## 4 Discussion
### Relationship Between Structural Parameter and Particle Size  
Loess is a fine-grained soil, and the moisture content has a very important effect on its mechanical and strength properties. The plastic limit of soil is an index of the soil reaching the plastic state. The liquid limit of soil is the moisture content of the boundary of the soil required to reach the liquid-plastic state. The plasticity index is the range of change between the upper and lower limits of the plastic state of soil, which mainly reflects the water-holding capacity of soil particles. The finer the soil grain, the greater the ability to absorb and hold water, and the greater the plastic limit, liquid limit, and plasticity index of the soil. The structural parameter \(m_{\tau}\) is closely related to the strength of loess. When the moisture content of the soil is close to the plastic limit, the soil is prone to plastic deformation, and the loess suffers less sensitivity to the effects of immersion and therefore less structural parameter. When the natural moisture content of loess is less than the plastic limit, the sensitivity of loess to soaking increases. Therefore, the difference between plastic limit and natural moisture content of loess soil samples was taken to investigate the effect of plastic limit on the structural properties of loess. The difference in moisture content is defined as follows:  
\[\Delta w=w_{p}-w \tag{6}\]  
Based on the test results of the structural parameter \(m_{\tau}\) with moisture content in Section 3.2.2, the relationship between \(m_{\tau}\) and moisture content difference was analyzed, as shown in Figure 7. The pattern of change of structural parameters with moisture content difference for different depths of Weinan loess samples from loess 1 to loess 5 shown in Figure 7 is approximately the same. The greater the difference between the plastic limit and the moisture content, the greater the value of \(m_{\tau}\); conversely, the smaller the value  
Figure 6: Variation of structural parameter with ratio of density to void ratio. of \(m_{\tau}\). When the moisture content is less than the plastic limit, the value of the structural parameter of loess is greater. The lower the plastic limit of the soil sample, the larger the slope of the change curve of moisture content difference with structural parameters, and the smaller the range of structural parameters. On the contrary, the smaller the slope of the change curve of moisture content difference with structural parameters, the smaller and larger the change range of structural parameters. A comparison of the structural parameters of the five soil samples as a function of the difference between the plastic limit and the moisture content is shown in Figure 8. It can be seen from Figure 8 that the growth rate of \(m_{\tau}\) with decreasing moisture content is smaller when the difference between plastic limit and moisture content is less than 7.5%. The growth rate of \(m_{\tau}\) with decreasing moisture content is greater when the difference between the plastic limit and moisture content is greater than 2.5%. At the same time, the test results of the change pattern of \(m_{\tau}\) with the difference of plastic limit and moisture content are more discrete. This shows that there are other factors besides moisture content (such as differences in density and structure of different loess) that can affect structural properties. To consider the effect of plastic limit and liquid limit on the structural properties, the test results of the variation of the structural parameters of plastic limit and liquid limit soils  
Figure 8: \(m_{\tau}\)-\(\Delta w\) of different loess samples. Figure 7: Variation of structural parameter with difference between plastic limit and natural moisture content. under different moisture content conditions are analyzed in Figure 9. It can be seen from Figure 9(a) that the structural parameters of loess generally show a monotonically increasing rule of change with increasing plastic limit.