Research on fine collection and interpretation methods of discontinuities on high-steep rock slopes based on UAV multi-angle nap-of-the-object photogrammetry

The rapid and accurate acquisition of discontinuity parameters of rock masses is of paramount significance for the stability assessment of rock slopes. However, the complex and hazardous terrain of high-steep rock slopes poses challenges to conventional survey methods. Given this, this study proposes the unmanned aerial vehicle (UAV) multi-angle nap-of-the-object photogrammetry technology. This method can comprehensively consider the terrain development characteristics of high-steep rock slopes and the orientation characteristics of dominant discontinuity sets. With trained pilots, high-quality image acquisition with millimeter resolution can be achieved. This method effectively overcomes issues like texture distortion, shadow obstruction, and low resolution commonly found in conventional UAV 3D models. Subsequently, a novel non-contact method is presented for obtaining discontinuity parameters, including orientation, trace length, spacing, aperture, and roughness, based on the 3D real-scene model of the slope. The feasibility and reliability of this method are verified by collecting 1728 discontinuities along the 1300-m length of a rock slope at a construction site on a railway in southeastern Tibet. A comparison with manually measured results indicates average differences of 2° for dip and dip direction; 1.3 cm and 9 mm for trace length and aperture, respectively; and 1.89 for JRC. The study also reveals that the effective resolution of the 3D model is approximately 1 to 2 times the theoretical resolution of a UAV image, providing crucial guidance for obtaining high-quality images on high-steep rock slopes. The method proposed in this study holds significant practical value in the stability assessment and disaster prevention of rock slopes.