Geotechnical Engineering

DHARM

56 GEOTECHNICAL ENGINEERING

reading on this account. For this purpose, the hydrometer is placed in clean distilled water at

different temperatures and a calibration chart prepared for the correction required. If the

temperature at the time of test is more than that of calibration of the hydrometer, the ob-

served reading will be less and the correction (Ct) would be positive and vice versa.

Deflocculating Agent Correction

The addition of the deflocculating agent increases the density of the suspension and thus ne-
cessitates a correction (Cd) which is always negative. This is obtained by immersing the hy-
drometer, alternately in clean distilled water and a solution of the deflocculating agent in
water (with the same concentration as is to be used in the test), and noting the difference in
the reading.
A composite correction for all the above may be obtained by noting the hydrometer
readings in a solution of the deflocculating agent at different temperatures. These with reversed
sign give the composite correction.
The corrected hydrometer reading Rh may be got from the observed reading Rh′ by ap-
plying the composite correction ‘C’:
Rh = Rh′ ± C ...(Eq. 3.33)
where C = Cm – Cd ± Ct
The next step is to determine the percentage finer of the particles of a specified size
related to any hydrometer reading.
Calculations: After obtaining the corrected hydrometer readings Rh′ at various elapsed
times t, and the corresponding effective depths He, Equations 3.20 and 3.21 may be used (He
being used for H), to obtain the corresponding particle size. Now Eq. 3.27 may be used as
follows for determining the percent finer than the particle size D:

N =

100

1

G

()G−. (γz – γw). V/W

But γz = Gss γw, where Gss = specific gravity of soil suspension

=^1 + 1000

F

HG

I

KJ

Rh

. γw, since Rh = 1000(Gss – 1).

∴ N =

100

1 1000 1 10

G

G

R V

W

G

G

V

W

. wh w Rh
()

.

()

..

γγ

−

×=

−

∴ N =

G

G

V

W

γwhR

()

..

− 110

...(Eq. 3.34)

Thus for each hygrometer reading, Rh, we obtain a set of values for D and N, fixing one

point on the grain-size distribution curve.

The grain-size distribution may thus be completed by the sedimentation analysis in

conjunction with sieve analysis for the coarse fraction.

3.8.4 Characteristics of Grain-size Distribution Curves

Grain-size distribution curves of soils primarily indicate the type of the soil, the history and

stage of its deposition, and the gradation of the soil, If the soil happens to be predominantly

coarse-grained or predominantly fine-grained, this will be very clearly reflected in the curve.