Nature and Phase Properties of Unsaturated Soil - Unsaturated Soil Mechanics in Engineering Practice - page 68

The degree of saturation S can be used to subdivide soils
into three main groups which can be described as follows.
Dry soils (i.e., S

There is one further arrangement of the volume-mass
properties that can be used to define the volumetric water
content of a soil:

0%) consist of soil particles and air
where essentially no water is present. Saturated soils (i.e.,
S

=

SwG s

θ =

(2.62)

S +

wG s

=

100%) consist of a soil where the voids are filled with
water. Unsaturated soils (i.e., 0% <S< 100%) consist of
a soil where both air and water occupy the void space of
the soil.
An unsaturated soil can be further subdivided depending
upon the continuity of the air phase and the water phase.
For example, at high degrees of saturation (i.e.,

Geotechnical engineers are most familiar with measuring
gravimetric water content while agriculture-related dis-
ciplines have commonly used volumetric water content.
Volumetric water content becomes the preferred term to
use when considering many unsaturated soil problems, and
it is for this reason that several relationships for converting
between volumetric water content and other volume-mass
properties have been presented.
Volumetric water content references the volume of water
to the total volume of the soil. However, there are situations
where the total volume of the soil may change during the
process under consideration. In this case, care must be taken
to ensure that there is consistency between the manner in
which the soil properties are measured and the manner in
which the soil properties are applied in an analysis.
When performing a test for the water content-soil suction
relationship (i.e., SWCC), the amount of water in the soil is
usually referenced to the original volume of the soil speci-
men. However, if the soil specimen changes volume as soil
suction is increased, there would be a discrepancy between
the original total volume and the instantaneous total volume.
This issue is further discussed when analyzing test results
associated with the measurement of the SWCC.

85% <
S< 100%), the water phase may contain occluded air bub-
bles and as a consequence the air phase is discontinuous.
Under these conditions the pore-air and pore-water combine
to form a compressible fluid phase. On the other hand, it is
possible at low degrees of saturation (i.e., 0% <S<

∼

15%)
that the water phase becomes discontinuous and liquid flow
essentially ceases through the soil.

∼

2.4.4 Volumetric Water Content
Volumetric water content θ is defined as the ratio of the
volume of water, V w , to the total volume of the soil, V :

V w
V

θ = (2.56)

The volumetric water content can also be expressed in
terms of porosity, degree of saturation, and void ratio. This
allows volumetric water content to be written as

SV v
V

2.4.5 Gravimetric Water Content
Gravimetric water content w is defined as the ratio of the
mass of water, M w , to the mass of soil solids, M s (Fig. 2.45).
Gravimetric water content is generally presented as a per-
centage [i.e., w (%)]:

θ = (2.57)

The above equation can be rewritten since V v / V is equal
to the porosity of the soil.

θ = Sn (2.58)

M w ( 100 )
M s

w = (2.63)

The volumetric water content equation can also be writ-
ten as

Gravimetric water content has been most commonly
used in geotechnical engineering. It is important to know
whether reference is being made to gravimetric water
content or volumetric water content when dealing with
unsaturated soils. The SWCC data are often plotted using
both volumetric water content and gravimetric water
content. However, many geotechnical engineering formu-
lations, particularly those associated with fluid flow, use a
volumetric water content representation when computing
unsaturated soil property functions or performing further
process analyses. The degree of saturation designation can
be the preferred representation of water in the soil when
there is volume change as soil suction is increased.

Se

θ =

(2.59)

1 + e

The relationship between the gravimetric water content w
and the volumetric water content θ can be written using the
basic volume-mass relationship (i.e., Se = w G s ):

wG s
1

θ = (2.60)

+ e

Recognizing that specific gravity G s can be written as
ρ s /ρ w (i.e., ρ s = density of

water) and using the definition for the dry density of a soil,
volumetric water content can be written in terms of dry
density and the density of water:

density of soil solids and ρ w =

2.4.6 Volume-Mass Relations
Density and unit weight variables provide information on
the relationship between volume and mass designations of

w ρ d
ρ w

θ = (2.61)