## 2 Unsaturated Subgrade Filler Triaxial Testing System
### Analysis of Experimental Results
#### 3.4.1 Sample Volume Change
Under identical loading history conditions, samples with higher moisture content show a larger reduction in specific volume, especially under 600 kPa confining pressure, at which the lubricating effect of water causes significant deformation during loading. Compared to 200 kPa, the change in specific volume under 600 kPa is more pronounced, especially under higher moisture conditions, where the volume compression effect is more evident. In Figure 8b, the change in specific volume during unloading under 600 kPa confining pressure is similar to that during loading, with more noticeable changes observed under high moisture content conditions, indicating that moisture continuously affects the deformation recovery ability of the sample. As the moisture content increases, the elasticity of the sample decreases, indicating that moisture has a long-term impact on plastic deformation. The yield stress of each sample is determined using the Casagrande method for further analysis. The specific volume changes of the samples from sets 1 and 2 during the shear process under 5 kPa and 200 kPa net confining pressures are shown in Figure 9a,b. Figure 9a shows that the samples exhibit similar trends under the lower confining pressure of 5 kPa. At approximately 1% axial strain, shear compression occurs in the sample. As the axial strain increases, the influence of moisture becomes evident, causing the sample to undergo shear expansion. This suggests that under low confining pressure, the strain of the samples is more flexible, and the influence of moisture on deformation is significant, leading to alternating expansion and compression during shear. Under the 5 kPa confining pressure, the sample predominantly experiences shear compression at low strains, with shear expansion occurring at higher strains. Figure 9b illustrates that under the higher net confining pressure of 200 kPa, the trend of specific volume change becomes more complex. When the initial moisture content exceeds 13.41%, the sample first undergoes shear compression as axial strain or deviatoric stress increases. Compared to the 5 kPa confining pressure, shear compression is more pronounced and persistent under higher pressures, especially when moisture content is elevated, where the effect of moisture leads to distinct shear compression characteristics at high strains. However, when the initial moisture content is below 13.41%, the sample exhibits different behavior, first undergoing shear compression and then experiencing shear expansion at higher strains. This indicates that at low moisture content, the deformation mode is more complex, with insufficient moisture causing shear compression at low strains, followed by a gradual transition to shear expansion as strain increases. In summary, moisture content significantly affects the specific volume change in unsaturated soil. Under 5 kPa confining pressure, the specific volume change is relatively gradual, and samples with higher moisture content are more prone to shear expansion. Under the higher 200 kPa confining pressure, samples with moisture content exceeding 13.41% exhibit more pronounced shear compression, while those with moisture content below 13.41% first undergo compression followed by expansion. Under higher confining pressure, the effect of moisture content on deformation becomes more complex, possibly due to the distribution of moisture within the sample and changes in pore pressure. Figure 8: Comparison of the specific volume of samples under different isotropic pressure and water content conditions. Notes: (**a**) A load of 200 kPa (Set 2), (**b**) a load of 600 kPa (Set 3). Figure 9: Comparison of the variation in the specific volume of the sample during the triaxial shearing process. Notes: (**a**) \(\sigma_{3}=5\) kPa (Set 1), (**b**) \(\sigma_{3}=200\) kPa (Set 2). Figure 10 shows the fitted surface of deviatoric stress, axial strain, and moisture content for samples from sets 1 and 2, which were tested under 5 kPa and 200 kPa net confining pressures.