## 2 Test Principle and Scheme
### Test Scheme  
The air permeability measurement device used in this experiment was an improvement of the ZC-5 air permeability meter (Figure 3) [22; 23]. To meet the requirements of the loess permeability measurement range, a vacuum chamber (10,000 mL) was added [24]. Besides, the pressure measurement and control system (the infrared photoelectric measurement module) were improved for a better accuracy and reduced test duration. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Depth (m)**} & \multirow{2}{*}{\(G_{\text{s}}\)} & \multirow{2}{*}{\(W\) (\%)} & \multirow{2}{*}{\(W_{\text{l}}\) (\%)} & \multirow{2}{*}{\(W_{\text{p}}\) (\%)} & \multirow{2}{*}{\(W_{\text{p}}\) (\%)} & \multirow{2}{*}{\(\rho_{d}\) (g/cm\({}^{3}\))} & \multicolumn{3}{c}{**Particle Size Composition (\%)**} \\ \cline{5-8}  & & & & & & \multicolumn{1}{c}{**\(>\)0.075 mm**} & & \\ \hline
2 & 2.47 & 14.80 & 27.30 & 16.10 & 1.30 & 4.47 & 77.09 & 18.44 \\
4 & 2.70 & 16.92 & 29.12 & 17.50 & 1.41 & 2.78 & 73.05 & 24.16 \\
6 & 2.58 & 17.10 & 26.90 & 17.30 & 1.47 & 2.94 & 79.34 & 17.72 \\
8 & 3.24 & 17.50 & 28.70 & 17.80 & 1.58 & 2.65 & 76.52 & 20.83 \\
10 & 3.72 & 17.70 & 30.40 & 18.20 & 1.65 & 2.24 & 72.47 & 25.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The physical properties of the experimental soils. Figure 1: (**a**) The loess distribution map of China and (**b**) the geological map of the loess plateau. (**c**) Sample preparation process and pictures of the test instrument. Figure 2: (**a**) The compaction curve of Malan loess in the Chanhe section. (**b**) The particle size distribution curve and particle size grading accumulation curve. The device consists of two kinds of test probes, one of which is a hermetic cover used for in situ tests (the inner radius of the contact probe and the outer radius of the sealing area are 3 and 8 cm, respectively), and the other is a test tube with a sealed base for indoor tests (the base and the test tube are detachable and replaceable) (Figure 3b). For the two kinds of probes, one end of the sample is open to the atmosphere, the valve is closed at the probe, the vacuum pump is used to form negative pressure in the air chamber (the buffer chamber and gas cylinder), and the valve is opened during the test; thus, air can only enter the air chamber after the sample has been tested. The intelligent measurement and control system are connected to a pressure sensor (the pressure gauge of the infrared photoelectric measurement module) to continuously monitor the negative pressure in the vacuum system. In situ tests were carried out at depths of 2, 4, 6, 8, and 10 m from the surface of the Xi'an loses profile, and soil samples were taken manually. Undisturbed soil samples with different depths were cut into cylindrical samples with a diameter and height of 8 cm, and then fixed in the sample cylinder, the wall of which was sealed with sealant. The operation procedure of the indoor test was the same as that of the in situ measurement. In undisturbed loess laboratory tests, the effect of size is not significant and can be ignored [25]. Undisturbed soil samples for the air permeability test were first vacuumed for 1 h in a vacuum saturator, and were then soaked in deionized water for 10 h. The gas leakage was measured immediately after removal. Considering the sidewall leakage of gas, attention should be paid to the phenomenon of \"gas slippage\". Therefore, a layer of Vaseline was applied around the ring knife to act as a sealant [26]. During the preparation of remolded samples, undisturbed Malan loess samples from Xi'an were dried, crushed, and screened (0.5 mm), and loess samples with moisture contents of 11.50%, 13.50%, 15.50%, 17.50%, and 19.50% were prepared with deionized water. After mixing, samples were placed in a moisturizing dish for 48 h to ensure a uniform moisture content.