Stress-Deformation Analysis for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 725

Pressure plate extractors such as the type manufactured by
SoilMoisture Equipment Corporation, Santa Barbara, Cali-
fornia, are commonly used to measure the SWCC. Pressure
plate extractors can be designed for testing multiple soil
specimens or single soil specimens. One of the test proce-
dures for pressure plate tests is given in ASTM D2325. The
pressure plate extractor consists of a high-air-entry ceramic
disk contained within an air pressure chamber. The high-air-
entry disk is saturated and is always in contact with water
through a compartment below the disk. The compartment is
usually maintained at zero water pressure.
Further details concerning pressure plate apparatuses and
associated test procedures appropriate for geotechnical engi-
neering are described in Chapter 5. Some of the apparatuses
described in Chapter 5 have been specifically designed for
geotechnical engineering applications. Individual soil spec-
imens are generally tested, and in some cases, both volume
change and water content change are measured.
The water content corresponding to high-suction values is
generally obtained by allowing small soil samples to equili-
brate in a high-suction humidity environment. The humidity
environment is created using saturated salt solutions at the
bottom of vacuum desiccators. Details of the vacuum des-
iccator procedure are presented in Chapter 5.

13.8.6 Determination of Volume Change Indices
The measurement of the volume change indices is illustrated
using two soils: silt and glacial till tested by Ho (1988). The
index properties for the silt and glacial till are presented in
Table 13.4. An attempt was made to prepare soil specimens
with nearly identical initial conditions. Each soil was oven
dried and hand mixed with a predetermined quantity of dis-
tilled water. The wet soil was placed in a sealed plastic
bag and was left to cure for several days in a constant-
humidity and constant-temperature room. Specimens were
formed by static compaction at one-half standard AASHTO
compaction effort at either “dry-of-optimum” or “optimum”
initial water contents. The compaction characteristics of the
silt and glacial till are given in Table 13.4.
Volume change and water content change tests (i.e.,
oedometer, pressure plate, and shrinkage tests) were
conducted on compacted silt and glacial till specimens with
initial conditions corresponding to both the dry-of-optimum
and optimum conditions. The laboratory data were used to
determine the volume change indices for the soils at both
compaction conditions.
The data for silt specimens compacted dry of optimum
illustrate the technique for obtaining the volume change
indices. The oedometer test results on the silt specimens
compacted dry of optimum are shown in Fig. 13.72. The
oedometer tests were performed using the constant-volume
test method. The loading curves correspond to curves 1 and
3 in Figs. 13.70 and 13.71, respectively, showing the C t and

13.8.5 Shrinkage Tests
Shrinkage test results show the relationship between void
ratio and the water content at various soil suction values. A
soil specimen can either be allowed to dry in the air or it can
be subjected to various suction values using a pressure plate
extractor. In either case, the void ratio and water content of
the soil specimen are measured at various equilibrium states.
The soil specimen can be covered at various time intervals to
allow soil suction to equalize throughout the soil specimen.
Measurements of void ratio can be made following test
procedures similar to those used when measuring the shrink-
age limit of a soil. Measuring the shrinkage curve involves
measuring the water content and volume of a soil specimen
as drying occurs. However, in the case of a shrinkage limit
test, only the initial and completely dry states are of interest.
In the case of a shrinkage curve test the void ratio and water
content are measured numerous times throughout the drying
process.
Historically, the mercury displacement technique has been
used for the measurement of the total volume of the shrink-
age specimens. The volume of the displaced mercury dur-
ing immersion can be converted to the total volume of
the specimen. However, this procedure is no longer advo-
cated because of the health risks associated with handling
mercury. Direct measurements of total volume can also be
performed using calipers. The shrinkage curve is constructed
by plotting the decreasing void ratios against the decreasing
water contents as the soil suction increases to completely
dry conditions.

D t G s deformation indices.
Similar oedometer compression results for the silt com-
pacted at optimum water content are shown in Fig. 13.73.
Oedometer compression results for the glacial till compacted
at dry-of-optimum and optimum water contents are shown
in Figs. 13.74 and 13.75, respectively.

Table 13.4 Index Properties of Silt and Glacial Till
Used in Test Program

Silt Glacial Till

Liquid limit, % 26.7 33.2

Plastic limit, % 14.9 13.0

Plasticity index 11.8 20.2

Sand sizes, % 25.0 32.0

Silt sizes, % 52.0 39.0

Clay sizes, % 23.0 29.0

Specific gravity, G s 2.72 2.76

Half standard effort
compaction:
w opt 19.0% 18.75%

16.65 kN/m 3 17.12 kN/m 3

γ d, max