Investigation of rock mass deformation through nap-of-the-object photogrammetry: a case study of Hongyun Golden Peak in Fanjing Mountain, China

Investigating rock mass structures and collapse hazards in high and steep mountainous regions often poses significant challenges, especially when traditional manual surveying methods are inadequate. Hongyun Golden Peak, a unique columnar geological body with a height of 104 m in Fanjing Mountain, Guizhou Province, China, attracts a large number of tourists every year. To analyze the rock mass structural characteristics and collapse hazards of Hongyun Golden Peak more intuitively, this paper employed Nap-of-the-object photogrammetry (NOOP), a technique that has recently been widely used in the reconstruction of ancient buildings. By flying a drone close to the target surface, high-resolution digital images with sub-centimeter accuracy and processed them with specialized software were obtained to generate a realistic 3D surface model of Hongyun Golden Peak. Based on the 3D model, a survey and statistical analysis of the rock mass structure were conducted, revealing that Hongyun Golden Peak is controlled by three sets of joints. The intersection of two joint sets has formed nine large tensile fractures (F1–F9) and identified 12 potential collapse rock blocks that pose a threat to tourist safety. Numerical simulation analysis indicated stress concentration at an elevation of approximately 2270 m, with the maximum displacement occurring at around 2300 m. Comparative analysis demonstrates that the NOOP technique is highly accurate and operable. With the continuous development of this technology, it shows great promise for applications in rock mass structure investigations and collapse hazard studies.