Environmental Engineering Reference
In-Depth Information
Figure 13.3 Test paths followed during study of constitutive sur-
face uniqueness for unsaturated soil specimens (after Barden et al.,
1969a).
control of the total stress, pore-air, and pore-water pressures.
Constitutive surfaces were traced for different test paths com-
prised of various combinations of the stress state variables
σ y u a and u a u w (Fig. 13.3). In all cases, the pore-air
pressure was held constant, while the total normal stress and
the pore-water pressure were varied.
A preliminary study using smaller stress increments than
those shown in Fig. 13.3 indicated that specimens with sim-
ilar initial water contents had similar volume changes inde-
pendent of the test paths. Soil specimens for the remainder
of the program were prepared in an “identical” manner.
Figure 13.4 presents the soil structure constitutive surface
for the illite clay plotted with respect to two independent
axes: σ y u a and u a u w . The results were obtained by fol-
lowing several loading paths from initial to final conditions
(i.e., from point A to point H ; see Fig. 13.3). The constitutive
surface showed uniqueness or loading path independence
as long as deformations were monotonic (Barden et al.,
1969a). Hysteresis between saturation and desaturation pro-
cesses was considered to be the major cause of loading path
dependence. An increase in the matric suction was found
to increase the stiffness of a flocculent soil compacted dry
of optimum water content. However, an increase in matric
suction had less effect on the stiffness for a soil with a dis-
persed structure compacted wet of optimum water content.
The soil structure stiffness increased with an increase in net
normal stress regardless of the type of soil structure.
A second independent constitutive surface was shown for
the degree of saturation of the soil. Figure 13.5 shows the
degree-of-saturation constitutive surface for a group of soil
specimens. The surface was traced by following several
loading paths from point A to point H (Fig. 13.5). Unique-
ness was shown with respect to the degree of saturation
provided the water phase deformation was monotonic.
Several other researchers have suggested the use of net
normal stress and matric suction as stress variables for
Figure 13.2 Void ratio and degree-of-saturation constitutive sur-
face for mixture of flint and kaolin under K 0 loading: (a) void ratio
constitutive surface; (b) degree-of-saturation constitutive surface
(after Matyas and Radhakrishna, 1968).
cases, matric suction u a u w was increased subsequent to
equilibrium under the initial stress state; however, in a few
cases, matric suction was decreased. The results showed
that the overall volume change of the specimen was stress
path dependent, being a function of whether the soil was
going toward saturation or away from saturation. Hysteresis
between the saturation and desaturation processes was
considered to be the major cause of stress path dependence.
It was concluded that the volume change behavior of an
unsaturated soil was best analyzed in terms of separate
components of stress, σ u a and u a u w .
Barden et al., (1969a) investigated the uniqueness of the
soil structure constitutive surface under K 0 loading condi-
tions. Three illite clay soils of low to high plasticity were used
in the study. Eleven groups of soil specimens were prepared
using either dynamic or static compaction. The specimens
were tested in a consolidation cell modified for independent
 
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