Revisiting the concept of inter-granular void ratio in view of particle packing theory

The equivalent inter-granular void ratio appears to be a pertinent density variable in analysing the mechanical response of silty sand. Using this density variable, the critical state locus (CSL) in the void ratio against mean effective stress plane for soil mixtures with various fines content can be normalised into a narrow band to give a single CSL. The equivalent inter-granular void ratio is defined as a function of fines content and a parameter b. However, the parameter b is vaguely defined as the fraction of active fines, which is considered to be a constant by some researchers and also as a function of fines content by other researchers. Until now, there are no conclusive ways of determining parameter b. In this respect, a new formula of the equivalent inter-granular void ratio is derived from the perspective of a recently proposed particle packing model. The new formula also shows that the parameter b is a function of specific volumes of coarse and fine particles. The two specific volumes in general are different for coarse and fine particles. If the two specific volumes are the same, the new formula is reduced to the same form of the classic formula. This new formula is then verified by several datasets of various sand-silt mixtures.