Civil Engineering Reference
In-Depth Information
Figure 26.4
Meniscuses in unsaturated soils.
so air will enter the soil if
4
T
(
u
a
−
u
w
)
>
(26.11)
(
v
−
1)
d
s
10
−
5
kNm
−
1
air at atmospheric pressure will enter
soil with a grain size of 0.001 mm and specific volume of 1.5 if the suction is about
600 kPa.
As air enters the soil the water content and degree of saturation reduce. There will
be a relationship between suction and degree of saturation which depends principally
on the grading of the soil. This relationship is known as the water retention curve or
the soil water characteristic curve. Figure 26.5 shows typical water retention curves
for a soil. There are different curves for drying and for wetting and there will be
different curves for different soils. When the soil is saturated the degree of saturation
is
S
r
×
Taking a value for
T
of about 7
1. At the air entry suction, which is related to grain size by Eq. (26.11), the
degree of saturation starts to reduce. As the soil dries it will compress but the change in
volume is not related to the change in water content as it was in saturated soil. There
is a minimum degree of saturation where there are stable meniscus water bridges.
At low water contents and low degrees of saturation suctions can be very large indeed,
especially in fine grained soils.
=
Figure 26.5
Water retention curves for unsaturated soils.
