# Analysis of Soil-Water Characteristic Curve and Microstructure of Undisturbed Loess  
[PERSON]  
1 Faculty of Urban Construction, Beijing University of Technology, Beijing 100124, China; [EMAIL_ADDRESS] (Y.T. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (W.C.) 1  
[PERSON]  
1 Faculty of Urban Construction, Beijing University of Technology, Beijing 100124, China; [EMAIL_ADDRESS] (Y.T. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (W.C.) 1  
[PERSON]  
2 College of Geosciences and Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450046, China; [EMAIL_ADDRESS] 2  
[PERSON]  
1 Faculty of Urban Construction, Beijing University of Technology, Beijing 100124, China; [EMAIL_ADDRESS] (Y.T. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (W.C.) 1  
[PERSON]  
1 Faculty of Urban Construction, Beijing University of Technology, Beijing 100124, China; [EMAIL_ADDRESS] (Y.T. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (W.C.) 1  
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the behavior and characteristics of soil under different water conditions. According to [PERSON] et al. [17] the SWCC of a soil's desaturation process is typically divided into three stages, which are delineated by three zones on the drying path of an SWCC: the boundary effect zone, transition zone, and residual zone. These stages are characterized by specific points on the SWCC, namely the air entry value (AEV) and the residual suction: (1) Boundary effect zone: In this zone, the volumetric water content of the sample remains relatively constant with increasing matric suction, indicating that the soil is essentially saturated; (2) Transition zone: Positioned between the AEV and residual suction, the transition zone is dominated by capillary action. Here, as suction increases, the free water in the macroporous space diminishes significantly, and gas gradually replaces the water. The slope of the curve in this zone reflects the soil's water-retention capacity; (3) Residual zone: This zone is characterized by a relatively low volumetric water content that decreases slowly with increasing matric suction. Water distribution in the sample becomes discontinuous, with adsorption effects influencing water retention in soil. Many researchers have utilized an SWCC to predict various parameters of unsaturated soil's behavior, including permeability coefficients, hydraulic conductivity, soil water storage, strength, and more. [18; 19; 20; 21; 22; 23; 24; 25; 26]. Additionally, an SWCC is instrumental in describing the waterforce-chemical coupling process in unsaturated soil [27]. These prediction parameters are based on the inherent connection between an SWCC and microstructure, especially the PSD. An SWCC is influenced by various factors, such as the soil type [28], soil structure [29; 30], stress history and stress state [18], density [31; 32], and more. Among these factors, the density, stress history, and stress state alter the SWCC by affecting the soil's structure [33]. Differences in the soil structure can cause significant variations in the SWCC of natural loess, and even small structural changes can lead to different SWCCs [34; 35; 36; 37; 38]. The undisturbed loess samples at different depths have different natural structures, so it is imperative to investigate the relationship between the soil microstructure and hydrological properties. Thus, developing technologies to study soil pore structures and mechanical properties is essential [39; 40; 41]. In soil structure research, SEM and MIP are commonly employed to analyze the soil pore size, particle geometry characteristics, and pore and particle morphology distributions [39; 40].