Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 33

First, the weight of the empty pycnometer is determined (W 1 ) in the dry condition. Then
the sample of oven-dried soil, cooled in the desiccator, is placed in the pycnometer and its
weight with the soil is determined (W 2 ). The remaining volume of the pycnometer is then
gradually filled with distilled water or kerosene. The entrapped air should be removed either
by gentle heating and vigorous shaking or by applying vacuum. The weight of the pycnometer,
soil and water is obtained (W 3 ) carefully. Lastly, the bottle is emptied, thoroughly cleaned and
filled with distilled water or kerosene, and its weight taken (W 4 ).

With the aid of these four observations, the grain specific gravity may be determined as
follows:

(a) Empty

pycnometer wt. W 1

(b) Pycnometer +

Dry soil wt. W 2

(c) Pycnometer + soil

+ water wt. W 3

(d) Pycnometer +

water wt. W 4

Fig. 3.3 Determination of grain specific gravity

From the readings, the wt of solids Ws = W 2 – W 1 , from (a) and (b)

Wt of water = W 3 – W 2 , from (b) and (c)

Wt of distilled water = W 4 – W 1 , from (a) and (d)

∴ Weight of water having the same volume as that of soil solids = (W 4 – W 1 ) – (W 3 – W 2 ).

By definition, and by Archimedes’ principle,

G = Weight of soil solids

Weight of water of volume equal to that of solids

=

()

()(

WW

WW WW

21

41 32

−

−−−)

=

()

()(

WW

WW WW

21

21 34

−

−−−)

∴ G =

W

WWW

s

s−−()^34

...(Eq. 3.1)

Ws is nothing but the dry weight of the soil.

Aliter. If the soil solids are removed from W 3 and replaced by water of equal volume, W 4
is obtained.

Volume of solids =

W

G

s

∴ W 4 = W 3 – Ws +

W

G

s

Hence, G =

W

WWW

s

()(s −− 34 )

, same as Eq. 3.1.