## 4 Discussion  
### Microstructure and Composition of Diatomite Soil  
The direct shear test and triaxial test results show that the diatomite soil exhibits different mechanical properties under different water content, the mechanical strength index of diatomite soil decreases with the increase in water content in diatomite soil. In this paper, the microstructure and element mineral composition of diatomite soil are studied by scanning electron microscope (SEM) and XRD experiments.  
Figure 4 shows the results of SEM, Figure 4a is 5000 times and Figure 4b is 10,000 times magnification. The SEM results of diatomite soil show that the diatomite soil contains many orderly arranged pore structures. The clay minerals in the whole diatomite soil show the structure of a parallel sheet and fold sheet. The aggregation among the grain groups is mainly in the form of a stacked polymerization arrangement. The overall structure of diatomite soil is presented as the dispersed structure. The clay particles and diatom debris grains are arranged nearly in parallel, and the contact form is surface-to-surface. This unique microstructure makes the diatomaceous soil relatively high in porosity, which shows a property of being easily soaked in water.  
Through the X-ray diffraction test, the elemental composition of the diatomite soil is analyzed. Figure 5 shows the XRD composition results. The XRD results show that there are many chemical elements in diatomite soil, including Si, K, Al, H, O, etc. The diatomite soil has different physical and mechanical properties because of the different ion content, these elements exist in the form of oxides in diatomite soil. The results show that diatomite soil has higher aluminum content. Aluminum is usually combined with other elements such as silicon and oxygen to form a variety of oxides and minerals. It is mainly composed of diatom remains (about 15-20%) and clay minerals (about 80-85%), and the content of diatom debris is slightly higher. The results show that the content of kaolinite and mica in diatomite soil is high, which is the main reason for its poor engineering properties.  
Figure 4: SEM results of diatomite soil. (**a**) 5000 times; (**b**) 10,000 times.  
There are many pores in diatomic soil, when the soil is relatively dry and the pores are large, the suction between particles in diatomic soil is small, they are separated from each other. The contact between particles is surface-to-surface contact. The structural force between layers is provided by the indirect contact surface of particles, and the contact area is large. The soil structure is stable, and the strength of diatomic soil is high. When the pores in the soil decrease and the pores are gradually filled with water, the suction between the molecules increases and the fine particles in the soil adhere to each other under suction, forming small aggregates. As a result, the contact area between soil particles gradually tends to point-to-point form. Because the content of kaolinite, mica, debris, and other minerals in diatomic soil is higher when water enters the soil, the soil will show certain volume expansion and softening characteristics, and its mechanical strength characteristics will decline.