Geotechnical Engineering

DHARM

52 GEOTECHNICAL ENGINEERING

The calculations completed for all the samples would provide information for the grain-
size distribution curve.

The pipette analysis, although very simple and direct in principle, is tedious and requires
very sensitive weighing apparatus. Accurate results are rather difficult to obtain. For
this reason, the hydrometer analysis is preferred in the laboratory.

Hydrometer Analysis

The hydrometer method differs from the pipette analysis in that the weights of solids
per ml in the suspension at the chosen depth at chosen instants of time are obtained indirectly
by reading the specific gravity of the soil suspension with the aid of a hydrometer.

1.040

1.030

1.020

1.010

1.000

–5 0.995

0

5

10

15

20

25

30

35

40

Rh Sp. gr.

Fig. 3.10 Hydrometer
Hydrometer is a device which is used to measure the specific gravity of liquids (Fig. 3.10).
However, for a soil suspension, the particles start settling down right from the start, and
hence the unit weight of the suspension varies from top to bottom.
It can be established that measurement of unit weight of the suspension at a known
depth at a particular time provides a point on the grain-size distribution curve.

Let W be weight of fine soil fraction mixed in water

V be the volume of suspension

Initially, the weight of solids per unit volume of suspension

= W/V

Volume of solids per unit volume of suspension =

W

VG..γw

Volume of water per unit volume of suspension = 1 –

W

VG..γw

Note. The readings may indicate di-

rectly the specific gravity, as shown on

the right; 1 may be subtracted from the

specific gravity and the resulting value

multiplied by 1000 and marked as the

hydrometer reading, as shown on the

left.