Environmental Engineering Reference
In-Depth Information
4.6.4 Estimation of Osmotic Suction
The salt content of a soil from a particular geological stratum
is usually found to be relatively constant, and as a result, the
osmotic suction component is relatively constant over a con-
siderable range of water contents. For example, the osmotic
suction of glacial till deposits in western Canada is com-
monly on the order of about 200 kPa and the osmotic suction
for Regina clay ranges from 150 to 200 kPa. Osmotic suction
measurements are not a routine part of geotechnical engineer-
ing practice; rather, typical values are often obtained from
special studies where the salt content has been measured.
The salt content of soils near ground surface can be altered
as a result of excessive evaporation or excessive infiltra-
tion. The conditions associated with excessive evaporation
of water at ground surface is a common occurrence. As water
evaporates from the ground surface, the salts are left behind
as a white deposit. The increased salts at ground surface
have the effect of increasing the total suction and reducing
the vapor pressure gradient between the soil and the atmo-
sphere. As a result, moisture loss from the ground surface
can be greatly reduced or “shut off.” White salt deposits on
the ground surface are an indication of an increased osmotic
suction (and total suction) and a subsequent reduction in
evaporation of water from the ground surface.
Special engineering studies might be encountered where
the salt content in the soil is altered over time. Under these
conditions it is osmotic suction that is the key variable being
altered and attention needs to be given to the engineering
significance of changing the salt content.
The concentration of salt in water can be designated in
several ways. Table 4.19 provides some typical values for
salt solutions found in bodies of water.
Osmotic suction can be estimated from the concentration
of salts found in the water. The same theory applies whether
the salt water is squeezed from the soil or exists in a large
Table 4.19 Typical Salt Concentrations in Bodies
of Water
..............................................
Concen-
Concen-
Electrical
tration,
s
tration,
s
s
Conductivity
Water Source
(g/g), %
(g/L)
(mmhos/cm)
10
−
8
Distilled water
0.0
0.0
5
.
5
×
Freshwater
0.02
0.2
0.005 - 0.05
Ocean water
3.5
36
48
Semisaline water
8.0
88
Potash brine
20
250
Dead-sea water
22
270
body of water. Osmotic suction can be designated in terms
of the molarity of a salt solution through use of the van't
Hoff equation (Campbell, 1985):
π
=
νCRT
K
(4.58)
where:
π
=
osmotic suction, kPa,
R
=
universal
(molar) gas constant
[i.e., 8.31432
J/(mol K)],
T
K
=
absolute temperature,
K
(i.e.,
T
K
=
273
.
16
+
T
,
temperature,
◦
C),
where
T
=
C
=
molarity of the salt solution, and
ν
=
number of osmotic active particles per molecule
(e.g., 2 for NaCl).
The amount of salt in a solution can vary over a wide
range, and therefore, it is useful to plot both the molarity and
the osmotic suction on a logarithm scale. Figure 4.108 shows
1,000,000
100,000
10,000
20
°
C
1,000
100
10
1
0.001
0.01
0.1
1
10
100
Molarity, M
Figure 4.108
Osmotic suction versus molarity of NaCl solution.
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