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Core Ideas A discrete element model was used to extract the pore structure of particle packings. Soil grain compaction and size mixing affect pore structure. Size mixing had a greater effect than compaction on the pore size distribution. Compaction caused the SWRCs to change uniformly with porosity. Compaction decreased the van Genuchten α parameter but had less effect on n. The hydraulic properties of unsaturated porous media very much depend on their pore structure as defined by the size, arrangement, and connectivity of pores. Several empirical and quasi‐empirical approaches have been used over the years to derive pore structure information from the particle size distribution. In this study, we used the discrete element method to simulate the pore structure of various sands as affected by compaction and particle mixing processes. We used five sands with different mean grain sizes to investigate the effects of different sand mixing ratios and degrees of compaction on pore structure as well as on the intrinsic permeability and the soil water retention curve. Average pore body and pore throat sizes were found to be determined mostly by the smaller particles as represented by the effective diameter D10. The effects of compaction on the average pore body and pore throat radii were used to simulate expected decreases in the permeability. We obtained mostly linear relationships between permeability and the average pore body and throat radii when mixing different unimodal sands. The intrinsic permeability of the coarser sands was found to be far more sensitive to porosity than the finer sands. Simulations of unsaturated conditions showed that the van Genuchten hydraulic parameter α increased nonlinearly with increasing grain size and mean pore body size of the sand mixtures. Compaction caused a linear decrease in α with decreasing porosity and pore body size. However, no clear correlation between the van Genuchten parameter n and porosity or D10 was found for the different compaction and mixing simulations.

In this study, a grain-scale modelling technique has been developed to generate the capillary pressure–saturation curves for swelling granular materials. This model employs only basic granular properties such as particles size distribution, porosity, and the amount of absorbed water for swelling materials. Using this model, both drainage and imbibition curves are directly obtained by pore-scale simulations of fluid invasion. This allows us to produce capillary pressure–saturation curves for a large number of different packings of granular materials with varying porosity and/or amount of absorbed water. The algorithm is based on combining the Discrete Element Method for generating different particle packings with a pore-unit assembly approach. The pore space is extracted using a regular triangulation, with the centres of four neighbouring particles forming a tetrahedron. The pore space within each tetrahedron is referred to as a pore unit. Thus, the pore space of a particle packing is represented by an assembly of pore units for which we construct drainage and imbibition capillary pressure–saturation curves. A case study on Hostun sand is conducted to test the model against experimental data from literature and to investigate the required minimum number of particles to have a Representative Elementary Volume. Then, the capillary pressure–saturation curves are constructed for Absorbent Gelling Material particles, for different combinations of porosity values and amounts of absorbed water. Each combination yields a different configuration of pore units, and thus distinctly different capillary pressure–saturation curves. All these curves are shown to collapse into one curve for drainage and one curve for imbibition when we normalize capillary pressure and saturation values. We have developed a formula for the Van Genuchten parameter (which is related to the inverse of the entry pressure) as a function of porosity and the amount of absorbed water.

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