Abstract: Mountainous highways are often threatened by rockfalls detached from high and steep slopes, while the steep terrain makes it difficult to obtain the information of discontinuities and to precisely identify the unstable rock mass. The reliability of risk assessment of the unstable rock mass and the effectiveness of its prevention and control are thus seriously affected. Therefore, taking the unstable rock mass on Heiduo high-steep highway slope, Diebu county, Gansu province as the research object, the paper utilized UAV photogrammetry technology to obtain the 3D point cloud data of the slope, and used the KNN algorithm and Least Squares method to automatically identify and obtain the occurrence information of the slope discontinuities, manually interpreted and identified the potential unstable rock masses and qualitatively analyzed their stability, and finally applied the RocPro3D software to simulate and analyze the kinematic characteristics of the unstable rock masses after failure. The results show that: (1) The slope structure and high-steep terrain are the basic geological conditions for the development of rockfalls on the Heiduo highway slope. (2) Six groups of predominant discontinuities including bedding were automatically identified, and 6 dangerous rock masses were interpreted, of which 5 were determined unstably; (3) The rockfalls sourced from the 5 unstable rock masses could move onto the highway, among which the WYT4 has the widest impact scope, and the movement distance of the blocks under the predominant path could reach as far as 619m, which inferred it is a most dangerous one. (4) The spatial distribution data of blocks of previous rockfalls can be used to effectively and reliably invert the kinematic parameters of potentially dangerous blocks. The results can provide basic theoretical guidance for the design of rockfall prevention and control of dangerous blocks on the high-steep slope, and the research method can provide a useful reference for the discontinuities' identification of the unstable rock mass and for the risk assessment of rockfall on similar high-steep slopes.

Key words: Drone photography, discontinuities, polar stereographic projection, rockfall dynamics, Bailong River