What is the difference between AASHTO and USCS soil classification?

AASHTO system is for finding out the suitability or otherwise of soils as sub-grade for highways only. USCS is for determining the suitability of soils for general use.

Both systems, however, has the same basis. They classify soils according to the particle size analysis and the plasticity characteristic.

  1. According to the AASHTO system, the soil is termed fine-grained, if more than 35% passes No. 200(0.075 mm)sieve, whereas in the ISCS if more than 50% passes that sieve. In that respect, the AASHTO system is somewhat better because the soil behaves as fine-grained when the percentage of fine is 35% and the limit of 50% in USCS is somewhat higher.
  2. In the AASHTO system, sieve No. 10(2 mm size) is used to divide the soil into gravel and sand, whereas in USCS, sieve no. 4(4.75 mm size) is used.
  3. In USCS, the gravelly and sandy soils are clearly separated, whereas, in the AASHTO system, a clear demarcation is not done.
  4. Symbols used in USCS are more descriptive and are more easily remembered than those in the AASHTO system.
  5. Organic soils are classified as OL and OH as peat (Pt) if highly organic in USCS. In the AASHTO system, there is no place for organic soil.
  6. USCS is more convenient to use than the AASHTO system. In the AASHTO system, the process of elimination is required which is time-consuming.

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