Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 184

CHAPTER 5

Soil-Water Characteristic Curves for Unsaturated Soils

5.1 INTRODUCTION unsaturated soil mechanics. Rather, it was a shift toward a

global economy and an emphasis on resource development
that began to bring about consideration of a science-based
understanding of unsaturated soil behavior. Many countries
of the world also began to press to be better stewards of
the environment. The design of containment facilities, cover
designs for landfills and mine wastes, the reclamation of
mining areas, and the remediation of spills were but a few
examples of unsaturated soil problems that needed to be bet-
ter understood. There was also a revisitation of compacted
soil behavior as well as more detailed consideration of the
behavior of residual soils, expansive soils, and collapsible
soils. These soils had become known as “problematic soils,”
and it was clear that there was need for a theoretical frame-
work that embraced all unsaturated soil behavior.
The progress toward establishing a scientific basis for
unsaturated soil behavior took time for two main reasons.
First, there was a lack of an appropriate science with a theo-
retical basis and, second, there was the lack of an appropriate
technology to render engineering practice financially viable
(Fredlund, 1979). It was the theoretically based, multiphase
continuum mechanics analysis of an unsaturated soil system
(Fredlund and Morgenstern, 1977) that provided the verifi-
cation of stress state variables for an unsaturated soil. The
analysis revealed that two independent stress state variables
should be used when describing unsaturated soil behavior.
Formulations that described each physical process associ-
ated with unsaturated soils were published. Fredlund and
Morgenstern (1976) formulated volume change behavior in
terms of independent stress state variables. Shear strength
behavior was also formulated in terms of stress state vari-
ables (Fredlund et al., 1978). Papagiannakis et al. (1984)
and Lam et al., (1987) published the formulations for steady-
state and transient seepage analysis for unsaturated-saturated
soil systems. Later, Wilson et al. (1994) published the solu-
tion of ground surface, climate-driven, moisture flux bound-
ary condition problems.
Unsaturated soil mechanics emerged in the 1960s and
1970s but there needed to be an improved understanding of
the linkages between theory, laboratory measurements, and

The SWCC provides a conceptual understanding between
the mass (and/or volume) of water in a soil and the energy
state of the water phase. The SWCC has proven to be an
interpretive model that uses the elementary capillary model
to provide an understanding of the distribution of water in
the voids. The effects of soil texture, gradation and, void
ratio have also become part of the interpretation of measured
laboratory SWCC data.
The SWCCs have an important role in the determination of
unsaturated soil property functions. The procedures that have
been proposed for unsaturated soil properties are approximate
but are generally satisfactory for analyzing unsaturated soil
mechanics problems. General agreement has emerged within
the geotechnical engineering profession with regard to proce-
dures for the measurement and interpretation of SWCC data
measured in the laboratory.

5.1.1 Background of Unsaturated Soil Mechanics
Early soil mechanics studies (e.g., 1930s and 1940s) focused
mainly on the behavior of saturated soils with positive
pore-water pressures. Common geotechnical engineering
examples involved the application of soil mechanics to
deep foundations and structures where positive pore-water
pressures were involved. Over time, it became common
practice to simply ignore negative pore-water pressures and
assume that consideration of positive pore-water pressures
would ensure a conservative and safe design. Many regions
of development were located in humid regions where the
groundwater table was high. Successes associated with the
application of saturated soil mechanics further promoted the
sole consideration of saturated soils. The ability to describe
the stress state of saturated soils in terms of effective stress
and the ability to model time-dependent behavior (e.g., the
consolidation process) provided technical tools that were
successful and financially rewarding.
Approximately one-third of the earth's surface is situ-
ated in arid and semiarid regions; however, this did not
appear to provide sufficient impetus for the development of