## 4 Conclusions  
This study used a homemade rainfall infiltration device to conduct a 1D vertical soil column infiltration test, investigated the infiltration process of soil columns with different densities with or without runoff, assessed the relationships between the advancing displacement and rate of the wetting front and time, studied the unsaturated infiltration characteristics, such as soil column infiltration, infiltration rate and moisture content change, and discussed the relationship between the permeability and the moisture content or the matric suction. The results show the following:  
Pore size distributions in this study exhibited bimodal distributions. The samples compacted at a low water content exhibited a double porous structure with two classes of predominant pores. Correspondingly, the compacted soils were observed to lose water in two clear stages during drying. Thus, the drying process corresponded to a double S-shaped SWCC. Water was lost more easily in the first stage than in the later stage because of the existence of a large volume portion of macropores or interaggregate pores. In the soil column vertical infiltration test, the relation between cumulative infiltration and time, as well as wetting front displacement and time, obeys the power function relationship as Q(t) = at and D(t) = at + b, respectively. In the initial stage, the soil column is relatively dry unsaturated soil, all the rainfall infiltrates into the soil, and at this time, the infiltration rate is theoretically equal to the rainfall intensity, called the rainfall intensity control stage. The time point of water drainage at the bottom of the soil column is used as the endpoint of the second stage. As infiltration progresses, the pores in the upper soil layer are gradually filled with water, the infiltration capacity decreases, and the soil infiltration capacity control stage starts. In this stage, infiltration rate of the soil column is mainly determined by the properties of the soil. Once water begins to drain at the bottom of the soil column, the third stage starts. When the entire soil column is saturated, the infiltration rate almost no longer changes, and the infiltration rate is equal to the saturation permeability. The hydraulic conductivity function for soil from permafrost regions is highly coincident with the van Genuchten model. The permeability of the soil column can change by multiple orders of magnitude, from \(10^{-4}\) to \(10^{-6}\) when soil moisture decreases from 50% to 20%, during the wetting process. For all moisture contents, the initial unsaturated permeability is not strongly sensitive to the presence of runoff but is related to the properties of the soil column, such as dry density, moisture content, soil quality, and external temperature. That is, the HCF calculated from the volumetric moisture content is more sensitive to changes in dry density than to changes in runoff or hydraulic head height. In the future, the hydraulic conductivity function in this study can be used to perform seepage simulation and stability analysis in unsaturated soil slopes induced by rainfall infil  
Figure 17: Unsaturated permeability of the C2 and C3 soil columns. tration. In addition, this work is significant in furthering understanding of the hydrological mechanisms of slope failure in permafrost regions. Conceptualization, L.L. ; Software, C.Y. ; Validation, X.L. and Y.H. ; Formal analysis, Y.H. ; Investigation, L.L. and L.W. ; Resources, C.L. ; Data curation, Y.L. ([PERSON]); Writing--original draft, L.L. ; Writing--review & editing, L.L. ; Visualization, Y.L. ([PERSON]); Supervision, X.L. and Y.L. ([PERSON]); Project administration, Y.L. ([PERSON]); Funding acquisition, L.L. All authors have read and agreed to the published version of the manuscript. This research was financially supported by the National Natural Science Foundation of China (Grant No. 42020101, 41877225, 42230712) and the Young Talent Fund of Association for Science and Technology in Shanxi (Grant No. CLGC20225).