Civil Engineering Reference
In-Depth Information
The 4.5 kg rammer method
In this compaction test, the mould and the amount of dry soil used are the same as for the 2.5 kg rammer
method but a heavier compactive effort is applied to the test sample. The rammer has a mass of 4.5 kg
with a free fall of 450 mm above the surface of the soil. The number of blows per layer remains the same,
27, but the number of layers compacted is increased to five.
The vibrating hammer method
It is possible to obtain the dry density/water content relationship for a granular soil with the use of a heavy
electric vibrating hammer, such as the Kango. A suitable hammer, according to BS 1377-4:1990, would
have a frequency between 25 and 45 Hz, and a power consumption of 600 to 750 W: it should be in good
condition and have been correctly maintained. The hammer is fitted with a special tamper (see Fig.
14.2)
and for gravels and sands is considered to give more reliable results than the dynamic compaction tech-
niques just described.
The British Standard vibrating hammer test is carried out on soil in the 152 mm diameter mould, with
a mould volume of 2305 cm
3
, the soil having passed the 37.5 mm sieve.
The soil is mixed with water, as for any compaction test, and is compacted in the mould in three approxi-
mately equal layers by pushing the tamper firmly down on to the soil and operating the hammer for 60
seconds, per layer.
The vibrating hammer method should only be used for fine-grained granular soils and for the fraction
of medium and coarse grained granular soils passing the 37.5 mm sieve. For highly permeable soils, such
as clean gravels and uniformly graded coarse sands, compaction by the vibrating hammer usually gives
more dependable results than compaction by either the 2.5 kg rammer or the 4.5 kg rammer.
14.2.2 Soils susceptible to crushing during compaction
The procedure for each of the three compaction tests just described is based on the assumption that the
soils tested are not susceptible to crushing during compaction. Because of this, each newly compacted
specimen can be broken out of the mould and mixed with the remaining soil for the next compaction.
This technique cannot be used for soils that crush when compacted. With these soils, at least five sepa-
rate 2.5 kg air dried samples of soil are prepared at different water contents and each sample is com-
pacted, once only, and then discarded.
Fig. 14.2
The vibrating hammer compaction test.
