Environmental Engineering Reference
In-Depth Information
3.6.1 Independence of Components of Soil Suction
Total suction
ψ
has two primary components of suction,
namely, matric suction
u
a
−
u
w
and osmotic suction
π
.
There may be a tendency to think that the components of
suction can always be added together and then used in a
combined form. However, each component of soil suction
may act in independent manners in the soil.
For example, it is unacceptable to use total suction to
model water flow in an unsaturated soil. It is possible for the
matric suction to result in water flow in a particular direction
while the chemical concentration gradient may result in flow
in another direction. The independent flow mechanisms can
only be taken into consideration if the matric suction and
osmotic suction are handled in an independent manner.
Equation 3.63 simply provides a mathematical relation-
ship between the components of suction and can only be
used for that purpose. Through use of thermodynamic con-
siderations, Edlefsen and Anderson (1943) showed that Lord
Kelvin's equation applied to both capillary phenomena as
well as osmotic phenomenon. However, this does not mean
that the components of suction can be added and then used
to describe all physical processes.
Total suction is measured through the assessment of vapor
pressure. Evaporation from a soil is controlled through a
vapor pressure gradient. The calculation of actual evapora-
tion or potential evaporation from a soil surface requires a
vapor pressure gradient in the soil or external to the soil.
Matric suction is written as the difference between the
pore-air pressure
u
a
and the pore-water pressure
u
w
.Matric
suction does not cause liquid or vapor flow. Rather, it is the
independent components of air and water that can be used
to calculate the flow of the independent phases. A gradient
in the air density or an air concentration gradient can cause
the flow of air through a soil. Water pressure combines with
elevation head to produce hydraulic head, which causes the
flow of water through a soil. It is possible to have water
flowing in one direction through a soil while having air
flow in another direction. In other words, total suction or
total potential should not be used to model the process of
water flow.
Osmotic suction is related to the salt concentration in the
free pore-water of a soil. Differences in salt concentration
at different points in a soil can cause the movement of salts
due to a concentration gradient. However, there may or may
not be any movement of water.
Equation 3.63 is of significant value when solving unsat-
urated soil mechanics problems; however, it is important
that the relationship be used in a proper manner. Formu-
lations that use total suction to model the flow of liquid
water through a soil would constitute a questionable usage
of Eq. 3.63. The physics associated with a particular phys-
ical mechanism must be adhered to in order to ensure the
correct usage of soil suction.
Figure 3.25
Total, matric, and osmotic suction measurements on
compacted Regina clay (from Krahn and Fredlund, 1972).
specimen is generally immersed in distilled water at the
start of the test (e.g., volume change test in an oedome-
ter). The matric suction is reduced to zero by immersing
the soil specimen. The osmotic suction in the sample may
also be changed in the process. It is not necessary to know
the change in osmotic suction provided changes occurring
in the field are simulated in the laboratory test.
In the case where the salt content of the soil is altered by
chemical contamination or deliberate chemical change, the
effect of the osmotic suction change on the soil behavior may
be significant. In this case, it is necessary to consider osmotic
suction as part of the stress state or as an independent stress
state variable. The significance of the osmotic suction com-
ponent can apply equally for saturated and unsaturated soils.
The role played by osmotic suction in influencing the
mechanical behavior of a soil may or may not bear a one-to-
one correspondence with the role played by matric suction.
The osmotic suction is more closely related to the diffuse
double layer around the clay particles whereas matric suc-
tion is mainly associated with the air-water interface (i.e.,
contractile skin). It is possible and likely more reasonable
to consider osmotic suction
π
as an independent, isotropic
stress state variable.
In the case where matric and osmotic suctions have the
same quantitative influence on the behavior of a soil, the
soil suction stress tensor can be written in the form
⎡
u
a
−
u
w
+
⎤
π
0
0
u
a
−
u
w
+
⎣
⎦
0
π
0
u
a
−
u
w
+
0
0
π
(3.64)
Research has been published that suggests it may be pos-
sible to algebraically combine matric suction and osmotic
suction when analyzing some geotechnical problems (Bai-
ley, 1965; Chattopadhyay, 1972).
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