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Index and classification properties of soils
Index properties of soils – phase relations
Soil texture and grain size
Grain size distribution and particle shape
1.3 Index properties of soils
Soil is a threephase material and its behavior depends on the interaction of its three phases. Any
mass of soil consists of a collection of solid particles with voids in between which can be filled
either with water, air, or filled partly with both water and air. Consequently, the total volume,
V
t
,
of the soil mass consists of the volume of soil solids,
V
s
, and the volume of voids,
V
v
. The
volume of voids is made up of the volume of water,
V
w
, and the volume of air,
V
a
. We can
schematically represent these three phases in a
phase diagram
shown in Fig. 34, in which each of
the three phases is shown separately. On the left side are the volumes of the three phases, while
on the right side are the corresponding masses of the phases. This diagram is depiction of a prism
the base area of which is one. In engineering practice, we usually measure the total volume, V,
the mass of water, M
w
, and the mass of dry solids, M
s
. Then we calculate the rest of the values
and the massvolume relationships that we need. Most of these relationships are independent of
sample size, and they are often dimensionless.
There are three
volumetric ratios
that are very useful in geotechnical engineering, and these can
be determined directly from the phase diagram in Fig. 34.
The
void ratio, e,
is defined as
s
v
V
V
solids
of
volume
voids
of
volume
e
=
=
.
Eq. 1.4
1
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View Full DocumentThe void ratio,
e
, is normally expressed as a
decimal.
The typical values of void ratios for
sands may range from 0.4 to about 1.0, while typical values for clays vary from 0.3 to 1.5
and even higher for some organic soils that have very open structure.
The
porosity,
n,
is defined as
(%)
100
(%)
100
x
V
V
x
volume
total
voids
of
volume
n
t
v
=
=
.
Eq. 1.5
Porosity is usually expressed in percentages. Void ratio and porosity are related in the
following manner:
e
e
n
+
=
1
and
n
n
e
−
=
1
Eq. 1.6
The
degree of saturation
tells us what
percentage
of the total volume of voids contains
water:
(%)
100
(%)
100
x
V
V
x
voids
of
volum
watewr
of
volume
S
v
w
=
=
.
Eq. 1.7
If the soil is completely dry, then S = 0% , and if the pores are completely full of water,
then the soil is fully saturated and S = 100%.
There is just one
mass ratio
used frequently in soil mechanics,
water content
(sometimes
called moisture content), but this property is about the most important property of soil:
(%)
100
(%)
100
x
M
M
x
solids
of
mass
water
of
mass
w
s
w
=
=
.
Eq. 1.8
Water content can range from zero (dry soil) to several hundred percent, up to 500%. The
natural water content for most soils is, however, well under 100%.
Other useful ratios in geotechnical engineering, as well as in many other engineering disciplines,
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 Fall '10
 VUCETIC
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