Geotechnical Engineering

DHARM

98 GEOTECHNICAL ENGINEERING

100

90

80

70

60

50

Clay %

40

30

Silty clay

Loam

20

10

0

100908070

SiltSilt

SiltySilty clayclay

SiltySilty clayclay

SandySandy loamloam LoamLoam

SandySandy clayclay clayclay LoamLoam

Silt %

6050403020100

100

90

80

70

Loam

Sandy clay

Sand %

60

50

40

30

20

10

0

Clay

S

Clay

S

SandSand

Fig. 4.2 Bureau of soils triangular chart for textural classification

4.5.5 Unified Soil Classification System

The Unified soil classification system was originally developed by A. Casagrande and adopted
by the U.S. Corps of Engineers in 1942 as ‘Airfield Classification’. It was later revised for
universal use and redesignated as the ‘‘Unified Soil Classification’’ in 1957.

In this system (Table 4.1), soils are classified into three broad categories :

1. Coarse-grained soils with up to 50% passing No. 200 ASTM Sieve
   (No. 75-μ IS Sieve)


2. Fine-grained soils with more than 50% passing No. 200 ASTM Sieve
   (No. 75-μ IS Sieve)


3. Organic soils
   The first two categories can be distinguished by their plasticity characteristics. The
   third can be easily identified by its colour, odour and fibrous nature.

Each soil component is assigned a symbol as follows :
Gravels: G Silt: M (from the Swedish Organic: O
word ‘Mo’ for silt
Sand: S Clay: C Peat: Pt
Coarse-grained soils are further subdivided into well-graded (W) and poorly graded (P)
varieties, depending upon the Uniformity coefficient, (Cu) and coefficient of Curvature (Cc):

Well-graded gravel, Cu > 4

Well-graded sand, Cu > 6

Well-graded soil, Cc = 1 to 3

Note that Cu is the same as U defined in Eq. 3.35.