# Analysis of Soil-Water Characteristic Curve and Microstructure of Undisturbed Loess
## 3 Results and Discussion
### Pore Structure and PSD Characteristics  
In this study, we examined undisturbed loess samples NO. 1 and NO. 2 from the same layer, as well as NO. 5 from a deeper layer (Figure 1). The SEM images obtained at magnifications of 500\(\times\) and 1000\(\times\) for these samples are shown in Figure 6. The images reveal particles of varying sizes loosely arranged, predominantly comprising angular debris particles with some sheet-like aggregates. Granular particle agglomerates are also evident, indicating the presence of aggregates. Particle contacts are mainly indirect or surface to surface, with fewer edge-face contacts and minimal point contacts. Pores of different sizes and shapes are observed between the soil particles. The comparison of the SEM images between NO. 1 and NO. 2 reveals that NO. 1 predominantly contains inter-aggregate or shelf pores between coarse particles, indicating better connectivity. This corresponds to \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & \(\theta_{s}\) & \(\theta_{r}\) & \(\alpha\) & \(n\) & \(R^{2}\) \\ \hline NO. 1 & 0.523 & 0.001 & 0.037 & 1.801 & 0.994 \\ \hline NO. 2 & 0.528 & 0.001 & 0.049 & 1.498 & 0.993 \\ \hline NO. 3 & 0.412 & 0.001 & 0.030 & 2.017 & 0.997 \\ \hline NO. 4 & 0.425 & 0.001 & 0.031 & 1.552 & 0.996 \\ \hline NO. 5 & 0.404 & 0.001 & 0.015 & 2.541 & 0.996 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SWCC fitting parameters of undisturbed loess samples. Figure 5: Variations in the fitting parameters \(a\) and \(n\) with respect to the dry density. (**a**) The relationship between \(\rho_{d}\) and parameter \(a\); (**b**) the relationship between \(\rho_{d}\) and parameter \(n\). a larger dominant pore diameter and higher pore distribution density in the PSD curve. NO. 2 exhibits a similar microstructure to NO. 1 but with smaller inter-aggregate pores. Figure 7a and b illustrate the representation of the pore size distribution curve using the cumulative pore size volume and pore size density distribution curves, respectively. Figure 7b illustrates that the PSDs of the five sets of undisturbed loess samples at various depths exhibit bimodal characteristics, indicating the presence of two families of pores within the samples. This bimodal distribution arises from the elementary units constituting the structure of undisturbed loess samples, primarily clay aggregates and clay-silt aggregates formed under electrostatic attraction and capillary attractions, as well as van der Waals forces. These aggregates, which form the skeleton structure of undisturbed loess, contribute to its double-layer pore structure, comprising the microstructure and macrostructure [62; 63; 64]. Pores within aggregates are classified as intra-aggregate pores, while those between aggregates are termed inter-aggregate pores, which have a bounding pore size of approximately 150 nm (Figure 7b). To facilitate the analysis of PSD changes due to burial depth variations, this study classifies undisturbed loess pores into four categories based on their radius, following the criteria proposed by [PERSON] (1988) [65]: micropores (d \(\leq\) 1000 nm), small pores (1000 nm \(<\) d \(\leq\) 4000 nm), mesopores (4000 nm \(<\) d \(\leq\) 16,000 nm), and macropores (d \(>\) 16,000 nm) (Figure 7a,b). The comparison of cumulative pore volume curves (Figure 7a) reveals a steep increase followed by a gradual increase in the mercury intrusion curves for  
Figure 6: SEM images of the microstructure of undisturbed loess under different magnifications. (a) 500\(\times\); (b) 2000\(\times\). undisturbed loess samples at different depths, with a subsequent significant decrease in the cumulative mercury intrusion amount and pore range as the burial depth increases. As depicted in Figure 7c, inflection points for NO. 1 and NO. 2 are at 17.3 \(\upmu\)m, while those for NO. 3, NO. 4, and NO. 5 are 24.1 \(\upmu\)m, 13.9 \(\upmu\)m, and 7.2 \(\upmu\)m, respectively. Generally, the pore size at the inflection point decreases with an increasing burial depth.