Gradation and Packing Characteristics Affecting Stability of Granular Materials: Aggregate Imaging-Based Discrete Element Modeling Approach

Unbound aggregate base and subbase in layered pavement structures provide an essential function of wheel load distribution through proper angular or crushed particle interlocking of coarse aggregate particles in the granular matrix. This paper used a validated model utilizing a particle image-aided discrete element method (DEM) to evaluate aggregate gradation and shape/morphological properties on pavement granular layer packing characteristics and load-carrying performance. The DEM packing simulations for different granular material gradations and particle shape categories were first performed to investigate the influence of the No. 4 (4.75-mm) sieve, separator of sand-sized and gravel-sized particles according to the Unified Soil Classification System (USCS), for typical dense-graded aggregates specified by the Minnesota Department of Transportation (MnDOT). Aggregate particles were modeled in the DEM as three-dimensional (3D) polyhedral discrete elements with both low and high angularity categories quantified by image analysis. According to the DEM simulation results, the No. 4 sieve (4.75-mm opening) size could be reasonably regarded as the breaking-sieve size that separated the load-carrying coarse fraction and void-filling fine fraction for the studied gradation band. The concept of gravel-to-sand (G/S) ratio, introduced in previous research studies by the authors, appeared to be working well for optimizing aggregate gradations for packing. A G/S ratio of around 1.5 was obtained from the DEM simulations using the 3D polyhedral particles according to the quantified imaging-based aggregate morphological indices to give the highest coordination number due to the achieved densest packing (indicated by the lowest porosity). The previously reported acceptable ranges of G/S ratio for the densest packing hence appeared to be sufficient criteria to optimize unbound aggregate performance. Next, DEM simulations were performed on different gradations with varying G/S ratio values to investigate an optimal gradation proposed for unbound permeable aggregate base (UPAB) materials. The verified G/S ratio-based gradation framework could be potentially applied to engineer aggregate gradations for improved particle contact and packing characteristicsespecially when the overall rutting potential influenced by shear strength characteristics needs to be minimizedyet still without significantly compromising desired drainage requirements.