## 3 Experimental Results and Analysis
### Effect of FT Cycles and Confining Pressure on the Mechanical Properties of Specimens
The strain softening behavior follows the same trend as the samples without FT cycles, increasing with water content and decreasing with confining pressure. However, the samples subjected to FT cycles show a more significant increase in strain softening compared to those that were not. After six FT cycles, under unchanged experimental conditions, as the water content increases from 7.6% to 11.6%, the average strain softening degree becomes 25.60% (w = 7.6%), 28.86% (w = 9.6%), and 32.38% (w = 11.6%). As the confining pressure increases from 50 kPa to 200 kPa, the average strain softening degree decreases to 32.10% (\(\sigma_{3}\) = 50 kPa), 28.93% (\(\sigma_{3}\) = 100 kPa), 28.37% (\(\sigma_{3}\) = 150 kPa), and 26.38% (\(\sigma_{3}\) = 200 kPa). After 15 FT cycles, the average strain softening degree increases to 27.90% (w = 7.6%), 32.00% (w = 9.6%), and 33.74% (w = 11.6%) as the water content increases. The corresponding values for the confining pressures of 50 kPa, 100 kPa, 150 kPa, and 200 kPa are 32.05%, 34.08%, 29.21%, and 29.52%, respectively. It is evident that FT cycles significantly affect the strain softening degree, with this effect stabilizing as the number of cycles increases. Samples with higher water content are more susceptible to FT damage, resulting in greater strain softening. However, when the water content reaches a certain level, this effect becomes more stable. The increase in strain softening as the water content rises from 7.6% to 11.6% is due to higher pore water pressures in higher water content samples, which makes the soil structure more vulnerable to damage, facilitating the reorientation of soil particles. Additionally, the lubricating effect of water reduces friction between the rough surfaces of soil particles, leading to a decrease in residual strength. As the confining pressure increases from 50 kPa to 200 kPa, the strain softening degree decreases because higher confining pressures compress the internal pores and cracks of the soil, reducing the particle spacing and improving the compaction of the sample. This makes the soil more resistant to deformation and failure. Notably, the effect of confining pressure on strain softening is more pronounced than the effect of water content when no FT cycles are applied, while for samples subjected to FT cycles, the influence of water content is more significant [28]. Figure 9 shows that for the samples without FT cycles, under the same experimental conditions, as the water content increases, the slope of the initial part of the stress-strain curve and the peak deviator stress gradually decrease. The degree of peak stress reduction is 9.66% when the water content increases from 7.6% to 9.6%, and 7.55% when the water content increases from 9.6% to 11.6%. As the confining pressure increases, both the slope and peak stress increase, with peak stress rising by 65.11% (from 50 kPa to 100 kPa), 33.47% (from 100 kPa to 150 kPa), and 28.71% (from 150 kPa to 200 kPa). Figures 10 and 11 show similar trends for the samples subjected to FT cycles: the slope of the elastic phase and the peak deviator stress both decrease with increasing water content and increase with rising confining pressure. After six FT cycles, under the same experimental conditions, the peak stress reduction is 15.66% (from 7.6% to 9.6% water content) and 9.29% (from 9.6% to 11.6%) [29]. As the confining pressure increases, the peak stress increases by 86.14% (from 50 kPa to 100 kPa), 31.27% (from 100 kPa to 150 kPa), and 26.38% (from 150 kPa to 200 kPa). After 15 FT cycles, the peak stress reduction is 15.60% (from 7.6% to 9.6% water content) and 10.08% (from 9.6% to 11.6% water content). The peak stress increases by 89.65% (from 50 kPa to 100 kPa), 32.61% (from 100 kPa to 150 kPa), and 31.64% (from 150 kPa to 200 kPa). The reduction in the slope of the curve as the water content increases from 7.6% to 11.6% is primarily due to the lubricating effect of water, which reduces friction between soil particles, accelerating plastic deformation.