Environmental Engineering Reference
In-Depth Information
CHAPTER 2
Nature and Phase Properties of Unsaturated Soil
2.1
INTRODUCTION
2.1.1 What Is an Unsaturated Soil?
An unsaturated soil is commonly defined as having three
phases: (1) solids, (2) water, and (3) air. It is, however, more
correct to recognize the existence of a fourth phase, namely,
the air-water interface or the contractile skin (Fredlund and
Morgenstern, 1977). The justification and need for a fourth
phase are discussed later in this chapter and the following
chapter on state variables.
The presence of even the smallest amount of free air ren-
ders a soil unsaturated. Even a small amount of air, likely
occurring as occluded air bubbles, renders the pore fluid
compressible. It is possible for a soil to remain essentially
saturated while the pore-water pressure becomes negative
relative to the air phase.
The zone immediately above the water table is referred to
as the capillary zone and is characterized as being essentially
saturated while having negative pore-water pressures. Soil in
the capillary zone is generally embraced within the context
of an unsaturated soil. It is the negative pore-water pressure
relative to the internal or external air pressure that qualifies
the soil as an unsaturated soil.
Generally it is a larger amount of air (i.e., about 15% air
by volume) that makes the air phase continuous throughout
the soil. The principles and concepts required to describe
the behavior of an unsaturated soil become necessary
as the pore-air and pore-water pressures begin to become
significantly different. Terzaghi (1943) clearly described the
important role of the air-water interface in understanding
unsaturated
The relationship of one phase of a mixture to another phase
in terms of mass and volume is discussed under the topic of
“volume-mass” relations. Generally each phase of an unsatu-
rated soil is considered to remain chemically inert while the
proportions of each phase may change as a result of a “pro-
cess.” It is possible, however, for air to move in and out
of the liquid water phase. There are also situations where
phase changes occur while processes are underway. Cases
where there may be phase changes are briefly discussed in
this chapter. The effect of temperature on each phase is also
discussed in this chapter.
The classification properties (e.g., grain-size distribution
and Atterberg limits) take on increased significance when
dealing with unsaturated soils. The additional importance
of classification properties to unsaturated soil behavior is
discussed in this chapter.
The concept of “state variables” is briefly introduced;
however, an entire chapter is later devoted to a more
thorough presentation on this important topic. Numerous
approaches could be taken when developing the discipline
of unsaturated soil mechanics. The approaches could
range from an empirical approach based primarily on
experience to a particulate mechanics or quantum mechanics
approach. The approach used throughout this topic can be
referred to as a macroscopic, phenomenological approach
to unsaturated soil behavior. In other words, the science is
developed around observable phenomena while adhering
to the principles of continuum mechanics. The continuum
mechanics approach starts with an understanding of the
state variables for the material under consideration. The
continuum mechanics approach has proven successful in
saturated soil mechanics and it would appear reasonable to
retain the same approach for unsaturated soil mechanics.
An attempt should be made to ensure there is a smooth
transition and consistency in rationale between saturated and
unsaturated behavior.
soil
behavior
(i.e.,
Chapters
14
and
15
of
Theoretical Soil Mechanics ).
2.1.2 Unsaturated Soil as Four-Phase Mixture
An unsaturated soil can be viewed as a four-phase sys-
tem because of the unique role of the air-water interface
or the contractile skin on soil behavior. The contractile skin
acts like a thin membrane interwoven throughout the voids
of the soil, forming a fixed partition between the air and
water phases. Stress state changes in the contractile skin can
 
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