Geoscience Reference
In-Depth Information
Two additional relationships, moisture content ( w ) and degree of saturation ( S ), relate the water
content of the soil and the volume of the water in the void space to the total void volume:
W
W
w
s
w
= 100%
(12.6)
and
V
V
w
v
S
= 100%
(12.7)
12.4.2 s peCiFiC g ravity
The specific gravity of a substance is the ratio of the unit weight of that substance to the unit weight
of water at 20°C. This is expressed in equation form as
w
w
= substance
water
SG
(12.8)
The specific gravity of a soil's solids is abbreviated G S . This value is the ratio of the unit weight
of the soil solids condensed into a solid mass to the unit weight of water. The equation for specific
gravity for soil's solids is
wV
w
ss
G
=
s
water
(12.9)
where
G S = Soil solids.
W S = Weight of a soil mass after drying.
V S = Solids volume.
A laboratory test may be performed to determine the specific gravity of the soil solids; however,
in some field situations, such data may not be available and an estimate must necessarily be used.
The specific gravity of a soil depends on the mineralogy of the soil grains. Most oils are a blend of
several basic minerals such as quartz, feldspar, hornblende, biotite, calcite, and others. A determina-
tion of the constituents of a soil is helpful in estimating a value for the soil's specific gravity. Table
12.1 provides the specific gravity values for some of the more important soil minerals.
TABLE 12.1
Specific Gravity of Select Soil Minerals
Mineral
Specific Gravity
Mineral
Specific Gravity
Montmorillonite
2.65-2.8
Dolomite
2.87
Kaolinite
2.6
Hornblende
3.2-3.5
Illite
2.8
Magnetite
5.17
Chlorite
2.60-3.00
Quartz
2.66
Calcite
2.72
Biotite
3.0-3.1
 
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