Civil Engineering Reference
In-Depth Information
5.5 Specific volume, water content and unit weight
Many important mechanical properties of soil depend on the closeness of the packing
of the grains, so that loose soils (i.e. where there is a high proportion of voids) will be
weaker and more compressible than dense soils. The state of a soil can be described
by the specific volume
v
given by
V
V
s
v
=
(5.1)
where
V
is the volume of a sample containing a volume
V
s
of soil grains. Sometimes
the voids ratio
e
is used instead of specific volume, where
V
w
V
s
e
=
(5.2)
and
V
w
is the volume of the voids which, in saturated soil, are filled with water. Since
V
=
V
w
+
V
s
,
v
=
1
+
e
(5.3)
For coarse-grained soils, where surface forces are negligible, the grains pack together
like spheres. The maximum specific volume of a loose assembly of uniform spheres is
1.92 and the minimum specific volume of a dense assembly is 1.35; common sands
and gravels have specific volumes in the range
v
1.3 to 2.0. For fine-grained clay
soils surface effects may be significant, especially at low stresses, and the maximum
specific volume of a recently sedimented clay will depend on the clay mineral and any
electrolyte in the pore water. Montmorillonite clays with large specific surfaces may
exist with specific volumes in excess of 10, while kaolinite clays which have smaller
specific surfaces have a maximum specific volume around 3. Under large loads the
specific volumes of clay soils may be reduced to as little as
v
=
=
1.2 as the flat clay
plates become nearly parallel.
Specific volume cannot be measured directly but it can be calculated from other
easily measured parameters. The most convenient is water content
w
, defined as
W
w
W
s
w
=
(5.4)
and unit weight
γ
defined as
W
V
γ =
(5.5)
where
W
w
is the weight of water evaporated by heating soil to 105
◦
C,
W
s
is the weight
of dry soil,
W
W
s
is the weight of a sample with volume
V
. Standard tests to
measure water content and unit weight are described in Sec. 7.3. For a typical clay soil
the water content might be in the range 0.20 to 0.70 (i.e. 20% to 70%) and the unit
=
W
w
+
