Effect of size on the compressive strength of coal

The compressive strength of coal depends on the distribution, type and condition of discontinuities. In the smaller specimen, the probability of finding larger discontinuities is smaller and the compressive strength is thus higher. All groups of laboratory and in situ test results were analyzed by a DataFit computer program separately. It was shown that the effect of size on the compressive strength of 10 different groups of coal is not the same. Compressive strength of different groups of coal specimens had a high scatter for the same size and strength-size relationship had high deviation when all groups of results were mixed. Compressive strength of specimens was divided by the compressive strength of a specimen having a size equal to d (in this analysis 50.8 mm) in any group of test results. Consequently the dimension of strength in all series of tests was omitted and the relationship between the ratio of compressive strengths and size for all groups of results was determined. From extrapolating laboratory and in situ test results, the relationship between compressive strength and size of all cubic coal specimens was derived.