Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 185

methods for solving practical engineering problems. A more
in-depth understanding of the SWCC provided a constitu-
tive framework for combining the theory of unsaturated soil
behavior and unsaturated soil properties. The SWCC pro-
vided the necessary linkages for the emergence of a science
for unsaturated soil mechanics along with economical proce-
dures for the implementation of unsaturated soil mechanics.

The capillary model provided a mathematical relationship
between the radius of curvature of the air-water meniscus and
the pressure difference between the air and water phases. The
difference between the air pressure u a and the water pressure
u w was shown to be balanced by the surface tension T s acting
at the wetting angle α along the solid surface. Terzaghi (1943)
and Taylor (1948) used the capillary model to explain the
differences in physical behavior between saturated and unsat-
urated soils. The capillary model assisted in explaining some
aspects of unsaturated soil behavior but was later found to
create serious limitations in the general application to unsat-
urated soil mechanics (Fredlund and Rahardjo, 1993a). The
capillary model was a micromechanics-type model that could
not be directly incorporated into a macrocontinuum mechan-
ics framework for solving practical geotechnical engineering
problems.
Buckingham (1907) is credited with introducing the con-
cepts of “capillary potential” and “capillary conductivity.”
These concepts were based on the ability of an unsaturated
soil to transmit and store water. Buckingham emphasized the
“energy” or “potential” of water in the soil. Water could be
stored and it was also assumed that it could flow in response
to a capillary potential. Buckingham (1907) held the view
that “the energy of the water in soil (was) representative of its
ability to hold water.” Buckingham (1907) held a phenomeno-
logical view of total potential suggesting that soil-water could
have various components. One component of energy was that
due to mechanical stresses across the air-water interface and
this led to the idea of capillary suction or matric suction,
which was defined as the difference between the pore-air,
u a , and pore-water, u w , pressures (u a −

5.1.2 Early Developments on the SWCC
in Soil Physics
Much of the theory and behavioral understanding of the
SWCC has arisen out of research in agricultural-related dis-
ciplines (e.g., soil science, soil physics, and agronomy). It is
important to acknowledge the important contributions made
in agriculture-related disciplines to unsaturated soil mechan-
ics. At the same time, it was important that there be a
reinterpretation of scientific findings to better fit geotechni-
cal engineering practice. Barbour (1998) provided a review
of the early understanding of the SWCC within soil physics.
Numerous researchers made significant contributions to our
understanding of the SWCC. Schumacher and Wollny, con-
sidered as the fathers of soil physics in Europe, made signif-
icant contributions in the 1800s to the understanding of soil
moisture and its movement in soils. In North America, King
(1899) and Slichter (1899) were forerunners in soil physics
while working for the U.S. Geological Survey.
Slichter (1899) developed a model for pore spaces in a soil
through use of a series of spheres packed together in vari-
ous arrangements. The geometry of the pore spaces was then
calculated. Briggs (1897) developed a centrifuge method for
determining the “moisture equivalent” of soils while working
for the Bureau of Soils of the U.S. Department of Agricul-
ture. An attempt was made to classify soils based on the
measured moisture equivalent. Briggs (1897) stated that “the
storage capability of soil is due to surface tension within films
of water around numerous capillaries.” Water in a soil was
classified on the basis of the mechanism by which the water
was held in the pores. Hygroscopic water was assumed to be
held on soil particles by attractive forces. Capillary water was
assumed to be held by surface tension forces. Gravitational
water was assumed to be water that could freely drain from
the soil because it was not held by the soil particles.
Haines (1927) further developed the concept of the air-
water interface introduced by Briggs (1897). The air-water
film or contractile skin was viewed as being wrapped around
the soil particles, thereby applying increased stress between
the particles. Consideration of the air-water interface led to
the use of a capillary model to describe soil behavior. The
capillary model was compared to a series of glass tubes of
varying diameters. Water would rise to differing heights in
the capillary tubes depending upon their respective diame-
ters. The capillary model was further expanded to include
individual tubes with varying diameter, thereby providing an
explanation of hysteresis associated with wetting and drying
processes.

u w ) . Energy due to
the chemistry of the soil-water system was considered as an
adsorption component or the osmotic suction, π .Thesumof
the matric suction component and the osmotic suction compo-
nent was referred to as the total energy or the total potential,
ψ , of the pore-water. Aitchison (1964) made use of the con-
cepts of Buckingham (1907) when introducing soil suction
into geotechnical engineering.
The concept of water having potential energy or water
potential was useful, but there appears to have been some
misuse of the physics associated with various processes
(Barbour, 1998). For example, it was suggested that com-
ponents of water potential were additive in a linear manner.
Some of the reported components of water potential were
(i) gravitational potential, (ii) osmotic potential, (iii) vapor
potential, (iv) matric potential, (v) hydrostatic pressure
potential, and (vi) overburden pressure potential, as well
as other potentials. In soil mechanics, the flow of water
through a saturated soil has been strictly related to hydraulic
head, which was a combination of a pressure head and an
elevation (or gravity) head. These two components were
linearly combined and operative in the vertical direction.
On the other hand, air flow, which is part of matric suction,
was considered to experience flow in response to its own
pressure gradient and was independent of water flow.