Unified soil classification system is adopted by ASTM D-2487-98 and IS: 1498-1970 for classification and identification of soils for general engineering purpose.
Soils are broadly classified into three divisions:
Coarse grained soils: In these soils, 50% or more of the total material by weight is larger than 75micron IS sieve size.
Fine grained soils: In these soils, 50% or more of the total material by weight is smaller than 75 micron IS sieve size.
Highly organic soils and other miscellaneous soil materials:
These soils contain large percentage of fibrous organic matter, such as peat, and the particles of decomposed vegetation. In addition, certain soils containing shells, cinders and other non-soil materials in sufficient quantities are also grouped in this division.
1. Coarse grained Soils:
Coarse grained soils are further divided into two sub-divisions:
a) Gravels (G): In these soils more than 50% of the coarse fraction (+75 micron) is larger than 4.75 mm sieve size. This sub-division includes gravels and gravelly soil, and is designated by symbol G.
b) Sands (S): In these soils, more than 50% of the coarse fraction is smaller than 4.75mm IS sieve size. This sub-division includes sands and sandy soils.
Each of the above sub-divisions are further divided into four groups depending upon grading and inclusion of other materials.
W : Well Graded
C : Clay binder
P : Poorly graded
M : Containing fine materials not covered in other groups.
These symbols used in combination to designate the type of grained soils. For example, GC means clayey gravels.
2. Fine grained soils:
Fine grained soils are further divided into three sub-divisions:
a) Inorganic silts and very fine sands: M
b) Inorganic clays: C
c) Organic silts and clays and organic matter: O.
The fine grained soils are further divided into the following groups on the basis of the following arbitrarily selected values of liquid limit which is a good index of compressibility:
i) Silts and clays of low compressibility:
Having a liquid limit less than 35 and represented by symbol L.
ii) Silts and clays of medium compressibility:
Having a liquid limit greater than 35 and less than 50 and represented by symbol I.
iii) Silts and clays of high compressibility:
Having a liquid limit greater than 50 and represented by a symbol H.
Combination of these symbols indicates the type of fine grained soil. For example, ML means inorganic silt with low to medium compressibility.
Unified Soil Classification System and Symbol Chart:
Fig: Unified soil classification system for coarse grained soils
Fig: Unified soil classification system for fine grained soils
Fig: Soil Plasticity chart as per Unified soil classification system