Geotechnical Engineering

DHARM

34 GEOTECHNICAL ENGINEERING

If kerosene is used,

G = WG

WWW

s k

s

−−() 34

...(Eq. 3.2)

where Gk = Specific gravity of kerosene at the temperature of the test.
Kerosene is used in preference to distilled water, if a density bottle is used; kerosene
has better wetting capacity, which may be needed if the soil sample is of clay. In the case of
clay, de-airing should be done much more carefully by placing the bottle in a vacuum desiccator
for about 24 hours. This procedure should be resorted to for obtaining the weights W 3 and W 4.

Conventionally, the specific gravity is reported at a temperature of 27°C. If the room
temperature at the time of testing is different from this, then temperature correction becomes
necessary. Alternatively, the weights W 3 and W 4 should be taken after keeping the bottle in a
constant temperature bath at the desired temperature of 27°C.

If the specific gravity, determined at a temperature of T 1 °C, is GT 1 , and it is desired to

obtain the specific gravity GT 2 at a temperature of T 2 °C, the following equation may be used:

GG

G

TTG

wT

(^11) wT
2
1
=.
()
() ...(Eq. 3.3)
where ()GwT 1 and ()GwT 2 are the specific gravities of water at temperatures T 1 °C and T 2 °C
respectively. (These should be the values for kerosene if that liquid has been used in plane of
water).
In other words, the grain specific gravity is directly proportional to the specific gravity
of the water at the test temperature. In the light of this observation, Eq. 3.1 is sometimes
modified to read as follows:
G =
WG
WGG
swT
s
.( )
−−() 34
...(Eq. 3.4)
where (Gw)T is the specific gravity of water at the test temperature.
If (Gw)T is taken as unity, which is true only at 4°C, Eq. 3.4 reduces to Eq, 3.1; that is to
say, Eq. 3.1 may be used if one desires to report the value of G at 4°C and if one would like to
ignore the effect of temperature. (The proof of the equations 3.3 and 3.4 is not difficult and is
left to the reader).
Since the specific gravity of water varies only in a small range (1.0000 at 4°C and 0.9922
at 40°C), the temperature correction in the determination of grain specific gravity is quite
often ignored. However, errors due to the presence of entrapped air can be significant.

3.5 Water Content

‘Water content’ or ‘moisture content’ of a soil has a direct bearing on its strength and stability.

The water content of a soil in its natural state is termed its ‘Natural moisture content’, which

characterises its performance under the action of load and temperature. The water content

may range from a trace quantity to that sufficient to saturate the soil or fill all the voids in it.

If the trace moisture has been acquired by the soil by absorption from the atmosphere, then it

is said to be ‘hygroscopic moisture’.