# Analysis of Soil-Water Characteristic Curve and Microstructure of Undisturbed Loess
## 3 Results and Discussion
### Pore Structure and PSD Characteristics
Notably, the dominant pore diameter corresponding to the larger peaks of NO. 1, NO. 2, and NO. 3 is 7.24 \(\upmu\)m, categorized as a mesopore, while NO. 4 and NO. 5 exhibit dominant pore diameters within the range of small pores, at 3.77 \(\upmu\)m and 3.71 \(\upmu\)m, respectively. For samples NO. 1 and NO. 2 from the same layer but different depths, the inflection pore sizes are 17.26 \(\upmu\)m and 7.24 \(\upmu\)m, respectively, consistent with the dominant pore sizes. However, NO. 4 and NO. 5 display inflection pore sizes of 13.94 \(\upmu\)m and 7.24 \(\upmu\)m, with dominant pore sizes of 3.77 \(\upmu\)m and 3.71 \(\upmu\)m, respectively. Interestingly, NO. 3 exhibits an inflection pore size of 24.14 \(\upmu\)m, aligning with NO. 1 and NO. 2, indicating consistency in the dominant pore sizes, except for NO. 3, which experiences a different depositional and pedogenic environment at the junction of loess and paleosol. Figure 8 demonstrates that the cumulative pore volume increases with the porosity, showing a quadratic growth relationship between the cumulative pore volume and void ratio. Figure 9 provides a comparison of the porosity of micro-, small, meso-, and macro-pores in undisturbed loess samples at different depths, revealing a gradual decrease in total porosity with an increasing burial depth, measured using MIP. Specifically, the meso- and macro-pore contents of four undisturbed loess samples, excluding NO. 3, decreased  
Figure 7: PSDs of undisturbed loess from different depths. (**a**) Cumulative pore volume curves; (**b**) pore size distribution curves; (**c**) pore radius versus dry density. with an increasing burial depth. Notably, although NO. 5 was buried deeper, its small pore content exceeded that of the other four undisturbed samples, indicating a higher burial depth impact on meso- and macropores compared to micro- and small pores. The porosity (\(n\)) represents the ratio of the total pore volume to the total volume of the soil sample, while the effective porosity (\(n_{e}\)) indicates the ratio of effective pore volume (total volume minus closed pore volume) to the total sample volume. The porosity corresponding to the cumulative intrusion rate of MIP represents the effective porosity (\(n_{e}\)). Additionally, Figure 9 summarizes the cumulative intrusion porosity (effective porosity, \(n_{e}\)), while Table 3 provides an overview of the pore parameters for the five undisturbed loess samples to analyze the pore measurement characteristics using MIP. It is observed that the \(n_{e}\) measured using MIP is lower than the \(n\) obtained through laboratory tests (Table 3), which is attributed to insolated and constricted pores, in addition to pores with a diameter outside the range of 3 nm to 360 \(\upmu\)m, which cannot be intrud  
Figure 8: Relationship between cumulative pore volume and void ratio of loess samples. Figure 9: Porosities of micropores, small pores, mesopores, and macropores, and \(n_{e}\) of the undisturbed loess samples from different depths. The effective porosities (\(n_{e}\)) measured using MIP for the five sets of undisturbed loess samples ranged from 41.33 to 53.33%, all of which were smaller than the actual porosities. To express the extent of mercury intrusion into soil pores, the effective rate of mercury intrusion (\(\varphi\)) is calculated as the ratio of the effective porosity (\(n_{e}\)) to the porosity (\(n\)) obtained using MIP. It was observed that the effective rate of mercury intrusion for the five undisturbed loess samples exceeded 89%. Specifically, the effective rate of mercury intrusion for NO. 1 and NO. 5 exceeded 94%, while for NO. 2 and NO. 3, it was around 91%, and for NO. 4, it was less than 90%. Compared with the literature, the effective porosity of undisturbed loess at a natural moisture content was higher than that of compacted loess samples at the same dry density [64].