Geotechnical Engineering

DHARM

IDENTIFICATION AND CLASSIFICATION OF SOILS 99

Fine-grained soils are subdivided into those with low plasticity (L), with liquid limit less
than 50%, and those with high plasticity (H), with liquid limits more than 50%. Dilatancy, dry
strength and toughness tests are to be used for filed identification.

The plasticity chart devised by Casagrande is used for identification of finegrained soils
(Fig. 4.3).

MHMH&OH&OH

CHCH

CLCL

ML &

OL

CLCL–ML–ML

120110100908070605040302010

Liquid limit

80

70

60

50

40

30

20

Plasticity index

10

0

A-line

IPL= 0.73 (W – 20)

Toughness and dry strength increase

Permeability & volume change

decrease

Comparing soils at equal w

Toughness and dry strength decrease

Permeability & vol. change increase

L A-line

Fig. 4.3 Plasticity chart (unified soil classification)

4.5.6 Indian Standard Soil Classification System

IS: 1498-1970 describes the Indian Standard on Classification and Identification of Soils for
general engineering purposes (first revision). Significant provisions of this system are given
below :

Soils shall be broadly divided into three divisions :

1. Coarse-grained Soils: More than 50% of the total material by weight is larger than 75-
   μ IS Sieve size.


2. Fine-grained Soils: More than 50% of the total material by weight is smaller than 75-
   μ IS Sieve size.


3. Highly Organic Soils and Other Miscellaneous Soil Materials: These soils contain
   large percentages of fibrous organic matter, such as peat, and particles of decomposed vegeta-
   tion. In addition, certain soils containing shells, concretions, cinders and other non-soil mate-
   rials in sufficient quantities are also grouped in this division.

Coarse-grained soils shall be divided into two sub-divisions :
(a) Gravels: More than 50% of coarse fraction (+ 75 μ) is larger than 4.75 mm IS Sieve
size.

(b) Sands: More than 50% of Coarse fraction (+ 75 μ) is smaller than 4.75 mm IS Sieve
size.