## 2 Soil Sample and Test Scheme
### One-Dimensional Compression Test Plan  
To investigate the compression characteristics of marine soil under varying salinity conditions, a series of one-dimensional compression tests were performed on samples prepared with NaCl solutions at concentrations of 0% (deionized water), 1.0%, 2.5%, 3.5%, and 5.0%.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Whole Rock Mineral** & **Content (\%)** & **Clay Mineral** & **Content (\%)** \\ \hline Quartz & 52.2 & I/S & 62 \\ Potassium feldspar & 1.8 & It & 26 \\ Plagioclase & 10.5 & Kao & 5 \\ Hornblende & 0.7 & C & 7 \\ Clay mineral & 34.8 & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mineral composition of whole rock and clay in soil sample.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Anion** & \
\begin{tabular}{c} **Content** \\ **(mg.** L\({}^{-1}\)**)** \\ \end{tabular} & **Cation** &
\begin{tabular}{c} **Content** \\ **(mg.** L\({}^{-1}\)**)** \\ \end{tabular} & **S Salinity** & **pH Value** \\ \hline \(F^{-}\) & 1.41 & Li\({}^{+}\) & 0.34 & \\ Cl\({}^{-}\) & 13,416.62 & Na\({}^{+}\) & 8502.19 & \\ Br\({}^{-}\) & 10.80 & NH\({}_{4}\)\({}^{+}\) & 0 & \\ NO\({}_{3}\)\({}^{-}\) & 4.48 & K\({}^{+}\) & 354.47 & 34,120 & 8.13 \\ SO\({}_{4}\)\({}^{2-}\) & 16.4 & Mg\({}^{2+}\) & 1022.2 & \\  & & Ca\({}^{2+}\) & 362.38 & \\ Total & 13,449.71 & Total & 10,241.57 & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Composition of the sea water.  
Figure 2: X-ray diffraction pattern of the soil samples. (**a**) X-ray diffraction pattern of whole rock non-clay minerals; (**b**) X-ray diffraction pattern of clay minerals.  
The reconstituted samples were prepared to replicate the in-situ soil conditions, matching both the natural moisture content (w = 23.74%) and wet density (\(\rho\) = 1.93 g/cm\({}^{3}\)). The specimens were formed in a standard cutting ring (61.8 mm diameter \(\times\) 20 mm height). Saturation was achieved through vacuum saturation at controlled NaCl concentrations, with a minimum saturation duration of 8 h to ensure complete pore fluid replacement. Following saturation, all specimens underwent ionic equilibration in their respective salt solutions for a minimum 7-day period [3;7], with solution renewal every 48 h to maintain constant concentration. This protocol ensured both chemical equilibrium and microstructural stability prior to mechanical testing.  
The one-dimensional compression test on reconstituted marine soil was conducted using an STK.YJZQG16-1 dual-chamber pneumatic oedometer (Figure 3). This system applies vertical loads to the soil specimen by regulating air pressure via an automated control system while simultaneously recording displacement data. The oedometer operates within a loading range of 0-1600 kPa, with a relative error of less than 1.0% and an accuracy of 0.3% of full scale (F.S.). The instrument is factory-certified and undergoes annual calibration to ensure measurement reliability.  
During the test, a loading ratio of 1.0 was maintained, initial loading from 12.5 kPa to 800 kPa, followed by unloading to the initial value of 12.5 kPa. Secondary compression was then conducted, involving gradual loading to 1600 kPa before final unloading. The stable compression time of each load is 24 h, and the specific loading scheme is shown in Figure 4.  
Figure 4: Loading scheme of one-dimensional compression test.  
Figure 3: STK.YJZQG16-1 dual-chamber pneumatic oedometer. (**a**) Pneumatic oedometer system; (**b**) Pressure regulation console of STK.YJZQG16-1; (**c**) Dual-chamber of STK.YJZQG16-1.