The unified soil classification system was first developed by Casa Grande and modified by the Bureau of Reclamation and crops of engineers of the United State of America.
This system of classification is the most popular one and is universally accepted for classification of soil for general engineering purposes. This system uses both particle size and plasticity characteristics of the soils. Furthermore, the system includes the field identification of soils. In this system, the group symbol letter consists of a primary and secondary descriptive letters. These letters and their meanings are given below:
Some primary letters and their meanings are:
| Symbols | Descriptions |
| G | gravel |
| S | sand |
| M | silt |
| C | clay |
| O | or.... |
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The unified soil classification system was first developed by Casa Grande and modified by the Bureau of Reclamation and crops of engineers of the United State of America.
This system of classification is the most popular one and is universally accepted for classification of soil for general engineering purposes. This system uses both particle size and plasticity characteristics of the soils. Furthermore, the system includes the field identification of soils. In this system, the group symbol letter consists of a primary and secondary descriptive letters. These letters and their meanings are given below:
Some primary letters and their meanings are:
| Symbols | Descriptions |
| G | gravel |
| S | sand |
| M | silt |
| C | clay |
| O | organic |
| `P_t` | peat |
Some secondary letters and their meanings are;
| Symbols | Descriptions |
| W | well graded |
| P | poorly graded |
| M | non-plastic fines |
| C | plastic fines |
| L | low plasticity |
| H | high plasticity |
In this system the Soils are classified into 15 groups. The soils are first classified into two categories
- Cross grained soil and
- fine grained soil
Course grained soils:
The soil is coarse grained,if more than 50% of the soil is retained on the slip of opening 0.075 mm (i.e, number 200 seive). Coarse grained soil is further divided into gravel and sand. If more than 50% of the course particle are retained on number 4 ( i.e, 4.75 mm) sieve, the soil is G. Otherwise termed as S. Gravel and sand are further divided into finer classes depending on the amount of fine particles in the soil.
Listen 5% fines: clean gravel or Clean Sand:
If the coarse grained soil contains less than 5% Fines and are well graded, they are given the symbols GW ( well graded gravel) and SW (well graded sand) and if they are poorly graded (P), they are given the symbols GP (poly graded symbol gravel) and ST (poorly graded sand).
more than 12% fines:
The soil in this category is divided into silty gravel (GM), clayey gravel (GC) or clayey sand (SC), depending on whether the consistency limit of the fine particles are located above the A-line (clayey) or below the A-line (silty) of USCS plasticity chart shown below.
The exact type of soil is determined from plasticity chart. The A-line has the equation `I_p=0.73 (W_l-20)`. It separates the clay from Silt. When plasticity index and liquid limit plots in hatched portion of the plasticity chart, the soil is given by double symbol CL-ML.
The inorganic soil ML and MH and organic soil OL, OH plot in the same zone of plasticity chart. The distinction between organic and inorganic soil is made by oven drying. If the oven drying decreases the liquid limit by 30% or more,the soil is considered organic(OL or OH)otherwise inorganic (ML or MH).
Fines between 5% and 12%:
If the percentage of fines in coarse grained soil is between 5 to 12% dual symbols such as GW-GM (between well graded gravel and silty gravel ), SP-SC (between poorly graded sand and clayey sand).
Fine grained soils:
Fine grained soils are for the divided into two types:
- soils of low compressibility (L) if the liquid limit is 50% or less. These are given by symbols ML, CL,OL.
- soils of high compressibility(H) if liquid limit is more than 50%. These are given by symbols MH,CH,OH.
Organic soils:
Organic soils are fine grained soils with organic characteristics such as strong odour, dark colour, highly compressible and visible presence of organic substances. Depending upon liquid limit, the soil can be organic soil of low plasticity (liquid level smaller than 50% denoted as OL) or Organic soil of high plasticity denoted as OH.
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