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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  GGravel  
SSand  
MSilt
CClay
OOrganic
ptPeat  
Secondary  WWell graded  
PPoorly graded  
MNon-plastic fines  
CPlastic fines
LLow plasticity  
HHigh plasticity  

In the USCS system, the soils are classified into 15 groups. The soils are first classified into three categories.

  1. Coarse-grained soil
  2. Fine-grained soil
  3. Highly organic soils

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  1. 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.

  1. 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.

  1. 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.

Unified soil classification system
Fig.1. Unified soil classification system

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 (USCS)
Fig.2. Plasticity chart (USCS)

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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