The concept of intergranular void ratio has become more popular in characterising the behaviour of soil mixtures of sand and fine particles up to a threshold transitional fines content. The transitional fines content at which these mixtures change from a sand-dominated to a fines-dominated behaviour is usually defined as the densest mixture. For samples having a less-than-transitional fines content, the fine particles can fall inside the void spaces created by the larger sand particles. Assuming all fine particles are inactive and hence treated as void, the compression curves of different sand-dominated mixtures can be represented by a single curve in term of the intergranular void ratio. More recently an additional 'b' parameter was defined as the fraction of the active fine particles out of the total fine content and only the inactive fine fraction was counted as void. The value of b was usually obtained from back-analysis or predicted using a semi-empirical approach. In this numerical study using the Discrete Element Method (DEM), various definitions of the intergranular void ratio e*are investigated and discussed, together with the micromechanical data showing the actual involvement of the fine particles in the force transmission. The results show that the value of b is related to the fraction of the fine particles involved in transmitting the strong, larger-than-average contact forces. The value of b is not constant but increases with stress level (decreasing void ratio) and fines content for samples having fines content less than the threshold value.