# Model Modification of the Soil-Water Characteristic Curve of Unsaturated Weak Expansive Soil
## 4 Analysis and Evaluation of the Pore Structure  
The structural characteristics of the soil skeleton comprise the internal factors affecting the physical and mechanical properties of the soil and the soil-water characteristics. All kinds of soils generally develop micropores and microcracks, forming their own special microstructure [21]. Therefore, it is necessary to analyze the pore structure and pore distribution law of expansive soil under different degrees of compactness and study the fractal characteristics of pores. Based on the MIP test data, the pore distribution curves under different dry densities were obtained, as shown in Figure 4.  
The sample's pore diameter ranges between 6 and \(10^{6}\) nm (Figure 4). The overall aperture distribution curve of the sample revealed three peaks (trimodal) distribution, corresponding to the aperture ranges of 6 to 77 nm (peak I), 550 to 3900 nm (peak II), and 6800 to 91,683 nm (peak III) (Figure 4b).  
An increase in the dry density of the sample does not significantly influence the position and slope of the small pore part of the curve, while the peak and peak width fluctuations are small. The cumulative decrease is 86.25% within the mL-g\({}^{-1}\) (1.4 g\(\cdot\)cm\({}^{-3}\)) to 0.0114 mL-g\({}^{-1}\) (1.9 g\(\cdot\)cm\({}^{-3}\)) range. Notably, there is a significant peak decrease with a marked left shift, which is accompanied by a slight decrease in peak width, and a pore size distribution curve (PSD curve) that gradually tends to a double peak.  
The soil skeleton inside the soil body is destroyed or recombined because of the increase in dry density of the expansive soil. This phenomenon causes significant changes in the pore structure, where some of the pore structure is destroyed, compacted, filled, or converted into other smaller diameter pores. Its impact on the pore volume content of the soil is shown in Figure 5. The total volume of the accumulated pores inside the expansive soil was reduced by 28.09%, while the cumulative reduction in total volume in large, medium, and small pores were 60.00, 12.36, and \(-\)3.16%, respectively. The volume of large pores decreased approximately linearly, which was obvious, while that of small pores increased. The destruction and reorganization of pore structure caused by the change in dry density of expansive soil significantly impacted the internal pore distribution and proportion of expansive soil.  
Figure 4: Pore distribution graphs at different dry densities. (**a**) Pore volume accumulation curve. (**b**) Pore distribution curve.  
The above results indicated that an increase in the dry density of the sample generates a relatively linear content of the pores inside the mud rock. Notably, the sample's dry density had no significant effect on the pore content or particle size distribution of the small aperture (6 to 300 nm), while the total volume basically remained unchanged. Moreover, the sample's dry density had little effect on the pore content and particle size distribution of the mid-hole (300-6800 nm). The dry density of sample preparation had a significant impact on large pores (>6800 nm), while the volume content of large pores decreased significantly. Particularly, 6800-10\({}^{5}\) nm pores were most significantly affected by the dry density of the sample