Water Retention Characteristics and Pore Structure of Binary Mixtures

The mixing of different sized soil particles is known to affect soil properties such as density, porosity, hydraulic conductivity, and soil water retention. To better understand how the mixing controls soil water retention characteristics, a set of laboratory experiments was performed using binary mixtures of coarse and fine particles with seven different mixing fractions. For each mixture, the water retention characteristics, porosity, and saturated hydraulic conductivity were obtained. To enhance the effects of mixing, coarse and fine sands with a relatively large particle size contrast were used. Results showed that at equal to or above the fine fraction of ∼0.3, the fine particles controlled the behavior. Starting from the fine‐only sample, introducing more coarse particles to the fine particles led to rather gradual changes in the water retention characteristics. Wall effects at the coarse–fine particle interfaces were presumed to be responsible for the changes in the water retention curves. The water retention curves for the mixtures with a fine fraction lower than 0.3 were controlled by the coarse particles yielding the displacement pressure similar to that of the coarse sand. Furthermore, it was inferred that different mixing fractions led to different spectra of pore sizes, suggesting that a parametric model with sufficient flexibility is needed to describe the water retention characteristics of binary mixtures.