Introduction of USCS ( Unified Soil Classification System )
The original form of this system was proposed by Casagrande in 1942 for use in the airfield construction works undertaken by the Army Corps of Engineers during World War II.
In cooperation with the U.S. Bureau of Reclamation, this system was revised in 1952. At present, it is used widely by engineers (ASTM Test Designation D-2487). The Unified classification system is presented in Table 2.
| Unit | 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 |
Table 1 – Symbols Used in USCS
a Gravels with 5 to 12% fine require dual symbols: GW-GM, GW-GC, GP-GM, GP-GC.
b Sands with 5 to 12% fines require dual symbols: SW-SM, SW-SC, SP-SM, SP-SC.
c Cu = D60 / D10 ; Cc =Â ( D 30 )2 / ( D60 x D10 )
d If 4 ≤ PI ≤ 7 and plots in the hatched area in below fig (Plasticity chart), use dual symbol GC-GM or SC-SM.
e If 4 ≤ PI ≤ 7 and plots in the hatched area in below fig (Plasticity chart), use dual symbol CL-ML.
Plasticity chart
For proper classification according to this system, some or all of the following information must be known:
1. Percent of gravel — That is, the fraction passing the 76.2-mm sieve and retained on the No. 4 sieve (4.75-mm opening)
2. Percent of sand — That is, the fraction passing the No. 4 sieve (4.75-mm opening) and retained on the No. 200 sieve (0.075-mm opening)
3. Percent of silt and clay — That is, the fraction finer than the No. 200 sieve (0.075-mm opening)
4. Uniformity coefficient (Cu) and the coefficient of gradation (Cc)
5. Liquid limit and plasticity index of the portion of soil passing the No. 40 sieve
The group symbols for coarse-grained gravelly soils are GW, GP, GM, GC, GC-GM, GW-GM, GW-GC, GP-GM, and GP-GC.
Similarly, the group symbols for fine-grained soils are CL, ML, OL, CH, MH, OH, CL-ML, and Pt.
• Fine Fraction = Percent Passing No. 200 Sieve
• Coarse Fraction = Percent Retained on No. 200 Sieve
• Gravel Fraction = Percent Retained on No. 4 Sieve
• Sand Fraction = (Percent Retained on No. 200 Sieve) – (Percent Retained on No. 4 Sieve)
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USCS Classifies Soils into Two Broad Categories
Coarse-Grained Soils—If more than 50% of the soil is retained on No. 200 (0.075 mm) sieve, it is designated as coarse-grained soil. There are 8 groups of coarse-grained soils.
Fine-Grained Soils—If more than 50% of the soil passes No. 200 sieve, it is called fine-grained soil. There are 6 groups of fine-grained soils.
Coarse-Grained Soils
The coarse-grained soils are designated as gravel (G) if 50% or more of a coarse fraction (plus 0.075 mm) is retained on No. 4 (4.75 mm) sieve; otherwise, it is termed sand (S).
If the coarse-grained soils contain less than 5% fines and are well-graded (W), they are given the symbols GW and SW, and if poorly graded (P), symbols GP and SP.
The criteria for well-grading are given in the above table 1. If the coarse-grained soils contain more than 12% fines, these are designated as GM, GC, SM, or SC, as per the criteria given.
If the percentage of fines is between 5 to 12% dual symbols such as GW-GM, SP-SM, are used.
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Fine-Grained Soils
Fine-grained soils are further divided into two types :
1. Soils of low compressibility (L) if the liquid limit is 50% or less. These are given the symbols ML, CL, and OL.
2. Soils of high compressibility (H) if the liquid limit is more than 50%. These are given the symbols MH, CH, and OH.
The exact type of soil is determined from the plasticity chart (above Fig). The A-line has the equation Ip = 0.73 (wt– 20).
It separates the clays from silts. When the plasticity index and the liquid limit plot in the hatched portion of the plasticity chart, the soil is given double symbol CL- ML.
The inorganic soil ML and MH and the organic soils OL, OH plot in the same zones of the plasticity chart. The distinction between the inorganic and organic soils is made by oven-drying.
If oven drying decreases the liquid limit by 30% or more, the soil is classified organic (OL or OH); otherwise, inorganic (ML or MH)
Highly Organic Soils
Highly organic soils are identified by visual inspection. These soils are termed peat (Pt).
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