DHARMINDEX PROPERTIES AND CLASSIFICATION TESTS 47
total sample. The percentage material finer than a sieve size may be got by subtracting this
from 100. The material passing the bottom-most sieve, which is usually the 75–μ sieve, is used
for conducting sedimentation analysis for the fine fraction.
If the soil is clayey in nature the fine fraction cannot be easily passed through the 75–μ
sieve in the dry condition. In such a case, the material is to be washed through it with water
(preferably mixed with 2 gm of sodium hexametaphosphate per litre), until the wash water is
fairly clean. The material which passes through the sieve is obtained by evaporation. This is
called ‘wet sieve analysis, and may be required in the case of cohesive granular soils’.
Soil grains are not of an equal dimension in all directions. Hence, the size of a sieve
opening will not represent the largest or the smallest dimension of a particle, but some inter-
mediate dimension, if the particle is aligned so that the greatest dimension is perpendicular
to the sieve opening.
The resulting data are conventionally presented as a “Particle-size distribution curve”
(or “Grain-size distribution curve”–the two terms being used synonymously hereafter) plotted
on semi-log co-ordinates, where the sieve size is on a horizontal ‘logarithmic’ scale, and the
percentage by weight of the size smaller than a particular sieve-size is on a vertical ‘arithmetic’
scale. The “reversed” logarithmic scale is only for convenience in presenting coarser to finer
particles from left to right. A typical presentation is shown in Fig. 3.9. (Results may be presented
in tabular form also).
Logarithmic scales for the particle diameter gives a very convenient representation of
the sizes because a wide range of particle diameter can be shown in a single plot; also a different
scale need not be chosen for representing the fine fraction with the same degree of precision as
the coarse fraction.
100806040200Percent finer by weight10 9 8 7 6 5 4 3 2 1 9 8 7 6 5 4 3 2 0.19 8 7 6 5 4 3 2 0.01
Sieve size (particle size) mm (log scale)
Fig. 3.9 Particle-size distribution curve
The characteristics of grain-size distribution curves will be studied in a later sub-section.