Environmental Engineering Reference
In-Depth Information
120
AEV=64 kPa
100
80
60
40
20
0
0.01
0.1
1
10
100
1000
Matric suction, (kPa)
(a)
100
80
60
40
20
AEV
0
0
50
100 150 200
Mean net stress (p - u
a
), kPa
250
300
350
(b)
Figure 5.92
SWCCs for compacted kaolin specimens tested in a modified triaxial apparatus:
(a) degree of saturation for specimen consolidated at 100 kPa, (b) change in AEV for various
consolidation pressures (after Thu et al., 2007).
temperature,
◦
C,
Drying SWCCs can be measured in a column test by satu-
rating the soil from the top of the column and then allowing
the water to drain down to an equilibrium hydrostatic state.
Equilibrium is generally attained within a few days. Samples
of the soil can be retrieved for water content measurements.
The distance from the water level at the bottom of the col-
umn is used to calculate respective matric suction values.
=
T
density of water, kg/m
3
,
ρ
w
=
ω
v
=
molecular mass of water vapor (i.e., 18.016 kg/
kmol),
u
v
=
partial pressure of pore-water vapor, kPa, and
u
v
0
=
saturation pressure of water vapor over a flat surface
of pure water at the same temperature, kPa.
The term
u
v
/u
v
0
is called relative humidity RH. At a
particular temperature the variables in front of the natural
logarithm of relative humidity (i.e., free-energy gas constant)
become a constant. The free-energy constant is
5.10 VACUUM DESICCATORS
FOR HIGH SUCTIONS
135,053
and it is possible to write total suction as a function of
relative humidity (or relative vapor pressure
h
r
):
−
The vapor pressure equilibrium technique can be used to
measure the water content versus total suction relationship in
the high-suction range. Desiccators can be used to establish
a partial vapor pressure in the soil (Edlefsen and Ander-
son, 1943; Richards, 1965). The thermodynamic relationship
between total suction (or the free energy of the soil-water)
and the partial pressure of the pore-water vapor can be writ-
ten as follows:
135
,
053 ln
u
v
u
v
0
ψ
=−
(5.80)
5.10.1 Use of Salts to Create
Constant-Relative-Humidity Environments
Constant-relative-humidity environments can be created in
the laboratory through use of either saturated or unsaturated
salt solutions. Any salt solution at a particular concentra-
tion and a constant temperature results in a fixed vapor
pressure environment under equilibrium conditions. Sulfuric
acid solutions can also be used.
Saturated
salt solutions provide a convenient, inexpen-
sive, and accurate controlled relative humidity environment.
ln
u
v
u
v
0
=
−
RT
κ
ρ
w
ω
v
ψ
(5.79)
where:
ψ
=
soil suction (or total suction), kPa,
R
=
universal (molar) gas constant [i.e., 8.31432 J/(mol
K)],
T
K
=
absolute temperature (i.e.,
T
K
=
273
.
16
+
T,K
),
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