Rainfall-Related Shear-Zone Weakening and Stability Degradation of a Reactivated Loess–Carbonaceous Slate Landslide

This study investigates the deformation evolution and stability degradation of a rainfall-reactivated loess–carbonaceous slate landslide in Luoda Town, Gansu Province, China. Field investigation, borehole logging, water-content-controlled direct shear tests, GNSS monitoring, rainfall analysis, and FLAC3D modeling were integrated to examine the weak shear zone developed near the lithological contact. The landslide comprises loessial–colluvial deposits overlying weathered carbonaceous slate and exhibits a progressive rotational-slide pattern characterized by rear tensile cracking, middle translational movement, and frontal compressional bulging. As specimen water content increased from 17% to 24%, the cohesion and internal friction angle of the shear-zone soil decreased from 22.6 to 14.5 kPa and from 17.0° to 9.7°, respectively. GNSS monitoring identified steady creep, accelerating creep, and rapid failure, with G1 accelerating earlier than G2. The strongest observed rainfall–displacement correlations occurred at antecedent windows of 48 h for G1 and 120 h for G2, indicating spatially variable rainfall responses. Across the corresponding laboratory-derived strength states, the calculated factor of safety decreased from 1.512 to 0.898. These results indicate that the 2021 reactivation was controlled by a weak shear-zone layer near the lithological contact, with rainfall-related wetting likely contributing to strength degradation and progressive deformation.