## 3 Experimental Results and Analysis
### Effect of FT Cycles and Confining Pressure on the Mechanical Properties of Specimens
Based on the results of triaxial tests under different water contents and confining pressures, the Mohr stress circles and failure envelope for coarse-grained soil samples from the southeastern Xizangan Plateau were plotted. The shear strength parameters, cohesion (c), and internal friction angle (\(\varphi\)) were calculated using the Mohr-Coulomb failure criterion and are presented in Figure 4.2  
As shown in Figure 13, the cohesion and internal friction angle of the coarse-grained soil samples from the southeastern Xizangan Plateau decrease to varying degrees as the water content increases from 7.6% to 11.6%. The reduction in cohesion is more pronounced compared to the internal friction angle. At a water content of 7.6%, the cohesion of the soil ranges from 20.99 kPa (FT = 15) to 33.92 kPa (FT = 0), with an average of 24.25 kPa. At a water content of 9.6%, the cohesion ranges from 14.34 kPa (FT = 15) to 25.50 kPa (FT = 0), with an average of 19.27 kPa. At a water content of 11.6%, the cohesion ranges from 9.08 kPa (FT = 15) to 20.17 kPa (FT = 0), with an average of 13.65 kPa. The reduction in cohesion from a water content of 7.6% to 11.6% is between 40.54% and 60.28%, indicating a significant deterioration in cohesion after FT cycles. For the internal friction angle, at a water content of 7.6%, it ranges from 18.02\({}^{\circ}\) (FT = 15) to 25.43\({}^{\circ}\) (FT = 0), with an average of 19.51\({}^{\circ}\). At a water content of 9.6%, the internal friction angle ranges from 16.14\({}^{\circ}\) (FT = 10) to 24.56\({}^{\circ}\) (FT = 0), with an average of 18.66\({}^{\circ}\). At a water content of 11.6%, the internal friction angle ranges from 16.00\({}^{\circ}\) (FT = 10) to 23.49\({}^{\circ}\) (FT = 0), with an average of 18.17\({}^{\circ}\). The reduction in the internal friction angle from a water content of 7.6% to 11.6% ranges from 7.63% to 18.62%. Figure 4.3 Relationship between cohesion (c), the internal friction angle (\(\varphi\)), and water content. (**a**) Cohesion c and (**b**) the internal friction angle \(\varphi\). The deterioration of cohesion and internal friction angles with increasing water content in the coarse-grained soil from the southeastern Xizangan Plateau can be attributed to the fact that a water content of 7.6% corresponds to the optimal water content. At this point, the soil particles are more tightly arranged, and their interlocking is stronger, resulting in fewer pores and cracks. As a result, the coarse-grained soil exhibits higher apparent cohesion and internal friction angles. However, as the water content increases, the soil particles become more loosely arranged, with weaker interlocking and a reduced overall structure. This leads to an increase in pores and cracks, significantly reducing the apparent cohesion and internal friction angle. Additionally, the increased water content causes the finer particles, which are present in small amounts, to become muddied and lose their cohesive properties, further decreasing cohesion. The muddy fine particles, along with water, act as lubricants during the deformation and failure process, reducing the friction between soil particles, which further decreases the internal friction angle.