## 3 Experimental Results and Analysis
### Effect of FT Cycles and Confining Pressure on the Mechanical Properties of Specimens
The decrease in peak stress is mainly due to water lubricating the particles and increasing pore water pressures, which makes the particle spacing larger and facilitates particle interlocking and sliding. As the confining pressure increases from 50 kPa to 200 kPa, the increase in the slope and peak stress is attributed to the compression of soil particles, closing or compacting pores and microcracks, and improving the soil's resistance to shear failure [30]. Moreover, the effect of confining pressure on peak stress is more pronounced than the effect of water content, especially at lower confining pressures (\(\sigma_{3}\) = 50 kPa) and near the optimal water content (w = 7.6%). The peak values of the stress-strain curves of the samples were taken as the shear strength of the soil. The relationship between shear strength, water content, and confining pressure was plotted, as shown in Figure 12 (only the curves after six and fifteen FT cycles are presented due to the large volume of experimental data). Under the same experimental conditions, for the samples that did not undergo FT cycles, as the water content increased from 9.6% to 11.6%, the shear strength decreased by 22.27% (\(\sigma_{3}\) = 50 kPa), 15.25% (\(\sigma_{3}\) = 100 kPa), 16.70% (\(\sigma_{3}\) = 150 kPa), and 11.79% (\(\sigma_{3}\) = 200 kPa), with an average decrease of 16.51%. When the number of FT cycles was six, the shear strength reduction as the water content increased from 9.6% to 11.6% was 23.49% (\(\sigma_{3}\) = 50 kPa), 18.71% (\(\sigma_{3}\) = 100 kPa), 16.14% (\(\sigma_{3}\) = 150 kPa), and 12.24% (\(\sigma_{3}\) = 200 kPa), with an average reduction of 17.64%. After 15 FT cycles, the shear strength decreased by 24.11% (\(\sigma_{3}\) = 50 kPa), 20.08% (\(\sigma_{3}\) = 100 kPa), 15.09% (\(\sigma_{3}\) = 150 kPa), and 13.02% (\(\sigma_{3}\) = 200 kPa), with an average reduction of 18.08%. It is evident that, whether or not the samples underwent FT cycles, the shear strength consistently decreased with increasing water content, and the rate of reduction increased with more FT cycles [31]. This is due to the fact that, as the water content increases, water hinders inter-particle interlocking and friction, making it easier for soil particles to dislocate, slide, and override, leading to failure. Additionally, the increase in pore water pressure during shear exacerbates the disturbance and damage to the sample's internal structure, making failure more likely. After FT cycles, the detrimental effects of water on the soil are further emphasized. Under the same experimental conditions, for the samples that did not undergo FT cycles, as the confining pressure decreased from 200 kPa to 50 kPa, the shear strength decreased by 61.96% (w = 7.6%), 66.08% (w = 9.6%), and 66.48% (w = 11.6%), with an  
Figure 4.1 Relationship between shear strength and water content under different numbers of FT cycles. (a) FT = 0, (b) FT = 6, and (c) FT = 15.  
average reduction of 64.84%. When the number of FT cycles was six, the shear strength decreased by 65.56% (w = 7.6%), 68.96% (w = 9.6%), and 69.98% (w = 11.6%), with an average reduction of 68.16%. After 15 FT cycles, the shear strength decreased by 67.21% (w = 7.6%), 71.05% (w = 9.6%), and 71.39% (w = 11.6%), with an average reduction of 69.88%. It is clear that regardless of whether the sample underwent FT cycles, shear strength increased with increasing confining pressure, and the extent of this increase grew with the number of FT cycles. This is because, as the confining pressure increases, the sample is compressed, reducing pores and cracks and resulting in the tighter interlocking of soil particles, which strengthens the sample's structure and makes failure less likely. Furthermore, the confining pressure enhances the stress state of the soil particles [32], increasing their resistance to sliding and overturning, thereby improving the soil's resistance to deformation and failure.