Establishment and validation of a new index for structural evaluation of loess based on air permeability test

The structure of loess is a key factor influencing its mechanical and hydraulic properties. Research on structural evaluation indices has been increasing, and existing studies, each with their distinct contributions, have greatly advanced the understanding of soil structure. However, most approaches rely on testing undisturbed, remolded, and saturated samples, which involve complex procedures and numerous variables. Moreover, it remains difficult to accurately quantify the individual contributions of different influencing factors to soil structure. Therefore, developing a well-defined, easily measurable, and parameter-efficient structural index for loess that also accounts for force-water coupling represents a significant goal in current research. In this study, a transient vacuum decay air permeameter was used to investigate the air permeability of loess under varying moisture conditions and force-water coupling, and to explore its relationship with structural characteristics. The results show that for undisturbed loess of different textures, air permeability increased as water content decreases during drying, and decreased as water content increased during wetting, with drying permeability consistently higher than wetting permeability. At the same water content, air permeability decreased with increasing vertical pressure. For the same soil texture, higher water content led to lower air permeability under the same pressure and air-filled porosity. Based on these findings, a novel expression was proposed to describe the variation of the structure ratio with water content and pressure. Furthermore, using the Poulsen model, predictive analyses were conducted to relate structural parameters to moisture and air-filled porosity, as well as to moisture, pressure, and air-filled porosity. This provides a new approach for the quantitative characterization of loess structure. Since the determination of air permeability has obvious advantages such as non-destructive, efficient, economical, convenient and fast, it has a unique advantage to be used as an index for evaluating the structural properties of loess and is worth to be vigorously popularized.