Civil Engineering Reference
In-Depth Information
If we consider unit volume, the two systems give the density and the unit weight of the material:
Mass
Volume
M
V
Density
,
ρ
=
=
Weight
Volume
W
V
Unit weight
,
γ
=
=
Weight mass
=
×
9 81
.
As an example, consider water at 20°C:
Density of water
,
ρ
=
1000
kg/m
3
=
1 0
.
Mg/m
3
w
Hence the unit weight of water,
γ
w
,
=
1.0
×
9.81
=
9.81 kN/m
3
.
Soil densities are usually expressed in Mg/m
3
to the nearest 0.01.
Soil weights are usually expressed in kN/m
3
.
In soils work, it is generally more convenient to measure the density of a soil through test (e.g. Example
1.7)
then to perform the geotechnical analysis using the unit weight derived from the density value.
Note: Weight density
With the introduction of Eurocode 7 to geotechnical design the term
weight density
is likely to eventually
replace the term
unit weight
. The two terms are synonymous and since the term
unit weight
has been in
use for many decades it will certainly remain in use for many years to come and for this reason it is used
throughout this topic.
Unit weight of soil
As mentioned, the unit weight of a material is its weight per unit volume. In soils work the most important
unit weights are as follows:
Bulk unit weight (
γ
)
This is the natural
in situ
unit weight of the soil:
Total weight
Total volume
W
V
W W
V
+
+
s
w
γ
=
=
=
V
s
v
G V
γ
+
+
V
γ
S
(
G eS
e
+
+
)
s
s w
v w r
s
r
=
=
γ
w
V
V
1
s
v
Saturated unit weight (
γ
sat
)
Saturated weight
Total volume
γ
sat
=
When soil is saturated S
r
=
1, therefore
G e
e
+
+
γ
=
γ
sat
w
1
