Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 738

2000

Volume change - percent
Surcharge load = 7 kPa

100% saturation curve
( G s = 2.749)
Standard AASHTO
compaction curve

1800

1600

10%

8%

6%

1400

4%

0%

2%

1%

1200

1000 10

15 20

Initial water content ( w 0 ), %

25 30 35 40

Figure 14.6 Effect of initial water content and dry density on expansive properties of compacted
Porterville clay when wetted (after Holtz and Gibbs, 1956).

finding is in agreement with the observations made by Holtz
and Gibbs (1956), as shown in Fig. 14.7.

14.4 EVALUATION OF STRESS HISTORY
IN UNSATURATED SOILS

It has been common practice since the early days of soil
mechanics to retrieve undisturbed samples from a site and
then perform laboratory tests to determine relevant physi-
cal properties for the soil. The geotechnical engineer has
been interested in obtaining two primary pieces of informa-
tion when dealing with the predictions of volume change
(i.e., settlement or heave): (i) the volume change proper-
ties of the soil and (ii) the present stress state of the soil.
Soils are classified as normally consolidated, overconsoli-
dated, or underconsolidated. In each case the test results
provide information on the in situ stress state of the soil.
Laboratory tests on unsaturated soils allow information to
be obtained on the present stress state of the soil as well
as the volume change properties. The solution to volume
change problems requires that information be obtained on
(i) the present in situ stress state, (ii) the final stress state, and
(iii) the pathway followed between those two stress states.
The final stress state is generally assumed based on the local
experience of the geotechnical engineer.
Volume change problems associated with expansive
soils may have geometries that are one-dimensional, two-
dimensional, or three-dimensional. Geotechnical engi-
neering practice has primarily focused on the solution of
one-dimensional problems of volume change through use of
laboratory results from K 0 oedometer tests. The following
sections focus on the interpretation of one-dimensional
oedometer test results.
The methodology for measuring and interpreting the
swelling pressure of an expansive soil is of significant
importance. Numerous test procedures have been proposed
for the measurement of the swelling pressure of a soil. It is

132

123 66

45
33

124
69
41
28

71

63

36

32

15

15

14

14

Figure 14.7 Swelling pressure produced when wetting com-
pacted Porterville clay at various placement conditions (after Holtz
and Gibbs, 1956).

expand about 3% of its volume. On the other hand, expan-
sion is reduced to zero at 3% wet of optimum water content
and is increased to 6% when the water content is 3% dry of
optimum.
Figure 14.7 shows the effect of dry density and water
content on vertical swelling pressure. The vertical swelling
pressure was defined as the pressure developed by a spec-
imen placed in an oedometer ring and saturated without
allowing any volume change. The diagram shows that the
vertical swelling pressure is more sensitive to variations in
the initial dry density than it is to variations in initial water
content.
Chen (1988) also studied the effect of volume-mass prop-
erties on vertical and lateral swelling pressures. The vertical
swelling pressure was found to be essentially independent
of the initial water content and the surcharge load. This