## 5 Discussion
### Effect of NaCl Concentration on Particle Rearrangement  
The results of the one-dimensional compression tests and SEM scans indicate that NaCl concentration significantly influences the compressibility and microstructure of marine sedimentary soils. Previous studies have shown that the salinity of pore water plays a critical role in affecting the mechanical properties of marine soils, such as compressibility and particle arrangement. In particular, the significant increase in compressibility at a  
Figure 19: Bar Charts of directional probability entropy and Standard Deviation for Pores and Particles with NaCl Concentration. (**a**) Pore directional probability entropy and standard deviation; (**b**) Particle directional probability entropy and standard deviation. NaCl concentration of 2.5% can be attributed to the flocculation and particle rearrangement mechanisms that occur at this threshold. Interestingly, the study also reveals that the microstructural changes observed at a NaCl concentration of 2.5%, such as an increase in pore fractal dimension and particle anisotropy rate, correspond with the maximum compressibility. The dependent compressibility of clay on pore water NaCl concentration is due to the interplay between inter-particle forces and microstructural evolution. At low NaCl concentrations (<1.5%), clay particles maintain abundant unsaturated ion adsorption sites, allowing free interaction between electrolyte ions (Na\({}^{+}\) and Cl\({}^{-}\)) and particle surfaces. This condition promotes the development of an extended double electric layer (\(\sim\)10-100 nm thickness) where electrostatic repulsive forces dominate. As a result, particles maintain larger interparticle distances and form an open, anisotropic fabric characterized by predominant edge-to-edge and edge-to-surface contacts (Figure 20a). This loose microstructure exhibits high resistance to compression due to strong interparticle repulsion and structural anisotropy. As NaCl concentration increases into the intermediate range (1.5-3.5%), several concurrent changes occur in the clay-water system. The rising electrolyte concentration progressively saturates ion adsorption sites, leading to compression of the double layer (reduced to \(\sim\)1-10 nm thickness). This results in weakened electrostatic repulsion coupled with enhanced van der Waals attraction between particles. The changing force balance causes particle reorganization into denser configurations dominated by surface-to-surface contacts (Figure 20b). This microstructural transition significantly increases the soil's compressibility, reaching a maximum at the critical concentration of 2.5% NaCl, which represents an optimal balance between residual repulsive forces and growing attractive forces. At high NaCl concentrations (>3.5%), the system undergoes further transformation. The ion adsorption sites on particle surfaces become completely saturated, leaving insufficient negative charges to maintain stable inter-particle repulsion. With the near-complete collapse of the double layer (<1 nm thickness) and the virtual disappearance of electrostatic repulsion, attractive forces become overwhelmingly dominant. This condition triggers particle aggregation into flocculated structures primarily governed by edge-to-edge and edge-to-surface contacts (Figure 20c). Paradoxically, this microstructural reorganization leads to reduced compressibility as the rigid particle networks resist deformation more effectively than the transitional surface-to-surface configurations. The relationship between clay compressibility and NaCl concentration displays a distinct nonlinear characteristic, following a bell-shaped curve. The compressibility initially increases with rising NaCl concentration (0-2.5% range), peaks at the critical concentration of 2.5%, and subsequently decreases at higher concentrations (>2.5%).