## 3 Experimental Results and Analysis  
### Effect of FT Cycles and Confining Pressure on the Mechanical Properties of Specimens  
The stress-strain curves of coarse-grained soil from southeastern Xizang were plotted based on the results of unconsolidated undrained (UU) triaxial shear tests. These curves illustrate the effects of different confining pressures and FT cycles at moisture contents of 7.6%, 9.6%, and 11.6%, as shown in Figures 4-6. In conventional triaxial tests, the relationship between deviator stress (\(\sigma_{1}\)-\(\sigma_{3}\)) and axial strain (\(\epsilon\)) typically exhibits either strain softening or strain hardening behavior. In this study, all specimens displayed strain softening characteristics, where the deviator stress increased to a peak value and then gradually decreased with further axial strain. The relatively high apparent cohesion observed (20-35 kPa) may be attributed to the presence of fines, moisture-induced suction, and initial static densification during sample preparation. Similar magnitudes have been reported for weakly cemented or well-graded coarse-fine mixtures in seasonally frozen soils. Figure 4 shows the shear process of coarse-grained soil samples from the southeastern Xizangan Plateau with a moisture content of 7.6% under different confining pressures and  
Figure 3: Testing equipment. (**a**) Surface vibration compactor, (**b**) constant temperature and humidity chamber, and (**c**) strain-controlled triaxial apparatus. FT cycles, specifically zero, one, three, six, ten, and fifteen cycles. All the samples exhibited strain softening behavior, with the degree of strain softening increasing as the number of FT cycles increased and decreasing with higher confining pressures. Figures 5 and 6 display the shear process of the same coarse-grained soil samples with moisture contents of 9.6% and 11.6%, respectively, under varying confining pressures and FT cycles. The strain softening trend observed is consistent with that in Figure 4. In Figures 4-6, under identical experimental conditions, the average strain softening degree generally increased with the number of FT cycles from 0 to 15, as follows: 11.59% (FT = 0), 25.51% (FT = 1), 24.98% (FT = 3), 28.95% (FT = 6), 31.10% (FT = 10), and 31.21% (FT = 15). As confining pressure increased from 50 kPa to 200 kPa, the average strain softening degree gradually decreased, with values of 27.66% (\(\sigma_{3}\) = 50 kPa), 26.22% (\(\sigma_{3}\) = 100 kPa), 24.38% (\(\sigma_{3}\) = 150 kPa), and 23.31% (\(\sigma_{3}\) = 200 kPa). It is clear that compared to the samples that did not undergo FT cycles, those that did showed varying degrees of increased strain softening. After just one FT cycle, the samples exhibited significant strain softening. This is mainly due to the phase change of water during freezing, which causes an expansion in volume, weakening the connections and interlocking between soil particles. Additionally, pore spaces and micro-cracks expand, leading to structural damage to the soil, which cannot fully recover during the subsequent melting of the water. This increases the difficulty of reorienting the soil particles and reduces the residual strength, thereby increasing strain softening. With more FT cycles, the repeated phase change of water gradually deepens the  
Figure 4: Stress–strain curves of specimens with different FT cycles and confining pressures (moisture content w = 7.6%). (**a**) \(\sigma_{3}\) = 50 kPa, (**b**) \(\sigma_{3}\) = 100 kPa, (**c**) \(\sigma_{3}\) = 150 kPa, and (**d**) \(\sigma_{3}\) = 200 kPa. damage to the soil. After 10 cycles, however, since the moisture content is fixed, the total amount of ice-water phase changes is limited, resulting in only minor structural and pore changes in the soil. The internal structure of the soil reaches a new balance, and the effect of FT cycles on the sample stabilizes, causing strain softening to level off [26].