Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 221

desaturate as it is subjected to ever-increasing applied soil
suctions. Figure 5.48 shows the desaturation stages along the
desorption branch of the SWCC (White et al., 1970). There
are three identifiable stages of desaturation: the boundary
effect stage, the transition stage (i.e., with primary and sec-
ondary transition stages), and the residual stage of desat-
uration. The air-entry value separates the boundary effect
stage and the transition stage. The residual value separates
the transition stage and the residual stage. These stages can
be clearly defined on a plot of degree of saturation versus
logarithm of soil suction. The entire picture of unsaturated
soil behavior is most clearly understood when the SWCC
data are plotted from a low suction value (e.g., 0.1 kPa) to
the maximum value of 10 6 kPa.
A line joining the air-entry value and the residual value (i.e.,
on a degree of saturation versus logarithm of soil suction plot)
defines the primary information required fromSWCC. Similar
features can also be observed on the wetting SWCC branch.
However, the first break on the wetting curve (starting from a
high-suction value to a lower suction value) is called thewater-
entry value and the next break on the wetting curve defines
the point where only occluded bubbles remain in the soil. The
occluded bubble point on the wetting curve is referred to as
the “air closure” soil suction value.

applied soil suctions. Figure 5.49 shows the SWCCs for a
sand soil and a graded silt soil. The distinctive features that
must be identified along the desorption branch are the air-
entry value and the residual conditions (i.e., residual suction
and residual water content). It is useful to plot all data sets to
a similar abscissa and ordinate scale. It is suggested that the
abscissa be a logarithm scale in kilopascals that commonly
ranges from 0.1 to 10 6 kPa. For some coarse-grained soils
it might also be necessary to extend the soil suction scale
lower than 0.1 kPa. The variable and units used to designate
the amount of water in the soil should be clearly designated.
Each SWCC should be viewed as a desorption “fingerprint”
for the soil. When the volume change of the soil is small,
any of the three measures for the amount of water in the soil
(i.e., gravimetric water content, volumetric water content,
and degree of saturation) will yield a similar interpretation
of key breaking points on the SWCC.
The sand soil shown in Figure 5.49 has an air-entry value
of 1.3 kPa and a residual suction of 5 kPa with a residual
gravimetric water content of 7%. The graded silt has an
air-entry value of 3 kPa and a residual suction of approxi-
mately 100 kPa with a residual gravimetric water content of
11%. The slope of a line joining the air-entry value and the
residual point (i.e., slope at the inflection point) provides a
measure of the rate at which water is removed from the soil
as suction is increased past the air-entry value. The slope
of the SWCC indicates the rate of change of water storage
in the soil with respect to soil suction. The interpretation
of the SWCC for a low-volume-change soil is based on the
assumption that changes in void ratio approach zero as suc-
tion is increased. Consequently, dw or d θ or dS portray
similar information regarding unsaturated soil behavior.

5.5.4 Interpretation of SWCC Data
for Low-Volume-Change Soil
The interpretation of pressure plate data is often based on the
assumption that the overall volume change of the soil spec-
imen is negligible during the laboratory test. In agriculture-
related disciplines, the water content versus soil suction
relationship is of greatest importance and the volume change
of the soil is of little consequence. In geotechnical engineer-
ing applications, the amount of volume change in the soil
can be of significance when interpretating laboratory data.
Most sand soils (and even most silt soils) undergo only
a small amount of volume change over a wide range of

5.5.5 Interpretation of SWCC Data
for High-Volume-Change Soil
The interpretation of SWCC data for a soil that undergoes a
high volume change in response to changes in soil suction

Transition
stage

Boundary
effect stage

Residual
conditions

Figure 5.48 Definition of variable associated with typical SWCC.