# Analysis of Soil-Water Characteristic Curve and Microstructure of Undisturbed Loess
## 4 Conclusions  
In this study, we conducted an investigation into the SWCCs and microstructures of undisturbed samples buried at varying depths (samples NO. 1 to NO. 5 at depths of 3.4 m, 8.5 m, 13.2 m, 25 m, and 51.9 m). The SWCC data were obtained using the HYPROP device, while a microstructural analysis was carried out using SEM and MIP. Our findings provide valuable insights into the influence of the burial depth on the SWCC and pore size distribution characteristics. The following conclusions can be drawn:  
1. The soils buried at different depths exhibited similar grain compositions, with all loess samples showing bimodal particle size density distribution curves. This bimodal distribution indicates the presence of two hierarchical structures within the soil's microstructure and macrostructure. A deeper burial resulted in a higher dry density, and the saturated volumetric water content of the undisturbed loess decreased by 10.9%, and the AEV increased from 13.67 to 40.19 kPa. Additionally, the suction range of the transition zone decreased, exhibiting different slopes. 2. As the burial depth of the loess increased from 3.4 to 51.9 m, the SEM and MIP tests indicated a decrease in connectivity pores with an increasing depth, resulting in a decrease in \(n_{e}\) from 50.69 to 38.88%: a total reduction of 11.81%. The PSD curves of all loess samples exhibit bimodal characteristics, corresponding to intra-aggregate pores within aggregates and inter-aggregate pores between aggregates, with a boundary at 150 nm. The intra-aggregate pores show little variation, while there are significant differences in the inter-aggregate pores. The content of macropores (d > 16,000 nm) decreased by 3.6%, the content of mesopores (4000 nm < d \(\leq\) 16,000 nm) decreased by 12.1%, while the contents of small pores (1000 nm < d \(\leq\) 4000 nm) and micropores (d \(\leq\) 1000 nm) showed minor changes. 3. It is crucial to emphasize that the SWCCs of undisturbed loess at various depths were predominantly controlled by the pore size distribution. Specifically, the total porosity controls the saturated volumetric water content of the SWCC. The number of dominant pores or the peak point of the PSD curve governed the SWCC desaturation mode. The inflection pore of the PSD curve along the mercury intrusion direction corresponded to the AEV of the SWCC. The range of dominant pores determined the transition zone of the SWCC, while the size of the dominant pores determined the slope of the transition zone curve. Furthermore, the micropores distribution determined the morphology of the residual zone of the SWCC. These findings underscore the pivotal role of the pore structure in governing the soil's water retention behavior. * 4. Combining the macroscopic SWCC test results and microstructure characteristics of undisturbed loess samples buried at different depths, it can be concluded that the soil's water retention behavior is strongly affected by the soil's compaction effect. The larger the burial depth of the soil, the higher the dry density, which leads to a decrease in the total porosity of the soil, resulting in a decrease in the number of macropores and mesopores in the soil structure. This affects the morphological characteristics and model fitting parameters of the soil's SWCC, resulting in differences in the water holding capacity of the soil. Therefore, when analyzing the propagation process of water in thick loess layers, it is necessary to fully consider the influence of the soil burial depth. Conceptualization, F.D. ; Data curation, Y.T. and Z.Z. ; Formal analysis, Y.T. and Z.Z. ; Funding acquisition, F.D. ; Methodology, F.D. ; Project administration, F.D. ; Writing--original draft, Y.T. ; Writing--review and editing, Y.T., F.D., Z.Z., J.Z. and W.C. All authors have read and agreed to the published version of the manuscript. This work was supported by the Huaneng Group Technology Project (No. HNKJ22-H107), the Yunnan Province Major Science and Technology Special Plan (No.