# Model Modification of the Soil-Water Characteristic Curve of Unsaturated Weak Expansive Soil
## 7 Conclusions  
In this paper, indoor experiments were performed to investigate the pore distribution, soil-water characteristic curve development, and change law of weakly expansive soil of high-speed rail foundations under different dry initial densities. The soil-water characteristic curves were verified and fitted using the F-X model and a three-parameter model. Based on the results, the soil-water characteristic curve correction model of the mercury intrusion test (capillary principle) was proposed. The main conclusions are as follows:  
1. As the dry density increased, the total volume of pores in the swelling soil decreased by about 28.09%, the total volume of large pores became obvious and decreased by 60%, the volume content of small pores increased, and the PSD curve of expansive soil gradually changed from trimodal to bimodal.
2. The soil-water characteristic curve obtained by the capillary principle was always lower than the soil-water characteristic curve obtained by the test method. Four models were used to fit the soil-water characteristic curve law, and the three-parameter model with a better fitting effect was chosen to analyze the influence law of different dry densities with the soil-water characteristic curve. Based on the test method and the fitting curve of the pressure measurement method (capillary principle), a soil-water characteristic curve model that can describe the influence of soil dry density change was proposed.
3. Based on the capillary principle, the soil-water characteristic curve correction model can better reflect the development law of the soil-water characteristic curve in the transition section. In actual engineering, soil pore structure and distribution characteristics and the soil-water characteristic curve can be obtained only through mercury intrusion porosimetry, which can simplify the test and shorten the time-consuming analysis.
4. The modified soil-water characteristic curve model was tested using the experimental results obtained by previous researchers, and the correctness of the model was determined. However, because of the limitation of the test range, the model cannot predict the soil-water characteristic curve of the unsaturated residual suction section of expansive soil. In addition, expansive soil is widely distributed throughout the world, and the expansive soil used in this paper is specifically expansive mudstone, so the universality of the modified model must still be verified.  
Figure 10: MIP-SWCC correction result validation [23,24].  
**Author Contributions:** Conceptualization, L.M.; methodology, L.M., X.D. and Y.X.; validation, L.M.; formal analysis, L.M. and J.G.; investigation, X.D.; resources, L.M.; data curation, J.G.; writing--original draft preparation, J.G.; writing--review and editing, J.G.; visualization, J.G.; supervision, L.M.; project administration, L.M. and D.L.; funding acquisition, L.M. and D.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Research Technology Priorities Program of China State Railway Group Co., Ltd., the grant number is K2021G025; Innovation Project of Young Scientific and Technological Talents in Lanzhou City, the grant number is 2023-QN-50; Natural Science Foundation of Gansu, the grant number is 22 JR11 RA161.  
**Data Availability Statement:** The datasets used and analysed during the current study available from the corresponding author on reasonable request.  
**Conflicts of Interest:** The authors declare no conflict of interest.