## 3 Results and Mechanism Analysis
### Quantitative Index of Loess Structure Based on Air Permeability  
The remolding process of soil consists of the reconstruction of its structure. The structure of remolded soil is quite different under different moisture contents. Under compaction, the soil particles in remolded loess are randomly arranged, the pores in soil are relatively uniform, and the structure and degree of cementation are relatively poor as compared with those of undisturbed soil. The moisture content and dry density have a certain impact on remolded loess. As indicated by the \(w\)-\(\log k\) curves presented in Figure 10, when \(\rho_{d}<1.58\) g/cm\({}^{3}\), the air permeability of the soil sample was found to first increase and then decrease with the increase in the water content. When the water content \(w\) was less than the optimal water content of 17.5%, the air permeability was found to increase. However, when \(w>17.5\%\), the air permeability was found to decrease rapidly. When the dry density \(\rho_{d}\geq 1.58\) g/cm\({}^{3}\), the air permeability of remolded loess was found to decrease slowly with the increase in the water content. Figure 8: The air permeability values of the in situ contact samples and indoor undisturbed samples. Figure 9: The variation in the air permeability coefficients of remolded samples with different heights. With the increase in the buried depth of undisturbed air-dried loess, the overlying load of the soil increases, which results in particle compaction, a significant decrease in the numbers of macropores and mesopores, and an increase in the number of small holes and micropores. Macropores and mesopores play a leading role in the process of air infiltration [23]. The variation in the grain composition of Malan loess with changes in the buried depth was found to cause the air permeability parameters to decrease with the increase in the buried depth (Figure 11). From the comparison of the air permeability of the undisturbed and remolded loess samples presented in Figure 11, it can be seen that, under the condition of the same water content and dry density, the air permeability of the remolded loess samples was lower than that of undisturbed loess, and when \(\rho_{d}>1.58\) g/cm\({}^{3}\), the air permeability coefficient of remolded loess decreased more obviously. This is consistent with [PERSON]'s research, suggesting that the process of soil reconstruction by a given method is that the macropores in soil are filled, that is, the metastable structure could be further gradually weakened by the addition of silt particles [9]. The larger the dry density, the denser the remolded soil, the more soil skeleton particles per unit volume, the less effective the pores in terms of air permeability, the smaller the air permeability coefficient, and the more uniform the remolded structure. Because the macropores of the undisturbed samples were filled with water in the process of vacuum saturation, the effective permeability of the pores that facilitated the passage of air were reduced and the structure was relatively dense, so the measured air permeability coefficient of the undisturbed saturated samples was much smaller than that of the remolded samples. Because the measurement of the air permeability coefficient is highly economical, convenient, and time-saving, a quantitative index of the loess structure based on air permeability is subsequently proposed. Another clear benefit of this definition is to create a dimensionless physical descriptor for future and further comparisons.