Entropy is proposed as a quantity measuring the randomness of the packing structure of granular materials. The entropy of the particle size distribution is considered, which makes clear the relationship between present entropy and those in information theory, statistical mechanics and thermodynamics. It is shown that there are problems in similar types of existing theories and it is concluded that the entropy represents a “distance” between an observed distribution and a reference distribution. The entropy of the void fraction distribution is considered as an illustrative example.
