## 4 SEM Scanning Test Results and Analysis
### Effect of NaCl Concentration on Micro-Arrangement  
Anisotropy describes the isotropic identity of pores or particle units, and its formula is shown in (3):  
\[I_{n}=\frac{L-B}{L}\times 100\% \tag{3}\]  
\(L\) is the longest axis of particles or pores; \(B\) is the shortest axis of a particle or pore.  
According to Formula (3), when the anisotropy is 0, it indicates that the particles or pores are randomly distributed, that is, isotropic. A value of 100% indicates that the particles or pores are distributed in the same direction, that is, completely anisotropic.  
Figure 16: Variations in average aspect ratio and its increment for pores and particles with NaCl concentration. (**a**) Average pore aspect ratio, standard deviation and increment after compression; (**b**) Average particle aspect ratio, standard deviation and increment after compression.  
Figure 17: Variation curves of pore and particle shape fractal dimensions and standard deviation with NaCl Concentration.  
Figure 18 shows the variations in anisotropy and its increment for pores and particles with NaCl concentration.  
It can be seen from the figure that the anisotropy of pores and particles in pre-compression samples first decrease and then increase with the increase of NaCl concentration, and the minimum values are 43% and 40% at 2.5% NaCl concentration, respectively, indicating that they tend to be isotropic under the condition of 2.5%NaCl. After compression, the pore increment decreases with the increase of NaCl concentration, while the particles increase, indicating that NaCl promotes the co-directional arrangement trend of particles when soil is pressed, and inhibits the co-directional arrangement trend of pores.  
Directional probability entropy \(H_{m}\) is introduced to describe the degree of directional arrangement of pores. The larger the value, the worse the arrangement order, and the smaller the probability entropy, the more regular the arrangement, the stronger the order and orientation, and the more stable the structure. Divide 0-180 into \(n\) locations, and the calculation formula is shown in Formula (4):  
\[H_{m}=-\sum_{i=1}^{n}\frac{m_{i}}{M}\cdot\frac{\ln(\frac{m_{i}}{M})}{\ln(n)} \tag{4}\]  
\(m_{i}\) is the \(i\) number of particles or pores with the long axis direction within the range of 0-180; \(M\) is the total number of particles or pores.  
Figure 19 shows the relationship between the directional probability entropy of pores and particles and the change of NaCl concentration. As can be seen from the figure, the orientation probability entropy value of pores and particles in soil samples without the influence of salt is close to 1, indicating that the directions of pores and particles are highly random; With the increase of NaCl concentration, the \(H_{m}\) numerical values of pores and particles in soil samples decreased, and the \(H_{m}\) numerical variation range of particles was nearly 18 times that of pores, and the order and orientation of pore and particle arrangement increased, reaching the minimum value at 2.5% concentration. After 2.5% NaCl concentration, with the increase of salt concentration in pore water, the porosity and particles of soil samples all rebounded, the \(H_{m}\) of pores and particles weakened, and the arrangement became more and more chaotic. It can be shown that the change of NaCl concentration in pore water mainly affects the arrangement of soil skeleton particles.  
Figure 18: Variations in anisotropy and its increment for pores and particles with NaCl concentration. (**a**) Anisotropy, standard deviation and increment after compression of pore; (**b**) Anisotropy, standard deviation and increment after compression of particle.