## 3 Results and Mechanism Analysis
### Quantitative Index of Loess Structure Based on Air Permeability
\[m_{k}=\frac{m_{1}}{m_{2}}=\frac{k_{o}/k_{r}}{k_{s}/k_{o}}=\frac{(k_{o})^{2}}{k _{r}\text{-}k_{s}} \tag{3}\]  
In Equation (3), \(m_{k}\) is defined as the structural index of loess based on air permeability, \(m_{1}\) reflects the variability of the loess structure, which is positively correlated with \(m_{1}\), \(m_{2}\) reflects the structural stability of loess, which is negatively correlated with \(m_{2}\)[38; 39], \(k_{o}\) is the air permeability coefficient of undisturbed loess, \(k_{r}\) is the air permeability coefficient of remolded loess, and \(k_{s}\) is the air permeability coefficient of undisturbed saturated loess. Figure 10: The air permeability of remolded loess samples. The dry density of loess at different depths varies, as does the corresponding structure and the texture index. The measured data are presented in Figure 12. When the dry density was \(\rho_{d}<1.58\) g/cm\({}^{3}\), the air permeability and structural texture index \(m_{k}\) of the remolded loess were found to first decrease and then increase with the increase in the moisture content. When the dry density was \(\rho_{d}\geq 1.58\) g/cm\({}^{3}\), \(m_{k}\) was found to increase with the increase in the moisture content of the remolded loess, and the range of the increase in \(m_{k}\) varied with the dry density. The texture index of remolded loess was found to be different from that of undisturbed loess, and it also varied with the increase in depth in the same section. This internal reasoning is as follows: the greater the depth, the greater the overburden weight, the longer the sedimentation time, the fuller the consolidation, and the stronger the structure. In the process of various disturbances, the structure of loess changes, the strength of the soil decreases, and the structural potential of loess is released [40; 41]. The stronger the initial connection of undisturbed loess, the denser the internal structure of the soil, and the smaller the air permeability coefficient \(k_{o}\). Moreover, the greater the strength loss of remolded loess, the smaller the air permeability coefficient \(k_{r}\) with the increase in the dry density, and the greater the soil structural parameters. The greater the structural damage of saturated undisturbed loess, the greater the strength loss, the more obvious the attenuation of the soil structure, the smaller the air permeability coefficient \(k_{s}\) and the  
Figure 11: The air permeability of undisturbed, undisturbed saturated, and remolded loess samples (the dry density and moisture content of the remolded loess samples were consistent with those of the undisturbed loess samples). Figure 12: The quantitative index of loess structure based on air permeability. larger the soil structural parameters. Throughout the remodeling process, the structure of the original sample is gradually lost, and a secondary structure is gradually generated. With the gradual loss of structural strength, the structure reflected by this parameter also changes [3]. Meanwhile, undisturbed loess and remolded loess have a similar pore size distribution [42]. In undisturbed loess, the particles come into point-to-point contact with adjacent particles through cementation bonds, with abundant pores [43]. However, there are few cementitious materials in remolded loess [44], and the cohesion (c) is slightly lower than that in undisturbed loess [43]. The sample forms a chain arrangement structure, the grains are mainly in point-to-face and face-to-face contact, the pores become smaller, and the ventilation channel becomes narrower. Therefore, for undisturbed loess and remolded loess with complex structure and pores, it is reasonable to explore structure using the air permeability test [30].