Grain-size dependence of water retention in a model aggregated soil

We experimentally examined the amount of water retention in a model soil composed of aggregated glass beads. The model soil was characterized by two size parameters: size of aggregates $D$ and size of monomer particles (composing aggregates) $d$. In the experiment, water was sprinkled on the model-soil system that has an open top surface and drainable sieve bottom. When the sprinkled water amount exceeded a threshold (retainable limit), draining flux balanced with the sprinkled flux. The weight variations of retained and drained water were measured to confirm this balanced (steady) state and quantify the retained water. We defined the weight of the retained water in this steady state as $W_0$ and examined the relationship among $W_0$, $d$ and $D$. As a result, it was revealed that $W_0$ increases as $d$ decreases simply due to the capillary effects. Regarding $D$ dependence, it turned out that $W_0$ becomes the maximum around $D\simeq 500$~$\mu$m. The value of $D$ maximizing water retention is determined by the void formation due to the aggregated structure, capillary effect, and gravity.