Environmental Engineering Reference
In-Depth Information
construction procedure identifies the distinct residual suction
point common to most SWCCs.
Nonlinear unsaturated soil property functions are com-
monly used for seepage, shear strength, and volume change
problems when soil suction is less than 1500 kPa. The
SWCC for a soil is defined over the entire range of soil
suction; however, estimated unsaturated soil properties may
not be extended to suctions higher than residual suction.
The entire SWCC is used in the estimation of actual evap-
oration from a soil surface (Wilson et al., 1994). Evaporation
from the soil surface occurs as a result of a vapor pressure
gradient between the air and the soil. Vapor pressure, in
turn, is affected by both matric suction and osmotic suction
(i.e., total suction). Coupled heat and moisture flow analyses
make use of the entire SWCC when predicting liquid and
vapor flow. The osmotic effect of salts at the soil surface is
particularly important in the high-soil-suction range.
Certain phenomena appear to be dominant in the low-
suction range but disappear in the high-suction range. Other
phenomena become dominant in the high-suction range.
The matric suction range has been applied to many classic
geotechnical engineering analyses and the total suction has
likewise been applied to other geotechnical engineering
analyses.
A single SWCC with its mixed components of matric and
total suction has been successfully used in unsaturated soil
mechanics. It is anticipated that this will continue to be the
way in which the SWCC is plotted and used in geotechnical
engineering. There may be some exceptions that will arise
where it is important to understand the independent roles of
the components of soil suction.
equation can be viewed as the water retention curve for air
and can be written as follows:
ln
u
v
u
v
0
RT
K
v
w
0
ω
v
ψ
=−
(5.10)
where:
ψ
=
total suction, kPa,
R
=
universal gas constant [8.31432 J/(mol K)],
T
K
=
absolute temperature, K
(T
K
=
273
.
16
+
T
, where
temperature,
◦
C),
T
=
specific volume of water, m
3
/
kg,
v
w
0
=
ω
v
=
molecular mass of water vapor (18.016 kg/kmol),
u
v
=
partial pressure of water, kPa, and
u
v
0
=
saturation pressure of water vapor over a flat surface
of pure water.
u
v
/u
v
0
) can
be substituted into Eq. 5.10 and presented graphically
(Fig. 5.14). The semilogarithm graph shows an asymptotic
behavior as relative humidity approaches 100% and also as
relative humidity approaches zero.
Figure 5.14 shows that soil suction changes from zero to
2700 kPa as relative humidity changes from 100 to 98%. Soil
suction changes rapidly in the high-relative-humidity range,
the range of significant interest in geotechnical engineering.
The free-energy curve in the low-relative-humidity range
(i.e., below 50% relative humidity) is shown in an expanded
form in Fig. 5.15. The extrapolation of a straight-line portion
on the semilog plot of relative humidity values below 50%
gives a total suction of approximately 400,000 kPa at zero
relative humidity. However, the low-relative-humidity por-
tion of the curve bends toward 10
6
kPa and beyond. A soil
suction value of 10
6
is not a singular total-suction value since
the curve theoretically extends to infinity. Figure 5.16 shows
the data from Fig. 5.15 plotted using a log-log format. The
results show that a total suction (or free-energy value) of 10
6
kPa corresponds to a relative humidity value of 0.06%.
Values of
relative humidity (i.e., RH
=
5.2.4 Upper Limit for Soil Suction
The Gibbs free-energy state equation for water vapor has
formed the basis for the idea that the maximum soil suction
is approximately 10
6
kPa (Gibbs, 1873; Edlefsen and Ander-
son, 1943). The free-energy state equation is a thermodynamic
relationship between soil suction and the partial pressure of
water vapor (i.e., relative humidity). The Gibbs free-energy
100
80
60
40
20
0
10
6
10
7
1
10
100
1000
10,000
100,000
Total suction, kPa
Figure 5.14
Soil suction as a function of relative humidity in the ambient or internal pore-air.
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