## 3 One-Dimensional Compression Test Results and Analysis
### Effect of NaCl Concentration on Compressibility Index  
Coefficient of compression \(\alpha_{1-2}\) at compression pressure of 100-200 kPa can directly reflect soil compressibility. Figure 7 shows the relationship between \(\alpha_{1-2}\) and NaCl concentration. It can be seen from the figure that in the first loading stage, the compressibility of soil samples basically increases first and then decreases with the increase of NaCl concentration, and reaches the peak value of 0.609 MPa\({}^{-1}\) when NaCl concentration is 2.5%, showing high compressibility, while the compressibility is 0.35-0.45 MPa\({}^{-1}\) at other NaCl concentrations, showing medium compressibility. In the second reloading stage, the compressibility of soil samples is not affected by NaCl concentration, and it is stable at 0.1 MPa\({}^{-1}\). Pore water in the soil is discharged with the deformation of the soil, and the influence of salt in the pores on the compressibility of the soil is obviously weakened. Compression index and rebound index are important parameters to describe the volume change characteristics of soil during loading and unloading. Figure 8 shows Variation curves of compression index and rebound index of soil samples with NaCl concentration. It can be seen from the figure that the compression index in the loading stage is similar to the compression coefficient, while the rebound index does not change with NaCl concentration, indicating that the rebound of compressed soil is not affected by the salt in pore water. Figure 6: \(e\)-\(\log p\) relation curve of soil sample’s twice compression resilience. Figure 7: Variation curve of the coefficient of compression and standard deviation of soil samples varies with NaCl concentration. Figure 9 shows the relationship curve between consolidation coefficient and NaCl concentration of soil samples. It can be seen from Figure 9a that in the first loading stage, with the increase of NaCl concentration, the consolidation coefficient of soil samples first decreases and then increases. When NaCl concentration is 2.5%, the consolidation coefficient of soil samples is the smallest, and with the increase of compression pressure, the consolidation coefficient of soil samples changes more obviously. NaCl concentration in pore water of soil samples will affect the compression process of marine sedimentary soil. It can be seen from Figure 9b that the consolidation coefficient of soil samples is less affected by NaCl concentration at different compression pressures in the secondary loading stage, and reaches the maximum value of 6.5 \(\times\) 10\({}^{-4}\)-7.5\(\times\)10\({}^{-4}\) m\({}^{2}\)/s at the compression pressure of 200 kPa, which reflects that the compression speed of soil is the fastest at the compression pressure of 200 kPa. For marine soil, if the NaCl concentration of pore water is not considered, the characteristic parameters of consolidation coefficient obtained by one-dimensional compression test are unsafe for practical engineering calculation. Due to the instability of actual seawater composition, the compression characteristics of marine sedimentary soil with 2.5% NaCl salt sensitivity should also be considered. compression pressures of 100 kPa and 1000 kPa. As can be seen from Figure 10, with the increase of pore water concentration, \(C_{c}^{*}\) first increases and then decreases, while \(e_{100}^{*}\) first decreases and then increases, both of which reach the maximum at 2.5% NaCl concentration. Void index \(I_{\
u}\) is an important parameter to describe the change behavior of compression volume under different stresses \(p\) during soil compression. Normalize the void index \(I_{\
u}\) under different compression pressures, and get the Equation (1). Interestingly, as shown in Figure 11, the reconstituted marine sedimentary soil with different salt concentrations can finally fall on this curve after the compression pressure is greater than 100 kPa.