## 2 Unsaturated Subgrade Filler Triaxial Testing System  
The unsaturated subgrade fill triaxial testing system developed in this study is shown in Figure 1a. The pressure chamber's outer wall is made of acrylic, with dimensions of 300 mm in height, 180 mm in outer diameter, 9.6 mm in thickness, and a refractive index of 1.491. A total of 200 measurement targets are installed on the acrylic outer wall, including six circular regions (30 targets each) and two vertical stripe regions (10 targets each). Additionally, high-contrast measurement targets, designed for automatic software identification, are installed on both the acrylic outer wall and the rubber membrane surrounding the specimen. Figure 1b illustrates the schematic structure of the testing system. Digital image correlation (DIC) technology is used in this system to reconstruct the 3D model of the unsaturated soil specimen from images captured during the triaxial test and to compute soil volume changes via non-contact photogrammetry. Moreover, a high-suction tensiometer monitors matric suction changes during the triaxial test rather than directly controlling the suction. This method applies axial force and confining pressure to the specimen to simulate natural drainage conditions, closely replicating the deformation and drainage processes of subgrade fill under natural conditions. Figure 1c shows the positioning of the 3D camera during the measurement process. To ensure effective data capture and prevent potential issues with image quality, the following camera parameters were carefully set and adjusted throughout the experiment: (a) Horizontal angle: The horizontal angle between the camera and the sample's central axis is initially set between 15\({}^{\circ}\) and 30\({}^{\circ}\) to allow coverage of the side surface of the sample and avoid occlusion dead zones. The angle is then adjusted based on changes in the sample throughout the different deformation stages of the test, ensuring effective capture of morphological changes, the deformation process, and local deformation characteristics. (b) Distance between the camera and the sample: The distance between the camera and the surface of the sample is continually adjusted based on the required resolution and image coverage. During the tests, the distance from the camera to the surface of the sample was kept within a range of 30 cm to 50 cm to obtain clear images and avoid blurriness or distortion. The distance from the camera to the sample surface in the early stage of the experiment is approximately 40 cm. For cases in which the highest resolution is needed to capture local deformation features, the distance can be reduced to 30 cm; for cases in which a wider field of view is required to capture large deformation stages, the distance can be increased to 50 cm. Overall, this ensures adequate resolution and overlapping areas in all the different experimental stages. (c) Camera movement step size: To ensure coverage of the various areas of the sample, the camera is moved in the horizontal circumferential direction by 15\({}^{\circ}\) for each shot. After completing each set of images, the camera rotates a total of 15\({}^{\circ}\) relative to the center of the sample to take the next set of shots and continues this process until the complete 360\({}^{\circ}\) scan is completed or all deformation areas to be observed have been covered. (d) Camera height adjustment: The center of the camera lens is generally aligned with the central axis of the sample, corresponding to its midpoint. When significant shear bands or large deformations are observed, the height is adjusted in increments of \(\pm\) 20 mm to ensure that the pre-identified deformation areas are within the optimal imaging range, thereby improving the accuracy and effectiveness of the images. (e) Number of views and overlap rate: The minimum number of photos taken for each testing step was 24 to cover the target minimum of a 360\({}^{\circ}\) view.