Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 754

et al. (1980a) was applied to the test results. ( Note :The
construction procedure was described earlier in this chapter.)
The corrected swelling pressure from a constant-volume
oedometer test ( P 3 − 2 ) was earlier adopted as part of the
ASTM (1986) standard for testing expansive soils.
The results of the constant-volume tests are shown in
Fig. 14.31. Two specimens were subjected to a surcharge of
5 kPa. The remaining four specimens were each subjected to
a differing surcharge pressure of 100, 200, 400, and 600 kPa,
respectively. After wetting was complete, one of the two
specimens with a surcharge pressure of 5 kPa was unloaded
to obtain its rebound curve. The uncorrected swelling pres-
sures P 3 − 1 ranged from 590 to 860 kPa and the corrected
swelling pressures P 3 − 2 ranged from 700 to 1010 kPa.

14.4.11.5 Comparison of Swelling Pressure
Measurements
The swelling pressures determined from each testing method
are shown in Fig. 14.33. Also shown are the relationships
between the swelling pressures and the surcharge pressures.
The first procedure used to define swelling pressure P 1 − 1
(i.e., loading-after-swell test procedure) shows a tendency to
decrease with surcharge pressure. Results of swelling tests
on clay from El Arahal (Justo et al., 1984) showed that the
swelling pressure P 1 − 1 was essentially independent of the
surcharge pressure.
The second procedure used to define swelling pressure
P 1 − 2 (i.e., loading-after-swell test procedure) shows a slight
increase in swelling pressure with surcharge pressure. Tests
conducted on various undisturbed expansive soils from the
Rocky Mountain area have shown that the surcharge pres-
sure does not have a significant effect on the swelling pres-
sure P 1 − 2 (Chen, 1988). Figure 14.33 shows that both P 1 − 1
and P 1 − 2 change somewhat with the surcharge pressure.
However, the changes do not appear to be large and an
average value for swelling pressure can be assessed.
The uncorrected swelling pressure P 3 − 1 and the corrected
swelling pressure P 3 − 2 show a tendency to increase with
increasing surcharge pressure when using the constant-
volume test procedure (Fig. 14.33). Komornik and David
(1969) arrived at a similar conclusion when examining
the effects of preloading on the swelling pressure of
undisturbed samples. The swelling pressure P 4 associated
with using test procedure 4 (i.e., unloading method) shows a
slight decrease in swelling pressure with surcharge pressure
(Fig. 14.33).
The measured swelling pressures for Regina clay can be
summarized as follows. The swelling pressure P 2 − 2 from

14.4.11.4 Unloading Test Procedure
Soil specimens were also tested using the unloading test
procedure (Feng et al., 1998). The soil specimens were first
consolidated under a surcharge pressure at the original water
contents. The surcharge pressure was selected as a value that
exceeded the estimated swelling pressure of the soil. The
specimen was inundated and swelling was allowed to take
place with the surcharge pressure applied. The specimen was
unloaded in increments to zero pressure or to a token load
when swelling ceased. The swelling pressure was defined as
the pressure (i.e., P 4 ) on the unloading curve corresponding
to the initial void ratio. This method is commonly used in
China and is documented as the National Standard of the
Peoples Republic of China (1969).
Figure 14.32 shows the void ratio versus vertical pressure
curves for four specimens tested using test procedure 4 (i.e.,
unloading method). The surcharge pressure did not have a
significant influence on the swelling pressure P 4 and the
amount of swell during unloading.

1.00

Surcharge pressure
5 kPa
5 kPa
100 kPa
200 kPa
400 kPa
600 kPa

0.95

0.90

0.85

0.80 e 0

(Initial void ratio)

0.75

0.70

P 3-1 = 590-860 kPa

0.65

P 3-2 = 700-1010 kPa

0.60

1 10 100 1000 10,000

Vertical pressure (kPa)

Figure 14.31 Swelling pressures measured when using constant-volume procedure with applied
surcharge pressures of 5, 50, 100, 200, 400, and 600 kPa (from Feng et al., 1998).