## 4 Conclusions  
Based on the experimental data and analysis of the triaxial testing system developed in this study, the following key findings and conclusions are drawn:  
1. The triaxial testing system developed in this study integrates high-suction tensiometer technology with digital image correlation (DIC), enabling the precise 3D reconstruction of sample models through non-contact photogrammetry. It effectively monitors the volume and suction changes of unsaturated subgrade fill samples. Compared with typical methods, which require several days to 24 h for the complete experiment, this system has reduced the experimental time by reducing the total test time to 6 h. This time reduction was achieved without compromising measurement accuracy, as demonstrated by side-by-side direct documentation using the same observation and validation methods as traditional studies. The time reduction represents a notable improvement in the testing process's efficiency. 2. As the moisture content increases, the specific volume of the sample gradually increases. When the confining pressure is below 200 kPa, particularly at 50 kPa, the changes in specific volume are smaller, with limited deformation, and the changes remain relatively minor at medium and low moisture content. As the confining pressure approaches 100 kPa, the specific volume curve steepens, and the sample undergoes significant plastic deformation. Under a confining pressure of 600 kPa, the effect of moisture becomes more pronounced, and the specific volume increases substantially,especially under high moisture content conditions, with more noticeable deformation. At low moisture content, the sample exhibits shear compression, whereas under high moisture conditions, the lubricating effect of water reduces particle friction, leading to shear expansion. 3. During isotropic loading, an increase in confining pressure causes a rapid decrease in suction. When a confining pressure of 200 kPa is applied, the suction decreases to about 420 kPa and then slowly recovers to equilibrium, indicating an adjustment in soil structure and moisture distribution. During triaxial shear loading, the decrease in suction is more pronounced. After applying deviatoric stress, the suction rapidly decreases to 380 kPa and ultimately stabilizes at equilibrium. Compared to isotropic loading, suction changes during shear loading are more complex, particularly under high moisture conditions. 4. In this study, the BBM model parameters were calibrated using triaxial test data. The results demonstrate that the model performs well in the medium to low stress range, especially when the mean net stress is 200 kPa, with only a 0.61% difference in specific volume between experimental and predicted values. However, in the high stress or high suction region, the prediction error increases, particularly when the stress is 400 kPa, with a difference of approximately 12.57%. As the mean net stress increases, the yield suction also increases, and there are significant differences in the slope and shape of the yield curves under different stress paths. The yield points in the high suction region are more concentrated, further highlighting the significant role of the stress path in yield behavior. Conceptualization, methodology, and software, Y.H. ; writing--original draft preparation, Y.H. ; funding acquisition and writing--review and editing, X.L. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Key Research and Development Program Project (Grant No. 2018 YFC1505305), the National Natural Science Foundation of China (Grant No. 51578195, 51608533), the Heilongjiang Province Applied Technology Research and Development Program (Grant No. GA19A501), and the 69 th Batch of General Projects of China Postdoctoral Science Foundation (Grant No. 2021M690840). Not applicable. Not applicable. Not applicable. Not applicable. Data Availability Statement: The original contributions presented in this study are included in the article.