Geotechnical Engineering

DHARM

IDENTIFICATION AND CLASSIFICATION OF SOILS 97

3. Massachusetts Institute of Technology (MIT) System of Classification of U.S.A.


4. Indian Standard Classification (IS : 1498-1970).
   These are shown diagrammatically (Fig. 4.1).

Gravel

Fine gravel

Coarse Medium Fine Very fine

Sand

mm2.0 1.0 0.50 0.25 0.10 0.05 0.005

Silt Clay

U.S.Bureau of soils and P.R.A. system of classification

mm2.0 1.0

Gravel Sand

0.5 0.2 0.1 0.050.020.0060.002 0.0006 0.0002

Ver ycoa-

rse

Coa

rse

Med

iumFine

Coa

rse fine

Coa

rse Fine Coarse Fine Ultrafine

Mo* Silt Clay

International classification

(Mo* is a swedish term used for glacial silts or rock flour having little plasticity)

mm2.0

Gravel Sand

0.6 0.2 0.06 0.02 0.006 0.002 0.0006 0.0002

Coarse Medium Fine Coarse Medium Fine Coarse Medium(Colloids)Fine

Silt Clay

M.I.T. Classification

GravelCobble

Coarse Fine Fine

Sand

mm 300 80 20 4.75 0.425 0.75 0.005

Silt Clay

Gravel

2.00 0.002

Coarse Medium

I.S. Classification (IS: 1498-1970)
Fig. 4.1 Grain-size classifications
A soil classification system purely based on grain-size is called a ‘Textural classifica-
tion’. One such is the U.S. Bureau of Soils and P.R.A. Classification depicted by a ‘Triangular
chart’ (Fig. 4.2), which ignores the fraction coarser than sand:
Any soil with the three constituents—sand, silt and clay—can be represented by one
point on the Triangular chart. For example, a soil with 25% sand, 25% silt and 50% clay will be
represented by the point ‘S’, obtained by the dotted lines, as shown by the arrows. Certain
zones on the chart are marked to represent certain soils such as sand, silt, clay, sandy clay,
silty clay, loam, sandy loam, etc. These have been marked rather arbitrarily. (‘Loam’ is prima-
rily an agricultural term).
Textural or grain-size classifications are inadequate primarily because plasticity
charateristics—consistency limits and indices—do not find any place in these classifications.