Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 192

soil specimen at the start of the test. Using the original volume
of the specimen as a reference gives rise to another definition
for volumetric water content. The reference volume for defin-
ing volumetric water content becomes important in situations
where the soil specimen undergoes significant volume change
as soil suction is changed.
The degree of saturation S references the volume of water
to the instantaneous volume of voids and therefore requires
a measurement of the instantaneous total volume of the soil
specimen:

referenced to a constant value. Only the gravimetric water
content is referenced to a constant value if the soil structure
is deformable.
Gravimetric water content has been the preferred designa-
tion for the amount of water in a soil since the inception of
soil mechanics in the 1930s. Agriculture-related disciplines
have mainly used volumetric water content. The amount
of water in the soil is generally referenced to the original
soil specimen volume and overall volume changes during
a test were not a major concern in the interpretation and
application of the test results. Each of the terms used to
represent the amount of water in a soil has advantages and
disadvantages when used in soil mechanics (see Table 5.1).
There are other designations of water content that have
also been used in unsaturated soil mechanics when describing
unsaturated soil property functions (D.G. Fredlund, 2002a).
Dimensionless gravimetric water content dg is defined as
any gravimetric water content divided by the original satu-
rated water content:

V w
V v

S = (5.3)

where:

S = degree of saturation.

Gravimetric water content is referenced to a mass of soil
solids while volumetric water content and degree of satu-
ration are referenced to volumes. Volumetric water content
and degree of saturation are referenced to instantaneous vol-
umes; however, these variables have often been referenced
back to the original volume with the assumption that overall
soil specimen volume changes are negligible.
All three designations of the amount of water in a soil are
the same if the volume of the soil specimen remains constant
as soil suction increases. If the soil specimen is essentially
nondeformable (i.e., V v =

w
w s

dg = (5.4)

where:

w = any gravimetric water content and

w s = gravimetric water content at saturation

V v 0 , where V v 0 is the original vol-
ume of voids), the measured amount of water in the soil is

The saturated water content corresponds to a somewhat
arbitrary value where the soil is allowed to imbibe water and

Table 5.1 Advantages and Disadvantages of Various Designations for Amount of Water in Soil

Designation Advantages Disadvantages

• Consistent with usage in classic soil
mechanics
• Most common means of measurement
• Does not require a volume
measurement
• Reference value is a “mass of soil”
which remains constant

• Does not allow differentiation between
change in volume and change in degree of
saturation
• Does not yield the correct air-entry value
when the soil changes volume upon drying

Gravimetric water
content, w

Volumetric water
content, θ

• Is the basic form that emerges in the
derivation of transient seepage and
fluid storage in unsaturated soils
• Commonly used in databases of results
obtained in soil science and agronomy

• Requires a volume measurement
• Rigorous definition requires a volume
measurement at each soil suction
• Is the designation least familiar and least
used historically in geotechnical
engineering

Degree of saturation, S • Most clearly defines the air-entry value

• Appears to be the variable most closely
controlling unsaturated soil property
functions

• Requires a volume measurement
• Although volume measurements are
required, the degree of saturation variable
does not quantify overall volume change