## 2 Unsaturated Subgrade Filler Triaxial Testing System
### Analysis of Experimental Results  
#### 3.4.1 Sample Volume Change  
The three-dimensional coordinates of 120 measurement targets on the sample surface were obtained by capturing images at various loading levels. A triangular mesh was then constructed by connecting adjacent points, reflecting the shape changes of the sample under different loading conditions. The deformation during the shear process was analyzed using sample 1 from Group 1 (moisture content: 15.89%) and sample 1 from Group 2 (moisture content: 15.90%), as shown in Figure 7. Figure 7,b illustrate the deformation of the two samples under net confining pressures of 5 kPa and 200 kPa and at different axial displacements. As seen in Figure 7, under a net confining pressure of 5 kPa, the sample largely maintained its cylindrical shape as the axial displacement reached 3.5 mm. As displacement increased, a distinct shear band formed, and the sample eventually failed under the applied deviatoric load. In contrast, Figure 7 shows that when samples with similar moisture contents were sheared under a 200 kPa net confining pressure, barrel-shaped deformation occurred without any observable failure surface as the axial displacement increased. At this stage, the diameter at the center of the sample was largest and tapered toward the ends. This deformation was caused by the friction between the soil and the loading platen, which restricted deformation at the ends. Additionally, the higher confining pressure effectively suppressed shear band formation. The experimental results are consistent with previous studies [11; 41; 42]. Figure 7: Deformation of the specimen under different axial displacement conditions. Notes: **(a)**\(\sigma_{3}=5\) kPa, **(b)**\(\sigma_{3}=200\) kPa. The volumes of the soil samples were measured under various loading conditions using the triangular mesh method. The total weight, moisture content, and soil density were measured, and the specific volume of each sample was subsequently calculated. Figure 8 illustrates the changes in the specific volume of samples from sets 2 and 3 under various isotropic pressures and moisture contents. The results indicate that as the moisture content of the samples increases, their initial specific volume also increases. At low moisture content, there is less water within the sample, resulting in greater friction between soil particles and a smaller specific volume. As moisture content increases, the lubricating effect of water reduces friction between particles, causing the sample to expand and the specific volume to increase. As shown in Figure 8a, when the total confining pressure is below 200 kPa, particularly below 50 kPa, the specific volume curve is flat, indicating that the sample's deformation is limited and remains stable. As moisture content increases, the specific volume rises slightly, with smaller changes observed, especially at medium to low moisture contents. However, when the total confining pressure approaches 100 kPa, the steepness of the specific volume curve increases, indicating significant plastic deformation of the sample. Under high moisture content conditions, the lubricating effect of water reduces friction between soil particles, leading to a more pronounced increase in the specific volume. As shown in Figure 8b, compared to 200 kPa, the confining pressure of 600 kPa has a more pronounced impact on the specific volume. As confining pressure increases, especially under high moisture content, the specific volume of the sample increases significantly. At higher confining pressures, the sample undergoes more plastic deformation, with moisture reducing interaction between soil particles, making the sample more susceptible to large-scale deformation. Under 600 kPa confining pressure, the specific volume curve becomes steeper, and the plastic deformation of the sample is more pronounced.