## 3 Experiment Results and Analysis  
The relational curves between principal-stress deviation (\(\sigma_{1}-\sigma_{3}\)) and axial strain \(\varepsilon_{1}\) of samples with 0.5% salt content are shown in Figure 1. Because the laws of stress-strain curve affected by FTCs were similar under different S, the stress-strain curve with 0.5% salt content is illustrated here. As shown in Figure 1, the principal stress deviation (\(\sigma_{1}-\sigma_{3}\)) had nonlinear growth with the increase of axial strain \(\varepsilon_{1}\). When \(\sigma_{3}\) was the same, the (\(\sigma_{1}-\sigma_{3}\)) -\(\varepsilon_{1}\) curve-variation tendency of the samples with different FTC was similar. All (\(\sigma_{1}-\sigma_{3}\)) -\(\varepsilon_{1}\) curves were of the strain-hardening type, and \begin{table}
\begin{tabular}{c c} \hline \hline
**Properties** & **Experiment Setting** \\ \hline Water content & 22\% \\ Compaction degree & 90\% \\ NaHCO\({}_{3}\):Na\({}_{2}\)SO\({}_{4}\) & 1:1 \\ Salt content & 0\%, 0.5\%, 1\%, 2\%, 3\% \\ Freeze-thaw cycles & 0, 1, 5, 10, 30, 60, 120 \\ Temperature range of each freeze-thaw cycle & \(-20\)-\(0\)\({}^{\circ}\)C \\ Duration of each freeze-thaw cycle & Freezing for 12 h and thawing for 12 h. \\ Confining pressure & 100, 200, 300 kPa \\ Sample size & 105 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sample properties and design of freeze-thaw cycle test. the shape of curves was hyperbolic. The stress-strain curve could be divided into three stages: the quasi-elastic stage, the elastic-plastic deformation stage, and the stable stage. In the repeated process of freezing and thawing, salt expansion, pore water migration, particle breaking and rearranging, and fracture development, gradually changed the force between soil particles, affecting the macromechanical behavior of saline soil [2; 28; 29]. The initial yield point of the samples decreased with more FTCs (Figure 1). The samples that underwent more FTCs were easier to damage, and their strength was lower. For strain-hardening soil, there is no peak on the stress-strain curves, so principal-stress deviation when the strain value is 15% is usually defined as the shear strength of Nong'an saline soil. It was recorded as \((\sigma_{1}-\sigma_{3})_{f}\) and called failure principal-stress difference, which is summarized in Table 2. \((\sigma_{1}-\sigma_{3})_{f}\) of soil varied with FTC change, as shown in Figure 2. The following exponential function was used for curve fitting:  
Figure 1: Stress-strain curves of samples with 0.5% salt content. (**a**) \(\sigma_{3}=100\) kPa; (**b**) \(\sigma_{3}=200\) kPa; (**c**) \(\sigma_{3}=300\) kPa. \[y=\mathrm{A_{1}}*\exp(\mathrm{B_{1}}*x)+\mathrm{C_{1}}*\exp(\mathrm{D_{1}}*x), \tag{1}\]  
where \(x\) is the FTC number that the samples underwent, and \(\mathrm{A_{1}}\), \(\mathrm{B_{1}}\), \(\mathrm{C_{1}}\), and \(\mathrm{D_{1}}\) are the fitting parameters. Fitting results are shown in Figure 2, and parameter values are shown in Table 3.  
The \((\sigma_{1}-\sigma_{3})_{f}\) of the samples with different S decreased rapidly within 10 cycles, especially about 31.22%-36.42% after the first cycle. At this stage, the structure of Nong'an saline soil was damaged. Then, it slowly declined between 10 and 60 cycles, and it declined faster after samples underwent 120 cycles (24.85%-57.13%). During the first freezing, water in the soil became ice and the volume expanded, which made small particles agglomerate and the aggregates separated from each other. Moreover, pore volume in the soil increased. When the ice melted, aggregate changes and the pore volume in the soil could not recover completely during the thawing process. After the first cycle, the structure of Nong'an saline soil was seriously damaged and its strength decreased rapidly. After 5 and 10 cycles, the micro and small pores in the soil gradually became medium and large pores, large particles or aggregates were separated, and small particles were reunited; finally, the soil entered