The Regulatory Mechanism of Base Structure Type on the Instability Evolution of Column-Shaped Unstable Rock

Column-shaped unstable rocks are commonly developed in canyons and karst regions, and their instability is strongly affected by the mechanical characteristics of the underlying base. In this study, the Zengziyan W12# unstable rock and the Wangxia W2 unstable rock in Chongqing, China, were selected as representative cases of a soft–hard interlayered base and a homogeneous weak base, respectively. Finite element models were established to compare the distributions of equivalent stress, shear stress, and displacement under self-weight loading. The results show that, in the soft–hard interlayered base, stress is mainly concentrated in the hard layers, and the high shear-stress zones are segmented by these interlayers. The instability-controlling location tends to shift upward to the degraded zone at the bottom of the unstable rock mass, indicating a fracture–collapse tendency. In contrast, the homogeneous weak base shows more continuous stress transmission and shear-stress development, which is favorable for the formation of a through-going shear zone within the base. These results indicate that the base structure type regulates the stress transfer path, shear-zone continuity, and instability-controlling location of column-shaped unstable rocks, thereby providing a mechanical basis for stability assessment and targeted prevention.