Civil Engineering Reference
In-Depth Information
Both unit weight and water content are obtained from direct measurements of dimen-
sions and weights of samples before and after drying. Specific volume and degree of
saturation can be calculated from these from
=
γ
w
G
s
(1
+
w
)
v
(26.5)
γ
and
wG
s
(
v
S
r
=
(26.6)
−
1)
The expressions in Eqs. (26.5) and (26.6) can be obtained from Eqs (26.1) to (26.4)
making use of Fig. 26.2. Notice that from Eq. (26.6), the specific volume depends on
both the water content and on the degree of saturation so, in an unsaturated soil, the
water content can change without any change in the volume of the soil.
26.4 Distribution of air and water in unsaturated soil
The way in which water and air are distributed through unsaturated soil is important.
Figure 26.3 illustrates an ideal soil. In Fig. 26.3(a) the water content and degree of
saturation are small. Water collects at the points of contact forming meniscus water
bridges. The air is continuous throughout the soil and air pressures are the same
everywhere. The water is not continuous. Water pressures depend on the radii of the
meniscuses, which may be different at different contact points and so water pressures
may vary through the soil. The meniscus water bridges have the effect of bonding the
grains together.
In Fig. 26.3(c) the degree of saturation is large. The water is continuous throughout
the soil, the water pressure is the same at any horizon and it varies with depth. The
gas, which is probably water vapour, is in bubbles and is not continuous. The pressure
in the gas depends partly on the sizes of the bubbles, which may differ throughout
the soil.
In Fig. 26.3(b) both the air and the water are continuous, in three dimensions,
throughout the soil. The water pressure is governed by the radii of the meniscuses
Figure 26.3
Distributions of air and water in unsaturated soils.
