Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 742

Figure 14.11 “Actual” or field stress path and “analysis” stress paths followed during wetting

of a soil.

can only perform laboratory testing in the total stress plane.
The assumption is made that it is possible to eliminate the
matric suction from the soil and still obtain the necessary
soil properties and stress state values from the total stress
plane. The interpretation of free-swell and constant-volume
tests are first discussed followed by information on other
test procedures that have been used for the measurement of
swelling pressure.

14.4.6 Constant-Volume Test
The constant-volume oedometer test (Fig. 14.12) involves
first subjecting a soil specimen to a token load and then
submerging the soil in water. The result is a release of the
negative pore-water pressure and a tendency for the soil
specimen to swell. As the soil specimen attempts to swell,
the load applied to the soil is increased to maintain the
specimen at a constant volume. This procedure is continued
until the specimen exhibits no further tendency to swell. The
applied load at this point is referred to as the “uncorrected
swelling pressure” P s . The specimen is then further loaded
and unloaded in the conventional manner used for a consol-
idation test. The term constant volume simply refers to the
initial conditions to which the soil specimen is subjected as
matric suction is released to zero stress.
The laboratory results from a constant-volume oedometer
test are generally plotted as shown in Fig. 14.12. The actual
stress paths followed during the test can be more clearly
visualized through use of a three-dimensional plot with each
of the stress state variables forming abscissas (Fig. 14.13).
It is important to understand the stress paths with respect
to both void ratio and water content when attempting to

Figure 14.12 Typical constant-volume one-dimensional K 0

oedometer test results.

interpret the laboratory test results. The void ratio and water
content stress paths are shown for the situation where there
is minimal disturbance due to sampling (i.e., due to stress
reversals). Even so, the loading path displays some curva-
ture as the total stress plane is approached. The actual stress
path can be even more affected as a result of sampling dis-
turbance (Fig. 14.14).
Geotechnical engineers have long recognized the effect of
sample disturbance when determining the preconsolidation
pressure for saturated clay. It is impossible for the soil speci-
men to return to its in situ stress state after sampling without
displaying some curvature along the void ratio-effective