Civil Engineering Reference
In-Depth Information
14.3.2 Relative compaction
Laboratory compaction tests use different compactive efforts from those of many of the big machines so
results from these tests cannot always be used directly to predict the maximum dry density values that
will be achieved in the field. What can be used is the
relative compaction
, which is the percentage ratio
of the
in situ
maximum dry density of the compacted fill material to the maximum dry density obtained
with the relevant laboratory compaction test. Specification by
end product compaction
is when the con-
tractor is directed to achieve a certain minimum value relative compaction and is allowed to select his
own plant.
14.3.3 Method compaction
The Highways Agency Specification for Highway Works (
2009a
), gives two tables (6.1 and 6.4) from which,
knowing the classification characteristics of the soil, it is possible to decide upon the most suitable com-
paction machine together with the number of passes that it must use. With this information a specification
by method compaction can be prepared. The tables are too large to reproduce here.
14.3.4 End-product compaction
The values of the maximum dry density and the optimum moisture content are obtained using the 2.5 kg
rammer or the vibrating hammer test, depending upon which is more relevant to the expected field
compaction. The value of the required relative compaction should be equal to or greater than the value
quoted in Table
6.1
of the SHW (2009a), usually 90 to 95%.
14.3.5 Air voids percentage
Another form of end-product specification is to instruct that a certain minimum value of air voids percent-
age is to be obtained in the compacted soil. This value of V
a
was for many years taken as between 5 and
10%, but more recent research has suggested that a value of less than 5% is required to remove the risk
of collapse on inundation sometime after construction (Trenter and Charles,
1996,
Charles
et al.
,
1998)
.
14.4 Field measurement tests
During construction, tests for bulk density and moisture content should be carried out at regular intervals
if proper control of the compaction is to be achieved. The number and spacing of tests is governed by
the nature of the particular project and by the type of the compaction activities taking place.
14.4.1 Bulk density determination
These tests are described in BS1377-9:1990.
Core-cutter method
Details of the core-cutter apparatus, which is suitable for cohesive soils, are given in Fig.
14.6.
After the
cutter has been first pressed into the soil and then dug out, the soil is trimmed to the size of the cutter
and both cutter and soil are weighed; given the weight and dimensions of the cutter, the bulk density of
the soil can be obtained.
Sand replacement test
For granular soils the apparatus shown in Fig.
14.7
is used. A small round hole (about 100 mm diameter
and 150 mm deep) is dug and the mass of the excavated material is carefully determined. The volume of
