse grained soils are those having 50 % or more retained on 0.075 mm sieve. They are further divided into Gravel and Sand. They are designated as Gravel (G) if more than 50% coarse fraction is retained on 4.75 mm sieve and otherwise, they designated as Sand (S). Both Gravel and Sand are further divided into sub groups depending on the whether the soil is well graded or poorly graded (decided based on Coefficient of Uniformity and coefficient of curvature) and also based on nature of fines in them. This sub- classification is tabulated below:

Fine  grained soils are those having 50 % or more passing through 0.075 mm sieve. They are further divided into Silt (S), Clay (C) and Organic (O) based on their liquid limit and plasticity Index. They are   further divided into sub groups based on liquid limit i.e. if liquid limit is less than 50 % then they are low plasticity (L) and designated as ‘ML- Silt with Low plasticity’, ‘CL- Clay with Low plasticity’, OL- organic soils with Low plasticity’    and if liquid limit greater than 50 % they are high plasticity and designated as ‘MH- Silt with high plasticity’, ‘CH- Clay with high plasticity’, OH- organic soils with high plasticity’.