Environmental Engineering Reference
In-Depth Information
correct air-entry value must be determined from the degree
of saturation versus logarithm of soil suction plot. The air-
entry value of clay soils can be extremely high, particularly
when the soil is in an initial slurry state.
Residual conditions may not be clearly defined on any
of the forms of the SWCC plot. Residual conditions would
appear to be near to (or below) the shrinkage limit of the
soil. The measured shrinkage limit for Regina clay was 13%
and the corresponding degree of saturation would appear to
be around 50%. In this case, residual suction would appear
to be on the order of 30,000 kPa. The air-entry values and
the residual values for the highly plastic clay illustrate the
significantly different SWCC behavior that can be observed
for sand soils and clay soils. The degree of saturation versus
soil suction plot provides the most valuable information for
the estimation of the air-entry value of a soil. The degree
of saturation versus soil suction plot is generally the most
meaningful relationship to use for the estimation of unsatu-
rated soil property functions.
It is also possible to plot volumetric water content versus
soil suction for Regina clay samples tested. The differentiation
(or slope) of the volumetric water content versus soil suction
plot provides the water storage function required as input for a
transient unsaturated soil seepage analysis. Volumetric water
content can be calculated in one of two ways: with reference
to either the original total volume or the instantaneous total
volume. The volumetric water content is defined as follows
when calculations are referenced to the original total volume:
4.0
Pressure
plate
extractor
3.5
Evaporation
3.0
2.5
Two specimens
2.0
Saturation line
1.5
1.0
0.5
0.0
0
20
40
60
80
100
Water content, %
Figure 5.51
Specific bulk volume versus gravimetric water con-
tent for Regina clay (from Fredlund, 1964).
V
w
V
t
0
θ
1
=
(5.62)
where:
θ
1
=
referential volumetric water content,
V
w
=
volume of water, and
V
t
0
=
total volume of the soil specimen at the start of the
test.
Figure 5.52
Degree of saturation versus gravimetric water con-
tent for Regina clay (from Fredlund, 1964).
Volumetric water content (i.e., referenced to the original
total volume) versus soil suction results in similar limita-
tions to those encountered when using gravimetric water
content. On the other hand, a plot of instantaneous volumet-
ric water content versus soil suction exhibits the character
of the degree of saturation versus soil suction plot. It is
important to specify the manner in which volumetric water
content is computed when high-volume-change soils are
being analyzed.
The air-entry value and the residual conditions provide
the most important information for generating unsaturated
soil property functions. Consequently, a plot of degree of
saturation (or instantaneous volumetric water content) is of
greatest significance to the geotechnical engineer. This is
disconcerting since it is difficult to directly measure the
overall volume of the soil specimen when using conven-
tional pressure plate equipment.
a graph that is similar in shape to the gravimetric water
content SWCC with the exception that the high-suction por-
tion remains horizontal at a void ratio corresponding to the
shrinkage limit (i.e.,
e
0
.
45). The large volume changes
that occur as soil suction is increased tend to mask the actual
air-entry value of the soil. The shrinkage curve of the soil
can be measured and used to assist in the interpretation of
the SWCC test results. It is possible to plot degree of sat-
uration versus the logarithm of soil suction and determine
the correct air-entry value of Regina clay.
Figure 5.13 (i.e., previously shown in Chapter 5) is a plot
of the degree of saturation versus soil suction for Regina
clay soil specimens. The results show that all soil specimens
produce essentially one unique line with a distinct break for
the air-entry value around 1500 kPa. In other words, the
=
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