## 4 SEM Scanning Test Results and Analysis
### Effect of NaCl Concentration on Microstructure  
Figure 14 shows the percentage of pores in soil samples before and after compression under different NaCl concentrations. It can be seen from the figure that the total number of small pores (\(d<10\) mm), medium pores (\(10\) mm \(\leq d\leq 20\) mm) and large pores (\(d>20\) mm) [26] in each soil sample is close to one third, and the proportion of small pores and macropores in soil samples fluctuates with NaCl concentration, and the proportion of medium pores first increases and then decreases with the increase of NaCl concentration, reaching a maximum of nearly 40 at 2.5% NaCl concentration. Some small pores and large pores are transformed into medium pores, and the transformation increases with the increase of NaCl concentration, and weakens after exceeding the threshold of 2.5%, and is not obvious until 5%. Total pore fraction is the ratio of the pore area of soil samples to the area of the whole SEM image. Figure 15 shows the variation of total pore fraction of each soil sample and its increment before and after compression with NaCl concentration. It can be seen from the figure that the total pore fraction of pre-compression soil samples first increases and then decreases with the increase of NaCl concentration, and the maximum value is 41.385% near 2.5% NaCl concentration, while the total pore fraction of post-compression soil samples has no clear correlation with NaCl concentration; The decrease of total pore fraction of soil samples before and after compression first decreases with the increase of NaCl concentration, and reaches the maximum value of 14.673% at 2.5% NaCl concentration,  
Figure 14: Percentage of small, medium, and large pores in pre-compression and post-compression soil samples under varying NaCl concentrations. Figure 13: Binarization threshold based on porosity (2.5% NaCl reconstituted soil). (**a**) Original image; (**b**) Binary image when \(Z_{i}=68\). which shows that the existence of pore water salt has an influence on the change of pore area of soil during compression deformation, and its promoting effect increases rapidly in the range of 2.5% NaCl concentration, and then decreases after exceeding 2.5% NaCl concentration. Therefore, it can be inferred that the effect of pore water salt in the middle concentration can be concentrated to the middle pore by changing the diameter of some pores, and it can also affect the narrowing of pores during compression, but its effect is not obvious at the lower or higher pore water salt concentration. Figure 16 shows the relationship between soil porosity and particle aspect ratio (the ratio of short axis \(B\) to long axis \(L\)) and NaCl concentration. As can be seen from the figure, the porosity and particle aspect ratio of each soil sample are mainly concentrated in the range of 0.52-0.65, and the structures are mostly oblate, with few elongated and equiaxed pores. After compression, compared with before compression, the increment of pore aspect ratio first decreases and then increases with the increase of NaCl concentration, while the increment of particle aspect ratio first increases and then decreases, all of which reach the maximum value at 2.5% NaCl concentration, which shows that when the NaCl concentration is less than 2.5%, the pore water salinity promotes the development of soil particles to round shape under compression, and the pores develop to long and narrow shape under compression, showing a reverse evolution after exceeding the NaCl concentration threshold of 2.5%. Fractal dimension of shape is one of the important parameters to comprehensively characterize the regularity of internal pores or particles and the complexity of external contour of soil, and its formula (2) is as follows:  
\[\lg P=\frac{D}{2}\cdot\lg A+C \tag{2}\]  
\(P\)-equivalent perimeter of pores or particles; \(A\)-the area of soil particles or pores; \(C\)-constant; \(D\)-fractal dimension of the shape of pores or particles.