Civil Engineering Reference
In-Depth Information
14.2 Laboratory compaction of soils
14.2.1 British Standard compaction tests
Three different compaction tests are specified in BS 1377-4:1990 and these are briefly described below.
The 2.5 kg rammer method
An air dried representative sample of the soil under test is passed through a 20 mm sieve and 5 kg is col-
lected. This soil is then thoroughly mixed with enough water to give a fairly low value of water content.
For sands and gravelly soils the commencing value of w should be about 5% but for cohesive soils it
should be about 8 to 10% less than the plastic limit of the soil tested.
The soil is then compacted in a metal mould of internal diameter 105 mm using a 2.5 kg rammer, of
50 mm diameter, free falling from 300 mm above the top of the soil: see Fig.
14.1.
Compaction is effected
in three layers, of approximately equal depth. Each layer is given 27 blows, which are spread evenly over
the surface of the soil.
The compaction can be considered as satisfactory when the compacted soil is not more than 6 mm
above the top of the mould (otherwise the test results become inaccurate and should be discarded). The
top of the compacted soil is trimmed level with the top of the mould. The base of the mould is removed
and the mould and the test sample it encloses are weighed.
Samples for water content determination are then taken from the top and the base of the soil sample,
the rest of the soil being removed from the mould and broken down and mixed with the remainder of
the original sample that passed the 20 mm sieve. A suitable increment of water (to give about 2% increase
in water content) is thoroughly mixed into the soil and the compaction is repeated. The test should involve
not less then five sets of compaction but it is usually continued until the weight of the wet soil in the
mould passes some maximum value and begins to decrease.
Eventually, when the test has been completed, the values of water content corresponding to each
volume of compacted soil are determined and it becomes possible to plot the dry density to moisture
content relationship.
105 mm
Mass
=
2.5 kg
Mould
volume
1000 cm
3
50 mm
Fig. 14.1
Equipment for the 2.5 kg rammer compaction test.
