11.1.1 British Standard compaction tests
Three different compaction tests are specified in BS 1377 and these are briefly described below.
An air dried representative sample of the soil under test is passed through a 20 mm sieve and 5 kg collected. 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 per cent but for cohesive soils it should be about 8 to 10 per cent 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. 11.1. 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 some 2 per cent 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.
In this compaction test the mould and the amount of dry soil used is 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.
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, 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 fi-,1 '..I
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