The influence of finer fraction and size-ratio on the micro-scale properties of dense bimodal materials

The properties of cohesionless bimodal materials are known to be influenced by the volumetric fraction of finer particles, F_{fine}, and the size ratio, χ = D_{coarse}/D_{fine}. Two aspects of this are: (i) an increase in χ will lead to finer particles packing more efficiently between the larger particles; (ii) there is a critical value of F_{fine} between 24 and 29% at which the finer particles will just fill the voids between the larger particles. Motivated by an interest in developing a comprehensive understanding of the mechanics of internal erosion of dam filters, in this paper Discrete Element Modeling (DEM) simulations are carried out on bimodal samples of spheres with χ of 2, 4, 6, 8 and 10 and F_{fine} of 20, 25, 30 and 35% by volume. Each sample is isotropically compressed to a very dense state and the number of particles in the simulations ranged from 307 to 54033. The number and magnitude of contacts between particles of the same and of different sizes is considered to give an insight into how F_{fine} and χ can affect the stress transfer characteristics of bimodal materials. In particular it is shown that for F_{fine} ≥ 30% fine and large particles contribute approximately equally to stress transfer, whereas for materials with F_{fine} ≤ 25%, an increase in χ is shown to reduce the contribution of the finer particles significantly. These findings can be linked to earlier experimental observations considering filter stability (Skempton and Brogan, 1994), but also give insight into the sensitivity of other mechanical properties to the finer fraction and the size ratio (e.g. effect on void ratio and position of the critical state line (Rahman et al., 2008)).