As per IS classification soil is broadly divided into three divisions:
1. Coarse grained soil: In these soils, more than half the total material by mass is larger than 75 micron IS sieve size. Coarse grained soils are divided into two sub divisions:
a) Gravels (G) - In these soils, more than half the coarse fraction (+ 75 micron) is larger than 4.75mm IS sieve size. The sub division includes gravels and gravelly soil, and is designated by symbol G.
b) Sands (S) - In these soils, more than half the coarse fraction (+ 75 micron) is smaller than 4.75mm IS sieve size. The sub division includes sands and sandy soils. Each of the above sub divisions are further sub divided into four groups:
i) Well graded, clean(W)
ii) Well graded with excellent clay binder (C)
iii) Poorly graded, fairly clean (P)
iv) Not covered in other groups (M)
These symbols are used in combination and designate the type of coarse grained soil e.g GC means clayey gravels.
2. Fine grained soils: In these soils, more than half the total material by mass is smaller than 75 micron IS sieve size. Fine grained soils are further divided into three sub divisions.
a) Inorganic silt and very fine sands (M)
b) Inorganic clay (C)
c) Organic silt and clays (O)
The fine grained soils further sub divided into following groups on the basis of liquid limit which is a good index of compressibility.
i) Silt and clays of low compressibility, having liquid limit less than 35% (L)
ii) Silt and clays of medium compressibility, having liquid limit greater than 35% and less than 50% (I)
iii) Silt and clays of high compressibility, having liquid limit greater than 50% (H)
Laboratory classification of fine grained soil is done with the help of plasticity chart.
Note: if the classification of fine grained soil is explained with the help of plasticity then appropriate marks should be given to the students.
3. Highly organic soils and other miscellaneous soil materials: These soils contain large percentages of fibrous organic matter such as peat, and the particles of decomposed vegetation. In addition, certain soils containg shells, concretions, cinders and other non-soil material in sufficient quantities are also grouped in this division.
Meaning of Terms:
i) GW – well graded Gravel
ii) SP – Poorly graded sands or gravelly sands
iii) ML – inorganic silt with low to medium compressibility.
iv) SM-ML - boundary classification between coarse grained and fine grained soils.