## 3 Results and Mechanism Analysis
### Analysis of the Air Permeability of In Situ, Undisturbed, and Remolded Loess Samples  
Figure 8 compares the results of the contact in situ air permeation test and undisturbed indoor air permeation test. At the depths of 2 and 4 m in the test site in Xi'an, the air permeability values of the in situ contact samples were \(1.1832\times 10^{-12}\) and \(9.0319\times 10^{-13}\) m\({}^{2}\), respectively, while the values for the indoor undisturbed samples at the same depths were \(1.3025\times 10^{-12}\) and \(9.9136\times 10^{-13}\) m\({}^{2}\), respectively, thereby presenting respective increases of 1.1008 and 1.0976 times. Good correspondence was found between the in situ and indoor air permeability test results, and the air permeability of indoor samples was slightly higher than that of in situ samples. At the depths of 6, 8, and 10 m, the air permeability of undisturbed soil was slightly less than that determined by the in situ test. Moreover, the air permeability was found to decrease with the increase in depth, i.e., the air permeability of surface soil was better than that of deep soil. This is the result of the years-long geological process of loess; from the loose accumulation of the surface layer to the repeated deposition of the inner layer, pores became relatively denser, the connecting force between skeleton particles was enhanced, and soil tended to be more uniform. Compared with the indoor measurement method, the in situ test has the advantage of undisturbed formation and can better reflect the actual formation situation [34].  
Figure 9 presents the variation curves of the air permeability coefficients of remolded loess samples with different overall heights and designed dry densities. It can be seen from the figure that, with the increase in the height of the remolded sample, the air permeability coefficient was found to gradually increase, i.e., with the increase in the overall height of the sample, the sample tended to be more uniform, and the more uniform the soil sample, the greater the air permeability coefficient. The pores of remolded soil are occupied by soil, water, and air, and due to the differences in interfacial tension, air always occupies large pores with priority, while water occupies small pores or pore throats with priority [35]. The more uniform the sample, the shorter the time taken for air to pass through soil, and the greater the air permeability coefficient. Furthermore, when the dry density and  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Designed Dry Density of the**} & \multicolumn{4}{c}{**Overall Height \(h\) of the Prepared Sample (cm)**} \\ \cline{2-6}  & **S** & **10** & **12** & **14** & **16** \\ \hline
1.5 & \(2.83\times 10^{-3}\) & \(2.60\times 10^{-3}\) & \(2.21\times 10^{-3}\) & \(1.82\times 10^{-3}\) & \(1.31\times 10^{-3}\) \\
1.4 & \(2.17\times 10^{-3}\) & \(1.91\times 10^{-3}\) & \(1.64\times 10^{-water content of the remolded soil sample are low, soil is loose, soil with a low water content has more pores, the pore distribution is more uniform, the actual airflow path is more tortuous, the air permeability is higher [36; 37], and the uniformity of the sample is obviously improved.