The Unified Soil Classification System
The Unified Soil Classification System (USCS) originally developed by Casagrande (1948). According to this system, the symbols of the various soils are as: Gravel (G), Sand (S), Silt or Silty (M), Clay or Clayey (C), Organic (O), Peat (Pt), Well graded (W), Poorly graded (P).
The soil is called coarse-grained soil if 50% or more soil is retained on the 0.075mm sieve. If the 50% or more of the coarse fraction is retained on the 4.75mm sieve, the soil is called Gravel. On the other hand if 50% or more of the coarse fraction is passed through the 4.75mm sieve, the soil is called Sand. Various types of coarse-grained soils are classified as: GW (Well graded Gravel), GP (Poorly graded Gravel), SW (Well graded Sand), SP (Poorly graded Sand), SM (Silty Sand), GM (Silty Gravel), SC (Clayey Sand), and GC (Clayey Gravel). In case of well graded gravels and well graded sands, less than 5% soils pass 75m sieve. However, In case of poorly graded gravels and poorly graded sands very little or no fines are present.
Fig. 4.1. Plasticity chart as per Unified Soil Classification System (USCS).
The soil is called fine-grained soil if 50% or more soil is passed through 0.075 mm sieve. The fine-grained soils are classified based on plasticity chart (as shown in Figure 4.1). The soil has low plasticity (CL: Clay with low plasticity, ML: Silt with low plasticity) if the liquid limit of the soil is less than 50% and if the liquid limit of the soil is greater than 50% the soil has high plasticity (CH: Clay with high plasticity, MH: Silt with high plasticity). However, more than one group can be termed as boundary soils (like GW-GM: Well graded gravel mixed with silt).