## 3 Experimental Results and Analysis
### Effect of FT Cycles and Confining Pressure on the Mechanical Properties of Specimens
When the soil undergoes shear failure, its stress-strain curve typically consists of five phases: compaction, elastic deformation, plastic deformation, strain softening, and instability failure. In the initial phase of the experiment, strain increases linearly and rapidly. Once the strain exceeds 1%, the curve enters a nonlinear stage, and the slope gradually decreases. Compared to finer-grained soils with higher cohesion, the elastic deformation phase of coarse-grained soil from the southeastern Xizangan Plateau primarily depends on the interlocking and static friction between soil particles, rather than cementation between agglomerates. Therefore, the elastic deformation phase is relatively short, quickly transitioning into the plastic deformation phase, where shear-induced weak zones and the interlocking and friction between soil particles (both static and dynamic friction) bear the applied stresses. As shown in Figures 4-6, under identical experimental conditions, as the number of FT cycles increases, the slope of the stress-strain curve and the peak stress gradually decrease. For example, in Figure 4, the degree of peak stress reduction is as follows: 8.41% (from zero to one FT cycle), 8.19% (from one to three cycles), 6.83% (from  
Figure 5: Stress–strain curves of specimens with different FT cycles and confining pressures (moisture content w = 9.6%). (**a**) \(\sigma_{3}=50\) kPa, (**b**) \(\sigma_{3}=100\) kPa, (**c**) \(\sigma_{3}=150\) kPa, and (**d**) \(\sigma_{3}=200\) kPa. three to six cycles), 8.41% (from six to ten cycles), and 4.47% (from ten to fifteen cycles). As confining pressure increases, both the initial slope and peak stress of the curve gradually increase, with peak stress rising by 67.35% (from 50 kPa to 100 kPa), 32.87% (from 100 kPa to 150 kPa), and 26.41% (from 150 kPa to 200 kPa). The gradual decrease in the slope of the curve as the FT cycles increase is partly due to the volume expansion of pore water during freezing, which enlarges the pores in the soil, increasing the distance between particles and making it easier for pore compaction and plastic deformation to occur during shearing. During the melting phase, the pore water becomes liquid, and due to factors such as gravity, the water migrates downward, causing redistribution. This results in a higher moisture content in the lower part of the sample, reducing the effective stress at the upper part of the sample during shearing. The excess moisture in the lower part further damages the soil during the next freezing cycle. Additionally, FT cycles cause larger soil particles to break and smaller particles to agglomerate, reducing the soil's homogeneity and affecting its physical and mechanical properties. The reduction in peak stress is mainly due to the accumulation of irreversible structural damage caused by FT cycles, which decreases the soil's resistance to deformation and failure [27]. After 10 FT cycles, the deformation and failure characteristics of the sample gradually stabilize. The increase in slope and peak stress as confining pressure increases is primarily due to the reduction in pore size between soil particles, increasing contact points, making the soil denser, and improving its resistance to deformation and failure. Figure 6: Stress–strain curves of specimens with different FT cycles and confining pressures (moisture content w = 11.6%). **(a)**\(\sigma\)3 = 50 kPa, **(b)**\(\sigma\)3 = 100 kPa, **(c)**\(\sigma\)3 = 150 kPa, and **(d)**\(\sigma\)3 = 200 kPa. The stress-strain curves of the samples all exhibit strain softening behavior, with the peak value of the curve being taken as the shear strength of the soil sample.