Civil Engineering Reference
In-Depth Information
Preparation of the soil at different water contents
It is important, when water is added to a soil sample, that it is mixed thoroughly to give a uniform disper-
sion. Inadequate mixing can lead to varying test results and some form of mechanical mixer should be
used. Adequate mixing is particularly important with cohesive soils and with highly plastic soils it may be
necessary to place the mixed sample in an air tight container for at least 16 hours, in order to allow the
moisture to migrate throughout the soil.
14.2.3 Determination of the dry density-moisture content relationship
For each of the three compaction tests described the following readings must be obtained for each
compaction:
M
1
=
Mass of mould
M
2
=
Mass of mould
+
soil
w
=
moisture content (as a decimal)
The bulk density and the dry density values for each compaction can now be obtained:
M M
Mg/m
−
2
1
ρ
b
=
3
(
for the
2 5
.
and
4 5
.
kg rammers
)
1000
M M
−
2
1
=
3
Mg/m
(
for the vibrating hammer test
)
230
5
ρ
b
ρ
=
w
Mg/m
3
d
1
+
When the values of dry density and moisture content are plotted the resulting curve has a peak value
of dry density. The corresponding moisture content is known as the optimum moisture content (omc). The
reason for this is that at low w values the soil is stiff and difficult to compact, resulting in a low dry density
with a high void ratio. As w is increased however, the water lubricates the soil, increasing the workability
and producing high dry density and low void ratio. However beyond omc pore water pressures begin to
develop and the water tends to keep the soil particles apart resulting in low dry densities and high void
ratios.
With all soils an increase in the compactive effort results in an increase in the maximum dry density and
a decrease in the optimum moisture content (Fig.
14.3)
.
Fig. 14.3
Typical compaction test results.
