Photogrammetric-Based Study of Refraction Correction Coefficients for Red Clay and Sandy Soils

The triaxial test based on photogrammetry can realize the real-time measurement of the local deformation of the specimen, but the refraction of light leads to the error in the measurement of specimen deformation. The introduction of the refraction correction factor (κ) can not only solve this problem, but also improve the test efficiency. In order to investigate the trend of the refraction correction coefficient in the axial and radial deformation measurements of different specimens, and to analyze the difference between the stress-strain curves measured by applying the refraction correction coefficient and those of the conventional triaxial tests, the photogrammetric-based consolidation and drainage triaxial tests were carried out on red clay and sandy clay, respectively. The results show that: (1) In the axial deformation measurement, the refraction correction coefficients of the different local regions of the red clay and the sandy clay converged to 1, and the refraction amplification effect on the axial deformation measurement of the specimen is negligible; (2) In the radial deformation measurement, the refraction correction coefficients of the red clay and sandy soil are 0.812 and 0.757, respectively. The refraction amplification effect has a greater impact on the radial deformation measurement of the specimen, and the refraction correction coefficients of the different specimens correspond to different refraction correction coefficients; (3) The change trends of the stress-strain curves measured by conventional triaxial and the application of refraction correction coefficient. The stress-strain curves measured by conventional triaxial and by applying the refraction correction coefficient have the same trend, and the bias stress measured by applying the refraction correction coefficient photogrammetry is relatively small in the middle and late stages of the test.