Unified Soil Classification System (USCS)
The unified soil classification system (USCS) was first developed by Casagrande in 1948, and late in 1952, was modified by the Bureau of Reclamation and the crops of engineers of the United States of America. USCS system is based on both particle size analysis and plasticity characteristics.
In this system, various symbols and abbreviations are used given in the table below.
Symbols | Description | |
Primary | G | Gravel |
S | Sand | |
M | Silt | |
C | Clay | |
O | Organic | |
pt | Peat | |
Secondary | W | Well graded |
P | Poorly graded | |
M | Non-plastic fines | |
C | Plastic fines | |
L | Low plasticity | |
H | High plasticity |
In the USCS system, the soils are classified into 15 groups. The soils are first classified into three categories.
- Coarse-grained soil
- Fine-grained soil
- Highly organic soils
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- Coarse-grained soil
If 50% of the soil is retained on ASTM(American Society of Testing Material) sieve No. 200 (0.075 mm, 75µ) sieve, it is known as coarse-grained soil.
There are 8 groups of coarse grained soils.
- Fined grained soil
If more than 50% of the soil passes through ASTM (American Society of testing material) sieve No. 200 (0.075 mm, 75µ) sieve, it is called fine-grained soil.
There are 6 groups of fine grained soils.
- Highly organic soils
Highly organic soils are identified by visual inspection, their colour, odor, and fibrous nature. Peat and muck are examples of highly organic soil. Peat contains carbonized or carbonaceous matter and is found in fibrous nature.

Coarse grained Soils
The coarse grained soils are designated as gravel (G) if 50% or more of coarse fraction (plus 0.075 mm) is retained on the No. 4 (4.75 mm) sieve, otherwise, it is termed as sand (S).
- If the coarse grained soils contain less than 5% fines and are well graded, they are given the symbols GW and SW, and if poorly graded (P), symbols GP and SP.
- If the coarse grained soil contains more than 12% fines, these are designated as GM, GC, SM, or SC as per criteria.
- If the percentage of fines is between 5 to 12%, dual symbols such as GW-GM, and SP-SM are used.
In summary for Coarse grained Soil:
1.Gravel
50% or more coarse retained on a 4.75 mm size sieve.
- Clean Gravel
- GW = well-graded gravel
- GP = Poorly graded gravel
- Gravel With fines
- GM = Silty Gravel
- GC = Clayey Gravels
2.Sand
More than 50% passing through 4.75mm sieve.
- Clean Sand
- SW= well graded sand
- SP = poorly Graded sand
- Sand with fines
- SM= silty sand
- SC = clayey sand
Fine grained Soils
Fine grained soils are further divided into two types:
- Soils of low compressibility (L) if the liquid limit is 50% or less. These are given by symbols ML, CL, and OL.
- Soils of high compressibility (H) if the liquid limit is more than 50%. These are given symbols MH, CH and OH.
Summary of Fined Grained Soil is given below:
- Silt and Clays with LL < 50%
ML = Inorganic silt of low Plasticity
CL = Inorganic clay of low Plasticity
OL = Organic silt of Low Plasticity
- Silt and Clays with LL>50%
- MH = Inorganic silt of High Plasticity
- CH = Inorganic clay of High Plasticity
- OH = Organic silt of High Plasticity
Highly Organic Soil
Peat and muck are examples of highly organic soil. Peat contains carbonized or carbonaceous matter and is found in fibrous nature.
The plasticity chart is used to describe the theme.

Plasticity chart
The plasticity chart analysis is used to find the exact type of soil. The A-line has the equation Ip = 0.73(W-20). It separates the clays from silt. When the plasticity index and liquid limit plots in the hatched portion of the plasticity chart, the soil is given the double symbol CL-ML.
The organic soil ML and MH and the organic soils OL, and OH plot in the same zones of the plasticity chart. The oven draying method is used to distinguish between the inorganic and organic soils. If the oven drying decreases the liquid limit by 30% or more, the soils is classified as organic (OL or OH); otherwise inorganic (ML or MH).
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