Effects of sand compaction and mixing on pore structure and the unsaturated soil hydraulic properties

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 a increased nonlinearly with increasing grain size and mean pore body size of the sand mixtures. Compaction caused a linear decrease in a 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.