## 2 Unsaturated Subgrade Filler Triaxial Testing System
### Analysis of Experimental Results
#### 3.4.2 Analysis of Suction Variation  
Figure 11 illustrates the suction response of the fourth sample from the second group during both the isotropic and shear loading processes. As the confining pressure increases, the suction decreases immediately, indicating that the pore water in the sample is influenced by the confining pressure, resulting in reduced suction during isotropic loading. Over time, suction gradually recovers and approaches equilibrium, which is consistent with trends observed in previous studies [33; 43; 44; 45; 46; 47; 48]. This suggests that suction variation in the sample under isotropic loading follows a regular pattern, where suction decreases and gradually recovers to equilibrium, reflecting adjustments in soil structure and water distribution. When a 200 kPa confining pressure is applied during triaxial shear, suction variation differs from that during isotropic loading. Initially, the applied deviatoric stress causes an immediate decrease in suction. Subsequently, under constant loading, suction continues to decrease slightly before eventually reaching equilibrium. Unlike isotropic loading, suction variation during triaxial shear is more complex, with a larger decrease in suction, likely due to soil deformation and pore water movement under deviatoric stress. As shear loading continues, the suction of the sample gradually stabilizes, indicating that the shear process has a more significant impact on soil moisture. During unloading, suction variation exhibits a clear rebound phenomenon. During initial unloading, suction increases rapidly, then gradually decreases, ultimately reaching a new equilibrium state. This process indicates that during unloading, the redistribution of water inside the sample temporarily increases suction. After unloading adjustment, suction gradually returns to equilibrium. Similar to the loading and shear processes, suction recovery after unloading also requires a certain amount of time, typically between 5 and 20 min, depending on the initial suction. Figure 10: Deviator stress–axial strain relationship during the triaxial shearing process. Notes: (**a**) \(\sigma_{3}=5\) kPa (Set 1), (**b**) \(\sigma_{3}=200\) kPa (Set 2). Suction in the equilibrium state is considered the representative value for each loading level. Figure 12a,b show the variation in suction of the samples from Set 2 and Set 3 during isotropic loading. Figure 12 shows that the suction of the samples decreases significantly as the isotropic load increases. This indicates that with increasing confining pressure during compression, the pore water pressure also increases, resulting in a decrease in suction. The magnitude of the suction decrease is closely related to the moisture content and initial suction of the samples. Samples with lower moisture content generally exhibit higher initial suction and experience a more significant decrease in suction during loading. As shown in Figure 12a, the fifth sample from Set 2 (w = 11.83%) and the sixth sample from Set 3 (\(w\) = 11.83%) have higher initial suction and display larger variations in suction under the same load. This suggests that samples with lower moisture content exhibit stronger capillary suction, which results in a more significant suction decrease during loading. In contrast, as the moisture content increases, the suction decrease becomes less pronounced, especially when the moisture content reaches 15.57%, at which point the suction variation stabilizes with almost no significant change. As shown in Figure 12b, irreversible suction changes occur during unloading under an average net stress of 300 kPa. With increasing moisture content, this irreversible suction change gradually decreases. Particularly at higher moisture contents, this change becomes almost imperceptible and is closely related to the stress history and moisture changes of the sample. Irreversible suction changes suggest that the microstructure of the sample undergoes irreversible changes during loading and unloading.