Close-range photogrammetry for 3D rock joint roughness evaluation

Abstract

Photogrammetry-based methods using everyday photographic equipment and easily available software have gained relevance for roughness measurement of rock joints, as an alternative to traditional approaches. However, the influence of some aspects, such as the number and orientation of camera poses, the parameters of the Structure-from-Motion, Multi-View Stereo and meshing algorithms, the resolution and accuracy of the reconstructed models, requires proper appraisal. To assess the surface roughness of a granite rock joint specimen using such photogrammetry methods, 3D models generated using different settings (i.e. camera sensors, camera poses, open/free software and workflows) were compared with a reference model obtained by contact digitalization. The results suggest that comparable results can be achieved if the photos adequately cover the specimen and, at least, equivalent vertex densities are attained. Percentiles p5 and p95 of surface deviations of the tested models to the reference mesh were, in general, lower than ±0.1 mm. Estimated roughness parameters showed reduced variability, with a minor impact on the shear strength evaluated according to the Grasselli’s GG-shear strength criterion, which was developed from the empirical correlation of roughness parameters with laboratory testing data. Results also showed that this methodology is far more accessible, easier to use, faster to implement, and less expensive than most currently available equipment and approaches.