## 2 Unsaturated Subgrade Filler Triaxial Testing System
For close-up images, the overlap rate was strictly maintained at \(\geq\)70%, with an overlap rate of 80% or more for specific areas of interest. This approach was designed to ensure that the subsequent imaging reconstruction process is seamless, smooth, and yields higher accuracy. The image overlap rate affects the comparison accuracy of the algorithm, as well as many of the tasks involved in stabilizing the 3D reconstruction for subsequent reconstruction processes. When the overlap is below 60%, feature point matching becomes very difficult. To support the robustness of the 3D reconstruction process, a number of effective countermeasures were put in place to ensure that the mean image overlap rate remained above 60%: (a) Setting overlap rates: To ensure that images were aligned correctly and generated quality 3D models during the stitching process, a photo-to-photo overlap rate of 60% from adjacent photos was required in this study. This overlap rate was based on the standard recommendation of DIC technology, while fully considering the requirements for feature point matching at a later stage and for 3D reconstruction. A rationally chosen overlap rate optimizes the image without sacrificing critical components, allowing for the adequate capture of feature points in the image and effective matching from different angles to improve stitching. (b) Angle and camera configuration: During multiple view acquisition, the angle between two adjacent cameras was maintained at 10\({}^{\circ}\)-20\({}^{\circ}\) apart from their respective shooting directions, ensuring at least a 60% overlap rate for each image. This angle selection maintained the continuity of each image, accurately including features from one image to the next without duplicating data from overlaps. This limited overlap of the samples creates fewer redundant overlapping areas while allowing for coverage of almost any flat surface that still has distinguishable feature points. Consideration was also given to the angle and position for shooting the sample. The final shooting process ensured that the arrangements and sequences of view-to-view camera shots produced clear images and angles that covered the surface area in question while preventing blind spots  
Figure 1: Overview of the testing system and setup. Notes: (**a**) self-developed unsaturated subgrade fill triaxial testing system, (**b**) schematic structure of the testing system, (**c**) positioning of the 3D camera during the measurement process. and coverage gaps in overlapping areas. Each camera's position and order pertaining to the images took into consideration the morphology of the subject material and experimental obligations. Based on the geometry of the subject material and utilizing precision objectives, a wider range from which to shoot was constructed as a circular approach with the cameras surrounding the material. While retaining as much usable image area as possible, the angles of each shooting situation and properly designated shooting sequences were managed so that any gaps due to omitted descriptions or lack of overlap did not result in white spaces or \"lost\" recorded aspects due to errors in the image acquisition order. (c) Redundant shooting strategy: A redundant shooting strategy was used as part of the experimental design to minimize the risk of insufficient overlap. That is, when the overlap area of adjacent images is too low, falling below the 60% threshold set for the experiment, additional shooting directions are taken, or the area is re-shot, to provide adequate support for all feature point locations in the reconstruction. Throughout the course of the experiment, repeated verification of the image stitching process and the integrity of the 3D reconstruction was performed to ensure that all overlap rates met the acceptable threshold and that there were no failures in the reconstruction due to low overlap areas.