Civil Engineering Reference
In-Depth Information
Compaction of fine grained soils is usually by rolling or impact while compaction of
coarse grained soil is usually by vibration. You can see this being done in roadworks
and in back-filling trenches. The objective of compaction is to remove air trapped
between the excavated lumps as they are first placed. This means that the lumps must
be relatively weak to allow them to fail and deform plastically. However, if they are
too weak the resulting fill will itself be weak. There is an optimum strength of the
lumps which results in the best fill for a particular effort in compacting it. Since soil
strength is related to its water content there is an optimum water content for placing
and compacting soil.
Do not confuse the words compression, consolidation and compaction: they
mean very different things and they describe very different processes. Compression
was described in Chapter 8: it relates volume change to change of effective stress.
Consolidation was described in Chapter 15: it relates volume change to time as excess
pore pressures dissipate and water is squeezed from soil. In these chapters I have
described compression and consolidation of saturated soil. Compaction is the pro-
cess of removal of air from an assembly of saturated lumps by mechanical work:
rolling, impact or vibration. The water content and the weight of soil grains do not
change during compaction but the volume decreases and the soil becomes stiffer and
stronger.
26.11 Compaction curves and behaviour of
compacted soil
The degree of compaction of soil is measured by the dry density
ρ
d
which is given by
M
s
V
ρ
d
=
(26.14)
where
M
s
is the mass of dry soil in a volume
V
. (The usual units for dry density are
Mg/m
3
.)
The degree of compaction of soil depends on the effort put into compacting it, either
in a laboratory test or in the ground, and on the water content. In a laboratory test
the compactive effort is provided by a number of standard hammer blows and in the
ground by passes of a roller or a vibratory compactor.
Figure 26.9 illustrates a typical compaction curve for a particular compactive effort
as the dry density
ρ
d
related to the water content. The dry density reaches a maximum
at the optimum water content. For greater or lesser compactive efforts the curve would
be shifted but it should retain the same basic shape.
The chain dotted line is the relationship between
ρ
d
and water content for a saturated
soil and the broken lines represent the relationships between
ρ
d
and water content as
the degree of saturation becomes less. At high water contents the degree of saturation
of a compacted soil become larger and approaches the fully saturated condition only at
large water contents. At water contents below the optimum the degree of saturation,
and the dry density, diminish rapidly. It is obviously best to compact soils at water
contents as close to the optimum as possible.
