Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 57

Typical grain-size distribution curves are shown Fig. 3.14. A soil is said to be “well-
graded”, if it contains a good representation of various grain-sizes. Curve marked (a) indicates
a well-graded soil. If the soil contains grains of mostly one size, it is said to be “uniform” or
“poorly graded”. Curve marked (b) indicates a unifrom soil. A soil is said to be “gap-graded”, if
it is deficient in a particular range of particle sizes. Curve marked (c) indicates a gap-graded
soil. A “young residual” soil is indicated by curved marked (d); in course of time, as the particles
get broken, the soil may shown a curve nearing type (a). Curve (e) indicates a soil which is
predominantly coarse-grained, while curve (f) indicates a soil which is predominantly fine-
grained. The more uniform a soil is, the steeper is its grain-size distribution curve.

c

b

a f

c

0.075

100 60 20 10 6 2 1 0.6 0.10.2 0.020.06 0.010.0060.0020.001

Grain size mm

100

90

80

70

60

50

40

30

20

10

0

Per cent finer (by weight)

d

Coarse=grained fraction

Serve analysis

Fine-grained fraction

Sedimentation analysis

Fig. 3.14 Typical grain-size distribution curves
River deposits may be well-graded, uniform, or gap-graded, depending upon the veloc-
ity of the water, the volume of suspended solids, and the zone of the river where the deposition
occurred.

Certain properties of granular or coarse-grained soils have been related to particle di-
ameters. Allen Hazen (1892) tried to establish the particular diameter in actual spheres that
would cause the same effect as a given soil, and opined that the diameter for which 10% was
finer would give this equivalence. It may be recalled that the effective diameter of a soil particle
is the diameter of a hypothetical sphere that is assumed to act in the same way as the particle
of an irregular shape, and that data obtained from sedimentation analysis using Stokes’ law
lead to effective diameters, De, of the soil particles. Thus, Allen Hazen’s D 10 = De. The effective
diameter is also termed the “Effective Size” of the soil. It is this size that is related to permeability
and capillarity. D 10 may be easily determined by reading-off from the grain-size distribution
curve for the soil.

An important property of a granular or coarse-grained soil is its “degree of uniformity”.
The grain-size distribution curve of the soil itself indicates, by its shape, the degree of soil
uniformity. A steeper curve indicates more uniform soil. Because of this, the grain-size distri-
bution curve is also called the ‘uniformity curve’.