Mass of gravel excluded = -j^ pd

Considering unit volume, the volume of soil that replaced the gravel = X, so: Volume of gravel omitted = ^

100 pwGg where Gg = particle specific gravity of excluded gravel.

Corrected maximum dry density =

100 100 pwGg 100 V PwGg

100 V PwGg


Corrected optimum moisture content = ——— w where w = the optimum moisture content obtained from the test.

Even when a gravel correction is applied compaction test results are not representative for a soil with X much greater than 25 per cent. BS1377 mentions that in such cases the CBR mould (a mould 152 mm in diameter and 127 mm high) can be used but gives little guidance as to procedure.

General practice has been to pass the soil through a 37.5 mm sieve before test. The compaction procedure is similar to that for the standard test except that the soil is compacted in the bigger mould in three equal layers with the same 2.5 kg rammer falling 300 mm but the number of blows per layer is increased to 62.

Correction for the excluded gravel (i.e. the particles greater than 37.5 mm) can be carried out in the manner proposed for the 20 mm size.

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