## 5 Discussion  
The proposed photogrammetry method is accurate, time- and cost-effective, and requires only a digital camera to capture images of the soil specimen during triaxial testing, from which accurate full-field 3D models of the soil specimen at different loading steps are reconstructed. The accuracy of the photogrammetry method can be independently verified by using various values. Another advantage of the photogrammetry method is that it can be utilized for deformation measurements on both saturated and unsaturated soils during triaxial testing with the same system setup. This study innovatively combines triaxial photogrammetry and finite element numerical simulation to explore the mechanical properties of undisturbed soft clay, offering valuable insights and methods for geotechnical engineering research. The photogrammetry method used in this research shows significant advantages. It can effectively overcome the limitations of traditional triaxial tests, such as difficulties in local strain measurement and end constraint issues. It provides more accurate and detailed deformation information by capturing real-time surface deformation images of soil samples and using image-processing techniques. For example, it can precisely measure the local peak stress of soil samples, which is more accurate than conventional triaxial tests. Additionally, it can be applied to both saturated and unsaturated soils with the same system setup, enhancing its versatility. However, the method also has some challenges. The spatial refraction correction model is crucial for accurate measurement, but the complexity of light refraction in the test environment may introduce potential errors. Although the model is designed to eliminate the refraction effect using the reversibility of light and Snell's law, in actual operation, factors such as the inhomogeneity of the test apparatus and the influence of soil-water-air interactions on light propagation may still affect the accuracy of the correction. Using the modified Cam-Clay model in the ABAQUS software 2022_09_29-02.11.55 183150 is reasonable in terms of the numerical simulation part. The model parameters, including the consolidation slope (\(\lambda\)), rebound slope (\(\kappa\)), and critical state slope (M), are carefully determined through laboratory tests, ensuring the reliability of the simulation results. The simulation can capture the pore pressure data well and predict the peak value of the stress-strain curve. The average absolute error of 0.45 kPa and relative error of 0.21% in stress-strain curve acquisition demonstrate its high accuracy. Nevertheless, the numerical simulation also has limitations. As shown in the comparison with conventional triaxial tests, there are differences in the stress-strain curves in the middle part of the loading process. The numerical simulation curves approach the peak values more rapidly under smaller axial strains, which may be due to the simplification of the soil model. Although widely used, the modified Cam-Clay model may not fully consider all the complex characteristics of undisturbed soft clay, such as the influence of microstructure and the coupling effect of multi-physical fields. In the comparative analysis of different methods, the study provides a comprehensive and in-depth understanding. The stress-strain curves obtained by the three methods show different characteristics. The photogrammetry method reflects the actual deformation more accurately, while the conventional triaxial test and numerical simulation somewhat overestimate the deviator stress values. Regarding shear strength indicators, the numerical simulation can effectively replicate the conventional triaxial test results for soft clay in areas with a confining pressure less than or equal to 100 kPa. However, photogrammetry is more accurate when the confining pressure exceeds 100 kPa.