Civil Engineering Reference
In-Depth Information
14.2.4 Percentage air voids, V
a
Saturation line
Figure
14.3
illustrates the saturation line, or zero air voids line as it is often called. It represents the dry
densities that would be obtained if all the air in the soil could be expelled, so that after compaction the
sample became fully saturated; this state is impossible to achieve by compaction either in the laboratory
or in the field, but with the compactive efforts now available it is quite common for a soil to have as little
as 5% air voids after compaction.
BS 1377-4:1990 expresses the percentage air voids as the volume of air in the soil expressed as a per-
centage of the total volume, rather than as a percentage of the void volume. Hence 5% air voids does
not mean the same as 95% degree of saturation.
Air voids line
Just as for the saturation line, it is possible to draw a line showing the dry density to water content rela-
tionship for a particular air voids percentage. (See Fig.
14.3.
)
Air volume
Total volume
V
V
1
1 1/
S
−
+
−
+
v
w
r
V
=
=
=
(
dividing by
V
v
)
a
V
V
e
v
s
hence:
V
n
a
V
= −
n
(
1
S
)
or
S
= −
1
a
r
r
In a partially saturated soil:
wG
S
s
e
=
r
and, substituting in the above expression for S
r
:
wG V
V
+
s
a
e
=
1
a
and substituting in the expression
ρ
w
G
e
ρ
=
d
1
+
G
wG V
V
ρ
w s
s
ρ
=
d
+
1
a
+
1
−
a
which eventually leads to:
1
1
−
V
a
ρ
=
ρ
d
w
w
+
G
s
It is seen that by putting V
a
=
0 in the above expression we obtain the relationship between dry density
and water content for zero air voids, i.e. the saturation line.
