## 1 Introduction
Despite its shortcomings in measuring volume changes, the double-chamber method was noted by [PERSON] and [PERSON] (2020) [PERSON] and [PERSON] (2020) as a good approach for determining the mechanical behavior of unsaturated soils in constant water content triaxial tests. This was particularly true when suction was low because the shear strength of the soil sample varied with water content, which complicated the influence of suction in controlled tests. [PERSON] et al. (2020) [PERSON] et al. (2020) developed a new method that incorporated high-suction tensiometers with triaxial shear tests to measure the mechanical behavior of soil samples under different moisture contents. In their work, they note the merits of constant water content triaxial tests, which are a useful method for evaluating unsaturated soils. However, while constant water content triaxial tests can measure the stress-strain behavior of unsaturated soils, they found that [PERSON] and [PERSON] (2020) [PERSON] and [PERSON] (2020) suggested the analysis is more complicated than that of suction-controlled triaxial tests due to the improper analysis of suction. [PERSON] et al. (2020) [PERSON] et al. (2020) utilized a modified state surface approach (MSSA) to address the issues they encountered with suction-controlled tests on unsaturated soils by explaining the elastoplastic behavior of unsaturated soils under constant water content triaxial tests. They concluded that the MSSA was a good method for resolving complex issues, such as the relationship between shear strength and volume change encountered in constant water content triaxial tests. In this research, a 3D settlement and consolidation testing system for unsaturated subgrade fill is presented, which incorporates high tensiometers and digital image correlation (DIC) non-contact photogrammetry. The system uses DIC and non-contact photogrammetry to accurately reconstruct the 3D model of the unsaturated soil specimens throughout triaxial testing, as well as to monitor volume changes in real time as a result of the test. The system incorporates a high tensiometer to measure matric suction during triaxial testing, rather than controlling suction, which allows for a more realistic representation of the 3D deformation process of the unsaturated subgrade specimen and suction changes during testing. The system also allows for axial force and confining pressure to be applied under unconfined conditions, which is closer to how soil behaves in its natural state. This system of testing can greatly enhance the accuracy of experiments compared to traditional methods, with much shorter testing times. Furthermore, variations in specific volume, suction response, and main parameters during shear failure for different moisture contents and total confining pressures are investigated. In order to relate the mechanical behavior of the specimens, the BBM model is used to calibrate them. This study presents crucial experimental data regarding the mechanical behavior of unsaturated subgrade fill under varying water content and loading conditions and provides important information pertaining to the mechanical properties of subgrade fill. Group A subgrade filler, which is widely used in practical engineering, was selected for the experiments due to its specific particle size distribution and physical properties. Therefore, the conclusions are mainly applicable to similar soil types and engineering conditions. Although the applicability is limited by variations in physical and mechanical properties among different soils, their fundamental mechanical behaviors are generally consistent. This suggests that the proposed testing method and the observed mechanical patterns can still be applied to similar soil conditions. Given the regional and project-based differences in the types and properties of subgrade fillers, future triaxial tests should be performed on a wider range of unsaturated sandy and silty soils to validate the applicability and generality of the conclusions.