# Analysis of Soil-Water Characteristic Curve and Microstructure of Undisturbed Loess
## 3 Results and Discussion
### Response of SWCCs to Pore Structure
A larger pore size range results in a broader transition zone and a longer desaturation path. This relationship is evident in the order of NO. 1 > NO. 2 > NO. 3 > NO. 4 > NO. 5, where the transition zone range decreases with an increasing burial depth. Therefore, variations in the pore size distribution influence the shape and extent of the transition zone in the SWCC, ultimately affecting the water retention properties of the soil. Specifically, the dominance of specific pore sizes plays a crucial role in determining how water behaves within the soil structure. Pores that fall within the range of the dominant pore sizes exert a strong influence on the movement and retention of water due to capillary action. Soils with a greater proportion of dominant pores typically demonstrate higher levels of volumetric water content. This characteristic is clearly illustrated when comparing samples NO. 1 and NO. 2, which display elevated saturated volumetric water contents in comparison to the other samples. Figure 11: Relationship between the PSD and SWCC of undisturbed loess soil samples from different depths. The distribution and morphology of dominant pores within undisturbed loess samples significantly impact their water retention characteristics. Specifically, water molecules residing within these dominant pores are strongly influenced by capillary action. Consequently, a higher proportion of dominant pores tends to correspond to a higher volumetric water content. For instance, samples NO. 1 and NO. 2 exhibit a greater saturated volumetric water content compared to the other samples, indicative of their higher concentration of dominant pores. Moreover, the strength of the capillary action can vary significantly among different dominant pores. In soils with bimodal dominant pores, the variation in capillary action results in five distinct transition zones with different slopes, as observed in Figure 11. This indicates the complex interplay between the pore size distribution and water retention behavior. The water molecules within the range of dominant pores are significantly affected by the capillary action, and the more dominant pores present, the higher the volumetric water content. For example, the saturated volumetric water contents of NO. 1 and NO. 2 are higher than those of the other three sets of samples. The strength of the capillary action produced by different dominant pores varies greatly, and the soil with bimodal dominant pores exhibits five transition zone with different slopes (Figure 11). Due to the different distribution of dominant pores and morphology in the micropore range of the five sets undisturbed loess samples at different depths, their SWCC morphologies in the residual zone are different. The dominant pore content in the NO. 2 aggregate is the highest, so its residual volumetric water content is greater than those of the other four sets of samples. Combined with the SEM images of the undisturbed loess shown in Figure 7, the results reveal that while the variation in pore size within the aggregates remains relatively small with an increasing burial depth, the connectivity of pores decreases notably. Samples NO. 1, NO. 2, and NO. 3 predominantly feature small pores as dominant pores, while NO. 4 and NO. 5 exhibit dominance in mesopores. This discrepancy in the dominant pore distribution leads to significant differences in the capillary action within the soil matrix.