Compaction-induced Anisotropy in Granular Materials: Effects of Grain Shapes on Fabric and Permeability Evolutions

Abstract

Particle morphology plays a vital role in altering the mechanical and fabric evolution of granular materials due to compression. In general, increasing particle shape irregularity, quantified by higher values of fractal dimension and surface roughness, enhances the fabric anisotropy and contact strength of a granular assembly. Localised deformation occurs when it is more energetically favourable than distributed compression in granular materials. The localisation is observed as zones where particles break and pores collapse in many experiments. However, localised deformation does not necessarily involve particle breakage; the mechanism driven by the release from particle interlocking without fragmentation remains less investigated. In this study, we reconstructed realistic irregular particles with controlled fractal dimension and surface roughness using Spherical Harmonics (SH). The particles are packed to resemble an oedometer test using the discrete element method (DEM). We measured anisotropic fabric and permeability in the virtual granular packings via fabric analysis and computational fluid dynamics (CFD). Results show that the release from interlocking between rough particles initiates compaction band formation without particle breakage, with increasing fractal dimension delaying band onset but accelerating propagation. The most permeable direction aligns with loading, and permeability anisotropy correlates strongly with pore fabric anisotropy. Compared with indirect influence through pore alteration, particle morphology has negligible direct impact on permeability anisotropy. This work provides microscopic insights into localised permeability evolution in deforming granular media, with implications for geotechnical and hydrological applications where the anisotropic flow rate through deforming granular media is of interest.