Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 744

stress plot (i.e., compression curve extending onto the
virgin compression branch). It has also been recognized that
sampling disturbance significantly affects the measurement
of swelling pressure (Fredlund et al., 1980a).
Sampling disturbance causes the conventional measure-
ment of swelling pressure P s to fall below the actual or
proper representation of the in situ swelling pressure P s .
The corrected swelling pressure better represents the in situ
stress state translated onto the total stress plane. The cor-
rected swelling pressure should be equal to the overburden
pressure plus the in situ matric suction translated onto the
total stress plane. The translated in situ matric suction is

called the “matric suction equivalent” u a − u w e (Yoshida

et al., 1983). The magnitude of the matric suction equivalent
will be equal to or lower than the in situ matric suction. The
difference between the in situ matric suction and the matric
suction equivalent is primarily a function of the degree of
saturation of the soil. The engineer desires to obtain the cor-
rected swelling pressure from an oedometer test in order to
reconstruct the in situ stress conditions. The interpretive pro-
cedure used to account for sampling disturbance is discussed
later.

Figure 14.16 Stress path followed for free-swell type of oedome-
ter test.

change and incorporates hysteresis into the estimation of
the in situ stress state. On the other hand, the test proce-
dures tend to compensate for the effects of sampling dis-
turbance. No empirical corrections need to be applied for
“sampling disturbance” when using the free-swell test to
measure swelling pressure.

14.4.7 Free-Swell Test
The soil specimen in a free-swell oedometer test is ini-
tially allowed to swell freely with only a token load applied
(Fig. 14.15). The term free swell simply refers to the ini-
tial conditions to which the soil specimen is subjected as
matric suction is released to zero. The load required to bring
the specimen back to its original void ratio is termed the
swelling pressure.
The stress paths followed by the soil can best be under-
stood from a three-dimensional plot of the stress variables
versus void ratio and water content, as shown in Fig. 14.16.
The free-swell test has the limitation that it allows volume

14.4.8 Correction for Compressibility
of the Oedometer Apparatus
The oedometer was originally designed and intended for
testing highly compressible clay soils. The compressibility
of components of the oedometer was of little concern when
testing highly compressible clay soils. However, unsatu-
rated, swelling soils generally exhibit low compressibility
and the compressibility of the oedometer can significantly
affect the interpretation of the laboratory test results. The
compressibility of the apparatus affects procedures used for
testing swelling soils; however, it is the constant-volume test
procedure that is most significantly affected by apparatus
compressibility.
The following procedure is suggested for taking the
compressibility of the oedometer into account and obtaining
the corrected swelling pressure from constant-volume test
results. Details of the testing procedure are presented in
ASTM D4546. Laboratory data need to be adjusted to
account for the compressibility of the components of the
oedometer apparatus. Desiccated, swelling soils have a low
compressibility, and the compressibility of the apparatus
can significantly affect the evaluation of in situ stresses as
well as the slope of the rebound curve (Fredlund, 1969).
The compressibility of the oedometer can be measured
by placing a steel plug as a substitute for the soil speci-
men. The measured deflections under applied loads with the
steel plug should be subtracted from the deflections mea-
sured when testing the soil specimen. Figure 14.17 shows
the manner in which an adjustment should be applied to
the laboratory data. The adjusted void ratio versus pressure
curve can be sketched by drawing a horizontal line from the

Figure 14.15 Typical free-swell one-dimensional K 0 oedometer
test results.