Influence of Particle Size Distribution on the Proportion of Stress-Transmitting Particles and Implications for Measures of Soil State

It is generally accepted that the use of void ratio and bulk density as measures of soil state have limitations in the case of gap-graded soils because the finer grains may not transmit stress. However, no research has explored systematically whether this issue also emerges for soils with continuous gradings. Building on a number of experimental and discrete-element method (DEM) studies that have considered the idea of an effective void ratio for gap-graded or bimodal soils, this paper extended consideration of this concept to a broader range of particle-size distributions. By exploiting high-performance computers, this study considered a range of ideal isotropically compressed samples of spherical particles with linear, fractal, and gap-graded (bimodal and trimodal) particle-size distributions. The materials’ initial packing densities were controlled by varying the interparticle coefficient of friction. The results showed that even for soils with continuous particle-size distributions, a significant proportion of the finer particles may not transmit stress, i.e., they may be inactive. Drawing on ideas put forward in relation to gap-graded soils, both a mechanical void ratio and mechanical bulk density that consider the inactive grains as part of the void space were determined. Even for the linear and fractal gradings considered here, the difference between the conventional measures and the mechanical measures was finite and density dependent. The difference was measurably larger in the looser samples considered. These data highlight a conceptual/fundamental limitation of using the global void ratio as a measure of state in expressions to predict granular material behavior.