# Analysis of Soil-Water Characteristic Curve and Microstructure of Undisturbed Loess
## 2 Materials and Methods  
### Materials  
The study site was the Jingyang South Loess Plateau, situated in Jingyang County, Shaanxi Province, China. The plateau's top elevation stands at 468 m, with a height variance ranging from 62 to 90 m and a slope gradient of 50\({}^{\circ}\) to 80\({}^{\circ}\). The base of the slope transitions into a river terrace with flat terrain, overlaid with distributed landslide deposits. The sampling location is situated within the loess plateau in Shutangwang Village, Gaozhuang Town (108\({}^{\circ}\)49\({}^{\circ}\)18\({}^{\prime\prime}\) E, 34\({}^{\circ}\)29\({}^{\circ}\)33\({}^{\prime\prime}\) N), on the right bank of Jingyang County, approximately 5 km south of the city. As shown in Figure 1a, the sampling profile exhibits a well-defined loess-paleosol layer, characterized by continuous and intact loess layers interspersed with discernible paleosol layers. The soil layer characteristics are distinctly discernible and facilitate easy differentiation. The stratigraphic profile illustrated in Figure 1b displays exposed loess strata comprising Q3 to L7 loess from Q3 to Q2, classified as the Late Pleistocene Malan loess, appearing light yellow with a consistent soil texture. S1 represents a brownish-red paleosol, characterized by calcareous nodules in the upper section and dense, hardened soil in the lower section. Below S1 lies the Middle Pleistocene Lishi loess and the paleosol series, with the Lishi loess exhibiting a grayish-yellow hue and uniform soil texture. The sampling points, indicated by red dots in Figure 1, are located in Shutangwang Village. In order to maximize the retention of the natural structure of the soil samples, we first removed the 0.5 m thick soil from the slope surface, and we employed a 250 cm\({}^{3}\) (50 mm high, 80 mm inner diameter) ring knife to extract undisturbed loess samples. Additionally, we collected loose soil in the vicinity of the sampling point to measure the basic physical properties of the loess in the laboratory.  
Figure 1: The topographic feature stratigraphic profile of the study area, numbers 1 to 5 represent sampling points for undisturbed samples at various depths. (**a**) Full view of the study area; (**b**) stratigraphic profiles of sampling sites.  
The physical properties of these samples were assessed in the laboratory and are summarized in Table 1. A laser particle size analysis was conducted on soil particles passing through a 2 mm sieve using the Mastersizer 2000. Figure 2 illustrates the cumulative particle size distribution curve and particle size density curve of the loess samples. The particle size density curves of loess soils at different depths exhibit bimodal distributions, with the first peak ranging from 28.28 to 39.91 um and the second peak from 316.98 to 893.37 um. The percentages of particle size at the first and second peaks range from 3.66 to 4.58 and 0.96 to 3.61%, respectively. Bimodal particle size distribution soils typically feature two levels of structure: the soil microstructure (the elementary particle associations within the soil aggregates) and macrostructure (the arrangement of the soil aggregates) [46]. Based on the particle size distribution curve, the percentages of various particle sizes of loess at different depths, as well as the control particle size (\(d_{60}\)), median particle size (\(d_{30}\)), and effective particle diameter (\(d_{10}\)) of each sample, were determined. Additionally, the coefficient of curvature (\(C_{c}\)), calculated as \((d_{30}\cdot d_{30})/(d_{10}\cdot d_{60})\), and the coefficient of uniformity (\(C_{u}\)), calculated as \(d_{60}/d_{10}\), were derived. Table 1 demonstrates that the loess exhibits good gradation at various depths.