Soil Classification: Unified soil classification system and AASHTO

Methods of classification discussed here are Unified soil classification system and AASHTO. Index properties like grain shape, distribution, consistency and plasticity index help us classify soils.

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How is soil classified?

• Coarse grained soil classification by Grain Size Distribution
• Fine grained soil classification by Consistency and Plasticity Index

What are the different methods?

• Unified soil classification system(USC)
• American Association of State Highways and Transport office system(AASHTO)
• IS Soil classification system(ISSCS)

Sieve Analysis for grain size distribution

Sieve analysis is used for grain size distribution. Different sizes of sieves are used to classify the soil.

Sieve analysis by mechanical shaking (10-15min).

• % retained on a sieve = (Weight of soil retain ×100)/(Total weight taken)
• Cumulative % retained = sum of % retained on all the sieves of larger size and that of a particular size.

• % finer than the sieve under consideration=100-cumulative %retained

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Unified soil classification system(USC)

This classification is done by Casagrande. Initially it was developed for air fields later modified for the embankments. The 4 major groups are

• Coarse grained
• Fine grained
• Orgainc
• Peat

• If the particles passing through 75microns are greater than 50%, then coarse grained
• If the particles passing through 75microns are less than 50%, then coarse grained.

American Association of State Highways and Transport office system(AASHTO)

8 major groups: A1 to A7 and A8 (Peat/Muck) separately.

GI=0.2a+0.005ac+0.01bd

• 0<=GI<=20
• If GI is negative it will be taken as 0.

• a= %passing through 75microns
• b=%passing through 75microns
• c=Liquid limit
• d=Plasticity Index

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