Multi-scale mechanics of polydisperse granular materials: From micro-scale and wave propagation experiments to DEM analysis

This paper presents a multiscale experimental study integrated with numerical simulations examining the mechanics of polydisperse granular mixtures composed of coarse-grained particles mixed with varying percentages of fines. The study includes macroscale wave propagation tests using bender elements on isotopically compressed granular samples to investigate the stiffness variation with changes in size ratio (SR) and fines content (FC). Empirical equations were developed to predict stiffness based on the experimental data, using the concept of equivalent void ratio to represent the influence of void ratio on stiffness. A newly introduced parameter called size disparity indicator was used to consider the coupled effects of size disparity and fines content on stiffness. Microscale assessment of the individual contacting grains revealed that stiffness (at grain scale) is affected by changes in grain size, even when the SR is less than 6, however, the experimental observations of the shear modulus behavior from the macroscale test results reveal minimal effect of FC. Furthermore, numerical simulations using the discrete element method were conducted on the polydisperse granular mixtures to demonstrate the coupled effect of SR and FC on the structural matrix formation, thereby influencing the force transfer among contacts resulting in varying stiffness behavior. Our findings provide valuable insights into the behavior of polydisperse granular mixtures in engineering applications.