Nature and Phase Properties of Unsaturated Soil - Unsaturated Soil Mechanics in Engineering Practice - page 48

shrinkage curve. The void ratio-soil suction represents the
limiting boundary condition associated with the void ratio
constitutive surface.
The degree of saturation versus soil suction can also be
plotted through use of the basic volume-mass relationship
(i.e., S

wG s /e ). The degree-of-saturation plot versus soil
suction provides the best measure of the air-entry value for
the soil. Consequently, the degree-of-saturation curve ver-
sus soil suction becomes an important relationship for the
estimation of other unsaturated soil property functions.

=

2.3 PHASE PROPERTIES
A portion of a mixture qualifies as an independent phase
if it has the following characteristics: (1) differing proper-
ties from the contiguous materials and (2) definite bounding
surfaces. These two conditions must be met in order for a
portion of a multiphase system to qualify as an independent
phase. It is easy to understand how a saturated soil con-
sists of two phases (i.e., soil solids and water). It is also
quite understandable that the air becomes another indepen-
dent phase when the soil becomes unsaturated. Each of these
phases (i.e., soil solids, water, and air) clearly meets the
requirements for designation as a phase.
It is also possible for a phase to change “state,” as is the
case when water freezes or vaporizes. Ice then becomes an
independent phase from that of the water when analyzing
engineering problems. Water can also exist in vapor form
and, as such, water can exist in all three states in an unsat-
urated soil.
Some special properties and interactions associated with
the interfaces between two phases are also discussed in this
chapter. The phase diagram (Fig. 2.29) shows the mass and
volume quantities associated with each phase of an unsat-
urated soil. The thickness of the contractile skin is in the
order of only a few molecular layers. Therefore, the physi-
cal subdivision of the contractile skin is unnecessary when
establishing volume-mass relations for an unsaturated soil.
The contractile skin is considered as part of the water phase
without producing significant error.
An unsaturated soil is a mixture of several phases. It is
important to establish the number of phases comprising the
soil since it has an influence on how the stress state of
the mixture is defined. It is important to first understand
the properties of each phase of an unsaturated soil. On the
basis of the definition of a phase, it is proposed that an
unsaturated soil actually consists of four phases rather than
the commonly referred to three phases. It is postulated that
the air-water interface (i.e., contractile skin) can legitimately
be referred to as a fourth phase in an unsaturated soil.
An unsaturated soil is a multiphase system comprising
(1) soil solids (particles), (2) water, (3) air, and (4) contrac-
tile skin. The contractile skin plays an important role from
the standpoint of stress state considerations. However, when
considering volume-mass relations for an unsaturated soil,

Figure 2.29 Phase diagram for unsaturated soil.

it is not necessary to separate the water in the contractile
skin from the remaining water mass (and volume). In other
words, an unsaturated soil can be considered to be a three-
phase system with respect to volume-mass properties but
needs to be considered as a four-phase system with respect
to stress state conditions (Fig. 2.30).

2.3.1 Solid Phase
There are several basic material properties that can be
defined for each phase. A few typical values are also
presented for each of the properties. Some of the main
material properties are (i) density (and specific volume)
and (ii) thermal properties.

2.3.1.1 Density of Solids
Density and specific volume are used to define the
volume-mass relationship property of each phase. Density,
ρ is defined as the ratio of mass to volume. Each phase
of a soil mixture has a specific density. The density of a
solid can also be referenced to the density of water to give

Figure 2.30 Element of unsaturated soil with continuous air

phase.