This paper provides a micro-mechanical commentary on the macroscopic behaviour observed in DEM simulations of the compression of individual crushable grains, and of triaxial tests on assemblies of both crushable and uncrushable grains. A fragmentation ratio is defined to describe the bond breakage processes, and the significance of other micro-mechanical parameters such as sliding contacts ratio, average coordination number, deviator fabric, and internal energies per unit volume is discussed. Three different modes of grain damage were observed: asperity breakage, internal shear cracking, and internal tensile cracking leading to fast fracture. Energy balances both for the compression of a single grain, and for triaxial tests on assemblies of grains, showed that the loss of elastic energy due to bond breakage was a negligible fraction of the significantly enhanced dissipation encountered with crushable materials. This extra dissipation was associated with frictional sliding triggered by the creation of new degrees of freedom among the breaking fragments. Different modes of grain breakage were found to be representative of different regions of soil states of stress defined with respect to the virgin compression line. The secondary role of elastic grain deformability, increasing coordination number but reducing dilatancy, has also been demonstrated.

The effect of particle-size distribution on the one-dimensional compressive behaviour of granular soil materials was investigated using the discrete-element method (DEM), and the results were compared with published experimental data with similar gradations. The particles used in this study were spherical, and their size range mimicked various power-law distributions. The contact formulation was calibrated such that the different compressibility and initial specific volume of non-uniform assemblies depended purely on the interaction of different particle sizes of the non-uniform gradations. The compressive behaviour of these assemblies can be classified as either big-particle-dominated or small-particle-dominated. The underlying micromechanical explanations for the effect of the particle-size distribution on the packing characteristics and on the compressibility are presented. Particles involved in the strong force transmission, which carried larger-than-average contact forces, were found to contribute to the stiffness of granular samples. When loose and dense samples with the same grading were compressed to a stage where their stiffnesses were identical, an identical size distribution was found for those particles involved in the strong force network. For some materials, there existed regions of particles that did not carry any strong force, and the non-affine movements of these small particles partially filling the void space between the bigger particles were especially significant. This behaviour was visualised in the DEM simulations. As these particles rearranged themselves, these regions contributed to a higher volumetric compression. It was also found that the coordination number of DEM particles significantly increased with better gradations. A change in the particle-size distribution of real sand through particle breakage would therefore lead to a higher coordination number of particles, and it would gradually reduce the probability of particle breakage during the later stages of the compression, as has been observed in other laboratory experiments.