## 2 Experimental Details  
### Sampling and Preparation of Samples  
The samples tested were acquired from the South Jingyang tableland (Figure 1), a site that has suffered a considerable number of landslide disasters since 1984 [33]. Field investigations evidenced that the vast majority of sliding surfaces were located between a loess layer and paleosol layer [28; 34]. Hence, samples of loess and paleosol were obtained to study their shear properties.  
Undisturbed bulk samples of loess (L\({}_{2}\) layer) and paleosol (S\({}_{2}\) layer) with a size of 200 \(\times\) 200 \(\times\) 200 mm\({}^{3}\) were collected, and the top sides of samples were clearly marked in the field. Thereafter, they were well sealed with plastic wrap and carefully transported to the laboratory to avoid any disturbance. In the laboratory, these samples were cut using ring knives to a diameter of 63.5 mm and height of 25.4 mm for direct shear tests. The diameter and height of the samples used for charting SWCCs were 61.8 mm and 20 mm, respectively. For the direct shear tests, loess and paleosol samples with initial moisture contents of 11, 13, 17, 21, 26, and 33% (by weight) were prepared by air drying or spraying distilled water. Afterwards, they were wrapped with plastic wrap and placed in an airtight moisturizing container for at least 48 h to achieve water homogenization. It is worth mentioning that samples used for measuring SWCCs should be fully saturated with degassed water before the test begins.  
Loess and paleosol samples' basic physical properties were determined in accordance with the Chinese National Standards (CNS) GB/T50123-2019 [37]. The grain size distributions, measured using a laser particle analyzer (Bettersize2600E), revealed that the loess and paleosol samples were predominantly composed of silt particles with proportions of 79.99-80.21% and 81.89-82.93%, respectively. Their physical properties are presented in Table 1. It can be seen that the paleosol samples had a higher dry density, moisture content, Arterberg limit, and silt and clay contents than the loess samples, and yet their specific gravity and sand content were lower than those of the loess samples. An X-ray diffraction test was implemented to analyze their mineral compositions, indicating that the two soils predominantly incorporated quartz, feldspar, and calcite, with small amounts of chlorite, illite, and amphibole (Table 2). The finding that the carbonate mineral content in the loess samples was higher than that in the paleosol samples (18.7% and 14.4%, respectively) agreed with the findings of the investigation by [PERSON] et al. [28].  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Sample** & **G\({}_{\text{s}}\)** & **\(\rho_{\text{d}}\)**/(g/cm\({}^{3}\))** & _w/\%_ & \multicolumn{3}{c}{**Afterberg Limits**} & \multicolumn{3}{c}{**Grain Size Distribution/\%**} \\  & & & _w\({}_{\text{L}}\)_/\%_ & _w\({}_{\text{P}}\)_/\%_ & **I\({}_{\text{P}}\)** & **Sand \textgreater{} 0.05 mm** & **Silt 0.002–0.05 mm** & **Clay \textless{} 0.002 mm** \\ \hline Loess & 2.71 & 1.49 & 13.4 & 29.6 & 17.0 & 12.6 & 11.20–11.49 & 79.99–80.21 & 8.52–8.59 \\ Paleosol & 2.67 & 1.51 & 17.9 & 31.8 & 18.3 & 13.5 & 3.18–3.80 & 81.89–82.93 & 13.89–14.44 \\ \hline \hline \end{tabular} Note: G\({}_{\text{s}}\), specific gravity; \(\rho_{\text{d}}\), dry density; \(w\), natural moisture content; _w\({}_{\text{L}}\)_, liquid limit; _w\({}_{\text{P}}\)_, plastic limit; _I\({}_{\text{P}}\)_, plasticity index.  
\end{table}
Table 1: Physical properties of loess and paleosol samples.  
Figure 1: The location and acquisition process of samples (modified according to [4; 35; 36]).