# Analysis of Soil-Water Characteristic Curve and Microstructure of Undisturbed Loess
## 2 Materials and Methods
### SWCC Measurement  
#### 2.2.1 Test Apparatus  
The measurements of the SWCCs for the undisturbed loess samples were conducted using the HYPROP system developed by the METER Group. The HYPROP system mainly consists of the soil sample, sensor unit, and a high-precision electronic balance. The soil sample intended for testing may be in either a remolded or undisturbed condition. The sensor unit consists of two tension shafts positioned at depths of 12.5 mm and 37.5 mm, respectively. The matric suction can be measured within a scale of 0-100 kPa with an accuracy of \(\pm\)0.05 hPa. The high-precision electronic balance with a weighing range of 0-2200 g and weight accuracy of 0.01 g is designed to monitor sample mass changes during the evaporation process. The matric suction of soil at a depth of 25 mm is equal to the mean of the recorded matric suction levels at elevations of 12.5 mm and 37.5 mm, which in accordance with the linear hypothesis posited by [PERSON] (1980) [48]. Meanwhile, the  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **NO. 1** & **NO. 2** & **NO. 3** & **NO. 4** & **NO. 5** \\ \hline Depth (m) & 3.4 & 8.5 & 13.2 & 25 & 51.9 \\ \hline In situ water content (\%) & 8.10 & 18.08 & 10.17 & 10.00 & 10.00 \\ \hline Dry density (g/cm\({}^{3}\)) & 1.27 & 1.30 & 1.50 & 1.51 & 1.59 \\ \hline Initial void ratio, \(e\) & 1.11 & 1.10 & 0.81 & 0.79 & 0.71 \\ \hline Specific gravity, \(G_{s}\) & 2.70 & 2.70 & 2.71 & 2.71 & 2.71 \\ \hline Liquid limit, \(w_{L}\) (\%) & 32.48 & 32.90 & 33.00 & 30.05 & 29.92 \\ \hline Plastic limit, \(w_{P}\) (\%) & 19.24 & 17.60 & 16.60 & 16.99 & 18.09 \\ \hline Plasticity index, \(I_{P}\) (\%) & 13.24 & 15.30 & 16.40 & 13.06 & 11.83 \\ \hline Clay (0–0.002 mm) (\%) & 2.58 & 4.56 & 2.26 & 3.71 & 2.69 \\ \hline Silt (0.002–0.075 mm) (\%) & 65.37 & 83.94 & 54.93 & 69.51 & 73.32 \\ \hline Sand (0.075–2 mm) (\%) & 32.05 & 11.50 & 42.81 & 26.78 & 23.99 \\ \hline Effective particle diameter (μm) & 6.42 & 3.98 & 7.66 & 4.96 & 5.33 \\ \hline Median diameter (μm) & 19.00 & 11.91 & 28.87 & 16.34 & 17.80 \\ \hline Control particle size (μm) & 50.50 & 28.86 & 88.12 & 42.32 & 40.95 \\ \hline Coefficient of uniformity, \(C_{u}\) & 7.87 & 7.24 & 11.50 & 8.53 & 7.68 \\ \hline Coefficient of curvature, \(C_{c}\) & 1.11 & 1.23 & 1.23 & 1.27 & 1.45 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical properties of loess samples.  
volumetric water content of soil at the midpoint (i.e., at a depth of 25 mm) can be ascertained by considering the evaporation mass of the saturated sample at that specific moment, which is in accordance with the principle of saturated evaporation proposed by [PERSON] and [PERSON] (2008) [49]. Using the difference method, the corresponding matric suction and volumetric water content were obtained at different time points and depths. Utilizing the HYPROP system for SWCC determination offers several advantages, including simple operation, short test cycles, modular expansion capability, simultaneous measurement of up to 20 samples, and non-perturbed measurement of the sample, allowing natural evaporation to establish the sample's SWCC.