Accuracy Analysis for 3D Model Measurement Based on Digital Close-range Photogrammetry Technique for the Deep Foundation Pit Deformation Monitoring

The deformation of deep foundation pits (DFP) may pose a huge potential threat to the safety of the bracing system and the surrounding environment. A large amount of deformation data facilitates the deformation prediction of DFP and the dynamic adjustment of the bracing scheme. This study proposes and validates a 3D modeling technique based on digital close-range photogrammetry, which can obtain sufficient deformation data and enhance the understanding of deformation patterns. Two typical sets of deformation experiments, namely the vertical deformation test and the horizontal deformation test, were conducted to examine the viability and accuracy of 3D model measurement. Furthermore, the factors affecting the accuracy of the measurement were discussed in detail. The results indicate that the measurement accuracy of 3D models can reach millimeter-level or even sub-millimeter level. The measurement accuracy is affected by the coupling of the pixel resolution of CCD, focal length, photographic distance, and photographic baseline. The increase in the pixel resolution of CCD and focal length improve the measurement accuracy of 3D model measurement, but increasing photographic distance shows the opposite trend. The ratio of photographic baseline to photographic distance (BDR) has a much greater effect on accuracy than other factors. The photographic distance with a resolution of less than 1.0mm and the BDR value of 0.6–1.8 are recommended. Based on the results, it is found that this technique can be adopted as an effective monitoring method in DFP deformation monitoring due to the high precision and labor-saving.