Analysis of the structural characteristics and deformation mechanisms of unstable rock masses in the red beds along the Renchi Expressway

This study analyzes unstable rocks in red-bed strata along the Renhuai-Chishui Expressway (Guizhou, China), integrating UAV tilt photogrammetry and PFC simulation, revealing: (1) Their development is controlled by the Cretaceous Jiading Formation's thick sandstone-thin mudstone interbedding; 93% occur on steep cliffs (>60°) with predominant NE aspect. (2) Cavity propagation (0→2.0 m depth) induces high stress concentration (Kt increases to 44.2), driving stress transition to tension in sandstone and triggering a triplet failure mechanism: top tension cracking, cavity compressive crushing, and basal support loss. (3) Weathering causes significant strength attenuation in Jurassic sandstone: uniaxial compressive strength decreases by 45.1%-58.7%, and cohesive strength loss reaches 46.2%. (4) PFC simulations replicate the progressive ‘compaction - penetration - instability’ process and the control mechanism of steep unloading fractures/cavities: toppling governed by gravity moment imbalance ([Formula: see text]), while falling failure results from tensile overstress ([Formula: see text]). These findings offer theoretical and technical support for early identification of high-positioned unstable rocks in red-bed regions.