# Model Modification of the Soil-Water Characteristic Curve of Unsaturated Weak Expansive Soil
## 1 Introduction  
Expansive soil, widely distributed worldwide, is a type of disastrous soil composed mainly of strong hydrophilic minerals in its clay composition. It exhibits significant characteristics such as \"water absorption expansion and water loss shrinkage\" and \"reciprocating deformation\" [1]. This poses a threat to various construction projects and their operations, with damages that are difficult to repair. As one of the crucial issues in the geotechnical engineering and geological engineering fields, expansive soil has caused severe problems during the early debugging stage of the Lanzhou-Xinjiang high-speed railway because of arch deformation caused by expansion and shrinkage, cracks, and over-consolidation [2], which seriously hindered its operation and development. Therefore, studying the soil-water characteristics of expansive soil is essential for providing technical support for designing new high-speed railways. At present, many scholars have studied soil-water characteristics. [PERSON] [3] studied the influence of dry density and sample water content on the macroscopic soil-water characteristics of loess. [PERSON] [4] measured the soil-water characteristic curve of expansive soil under different concentrations of a NaCl solution in pores. [PERSON] [5] established a new model to predict the constitutive relationship between matric suction and water content in unsaturated soils. [PERSON] [6] analyzed the influence of the initial void ratio on soil-water characteristics under high suction conditions through the soil-water characteristics test of compacted silt samples. [PERSON] [7] measured the soil-water characteristic curve of compacted loess silt with filter paper. The soil-water characteristic curve (SWCC) characterizes the relationship between matric suction and soil water content. It is widely used to describe the water state of soil according to the matrix suction of soil [8]. Some researchers used the SWCC to establish a thermodynamic frost heave model, which helped to evaluate themechanical properties of frozen soil [9; 10]. It is very important to measure and calculate the SWCC accurately and efficiently when studying the characteristics of unsaturated soil and solving engineering problems. [PERSON] [11], [PERSON] [12], [PERSON] [13], [PERSON] [14], and others provided guidance for practical engineering by conducting indoor and outdoor test calculations and predicting soil-water characteristic curves. The change in influence suction is reflected in the strength and deformation characteristics of soil, and the strength and deformation characteristics of soil are reflected in the microscopic pore structure of soil. The suction in the pore matrix is generated by the pore water and air pressure on both sides of the curved liquid surface adsorbed on the soil particle skeleton [15; 16; 17; 18]. However, previous studies have been limited to the macro factors affecting the soil-water characteristic curve. Studies on the hydraulic properties of soil should not ignore the influence of the microstructure, but it is difficult to meet the needs of scientific research and engineering because of the lack of micro-mechanism support. The microstructure of soil plays a decisive role in all aspects of the properties of soil. The mechanical properties of soil can be understood as the macroscopic performance of the microstructure. Therefore, to solve the above problems, this paper will analyze the influence of compaction degree change on the microscopic pore volume content, pore size distribution, and pore shape of expansive soil through a mercury intrusion test. In summary, we used a pressure plate instrument (0-0.5 MPa) and the filter paper method (0-40 MPa) to generate the soil-water characteristic curve. Then, we applied the mercury intrusion method (capillary principle) to determine the theoretical calculation value of the soil-water characteristic curve and determined the reason why the theoretical calculation value of the soil-water characteristic curve was too small.