Cracking and deformation behaviors of overhanging rock: Laboratory tests and optical monitoring

The destabilization of overhanging rock is a dangerous geological problem. In this study, a generalized model of typical overhanging cliffs from the Three Gorges Reservoir area in China with different fracture angles, fracture lengths, and free surface depths is constructed to investigate the cracking and deformation behavior of overhanging rocks. Laboratory tests and deformation field monitoring using the digital image correlation (DIC) method are performed on these specimens to reproduce the destabilization and failure process of overhanging rock under external loading. The influence of peak load is found to be the most sensitive to the fracture length, followed by the free surface depth, and to be the least sensitive to the fracture angle. The DIC-based strain fields reveal that the fracture angle and free surface depth significantly alter the crack propagation paths, whereas the influence of the fracture length is weaker. These parameters also affect the crack initiation time. The relative displacement evolution characteristics indicate that fracture angle, fracture length, and free surface depth affect the shape and size of the rotating block, the rotation center, and the rotation pivot point and degree, respectively. The grayscale characteristic evolution trends are similar for all examined overhanging rock specimens. The evolution of the grayscale indices based on DIC can be divided into high-frequency oscillation, smooth decline (or smooth downward concavity), and stable development stages. Furthermore, the multistage properties of the indices can be used to identify the fracture state of overhanging rocks, providing a theoretical basis for graded early warning of rockfall disasters.