Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 189

value for the estimation of unsaturated soil property func-
tions (M.D. Fredlund, 2000).
One of the obstacles hindering the implementation of unsat-
urated soil mechanics was the excessive cost and demanding
laboratory testing techniques associated with the experimen-
tal measurement of unsaturated soil properties. Nonlinear soil
properties meant that it was necessary to make a series of soil
property measurements on a particular soil specimen. In other
words, the unsaturated soil properties needed to be measured
under various controlled stress state conditions.
High-air-entry ceramic disks have played an important
role in testing unsaturated soils for more than 50 years
and still remain the basic means of separating pore-air and
pore-water pressures (i.e., axis translation technique). High-
air-entry disks have two primary limitations: an extremely
low coefficient of permeability and a gradual diffusion of air
through the water phase. The low coefficient of permeability
places a limit on the rate of pore pressure response as well
as the impeded release of water from the soil. The high-
air-entry disk places a restriction on the largest coefficient
of permeability that can be measured in the laboratory. The
air-entry value of the ceramic disk also limits the maximum
matric suction that can be applied.
The measurement of unsaturated soil property functions
becomes unacceptably costly for routine geotechnical engi-
neering purposes. Consequently, the need arises for practical
solutions for the determination of unsaturated soil property
functions. SWCCs have emerged as a practical and suffi-
ciently accurate tool for the estimation of unsaturated soil
property functions for most geotechnical engineering prob-
lems. The SWCC has become viewed as the “key” to the
implementation of unsaturated soil mechanics in engineering
practice (D.G. Fredlund, 2002a). For this reason, an entire
chapter is devoted to better understanding the theory related
to the SWCC as well as procedures that can be used for its
measurement and estimation. Each subsequent chapter will
show how the SWCC can be used for the estimation of a
variety of unsaturated soil property functions.

The term soil-water characteristic curve has been selected
as the preferred terminology for this topic on geotechnical
engineering with unsaturated soils. The preference is to use
“soil-water” because it is the water content of the soil that is
measured. The preference is to use the word “characteristic”
simply because it appears to have been the most common
term historically used in engineering. In addition, the term
implies that the curve describes some characteristic of the
soil. The word “retention” is more closely related to retain-
ing water for plant growth in agriculture or the retention of
water in materials other than soil (e.g., mine tailings).
SWCCs are pivotal to the solution of unsaturated soil prob-
lems. SWCCs constitute one of the bounding relationships
of the volume-mass constitutive properties for an unsaturated
soil (Pham, 2005). Figure 5.4 shows the volume-mass consti-
tutive surfaces for a clay soil. The degree of saturation-soil
suction relationship provides a definitive value for the air-
entry value for a clay soil that may undergo considerable
volume change prior to desaturation. If the volume change
of the soil is small, it is possible to use the gravimetric water
content versus soil suction relationship to determine the air-
entry value and residual water content for a particular soil.
The volume-mass constitutive surfaces for a sand soil are
illustrated in Fig. 5.5 (Pham, 2005). Gravimetric water con-
tent does not require a volume measurement in the laboratory
whereas degree of saturation and volumetric water content
require that the volume of the soil specimen be known. It is,
however, the volumetric water content that is most directly
required to represent the water stored in an unsaturated soil
and therefore consideration of the overall volume of the soil
is important in the determination of the SWCC.
Unsaturated soil properties are a function of all volume-
mass properties. The gravimetric water content versus soil
suction relationship is the easiest relationship to measure,
and for many applications (e.g., sands), it is an adequate rep-
resentation of the SWCC. Various measures of the amount
of water in the soil can be used in the estimation of the
unsaturated soil properties if the soil exhibits little volume
change as soil suction is changed. On the other hand, if
the soil exhibits substantial volume change when soil suc-
tion is changed, then greater care must be exercised when
establishing key variables associated with unsaturated soil
behavior.
Water storage is always measured as the slope of the vol-
umetric water content versus soil suction relationship. Most
permeability functions assume that it is the change in degree
of saturation that initiates the calculation of a significant
decrease in the coefficient of permeability. Figure 5.6 shows
a typical SWCC with its three distinct zones of desaturation.
The reliability of estimated unsaturated soil property func-
tions is quite closely related to the accuracy with which the
SWCC is defined.
The key transition points on the SWCC are the air-entry
value and the residual value for suction and water con-
tent. These transition points are defined on the degree of

5.1.7 Terminology and Definitions
There have been a number of terms that have been used
to describe the relationship between the amount of water
in the soil and soil suction. The variation in terminology
is somewhat related to the variety of disciplines that have
made use of the water content-soil suction relationship.
An attempt has been made to establish a level of consis-
tency with regard to the terminology used within the disci-
plines of geotechnical engineering. Some of the terms used
when referring to the relationship between the amount of
water in the soil and soil suction are as follows: (i) SWCC,
(ii) suction-water content relationship, (iii) retention curves,
(iv) moisture retention curves, (v) soil moisture retention
curves, and (vi) water retention curves (WRCs). These terms
can be independently applied to the drying and wetting water
content-soil suction curves.