Loose, cohesive granular assemblies: Constitutive law and microstructure evolution in quasistatic isotropic compression

DEM-simulated cohesive granular materials made of spherical beads of diameter a initially assembled in homogeneous solidlike loose states (solid fraction Φ as low as 20-30%), by ballistic cluster aggregation, are subjected to quasistatic isotropic compression. As the large initial voids gradually collapse under compression, void ratio e = −1 + 1/Φ decreases irreversibly as in cohesive soils. This plastic compression is controlled by dimensionless ratio P* = a2P/F0, comparing pressure P to contact cohesive strength F0. The compression curve, e versus log P*, is sensitive to the initial characteristic velocity during aggregation, which determines network connectivity and microstructure, and to the initial density. Despite the large heterogeneities, this curve is remarkably reproducible, independent of sample size and aspect ratio. Its slope (the compression index) varies with the initial density in the aggregation stage, and, surprisingly, correlates with small scale microstructural features much better than with large scale ones.