Detailed investigation and potential instability analysis based on FLAC3D-Massflow: a case study of Majia landslide

Since 2020, heavy rainfall has triggered two intensely deforming landslide bodies on a slope in Chaotian District, China, showing a "upper-lower superimposed" spatial distribution. Currently, research on the instability mechanisms and dynamic evolution of such landslides remains relatively limited. This study comprehensively employs high-precision UAV mapping and FLAC3D-Massflow coupled simulation technology, and combines them with field investigation, trench profile analysis, and borehole stratum exploration to construct a 3D geomechanical model of the study area. On this basis, the instability mechanisms and dynamic evolution of potential landslides with spatial superimposition characteristics were analyzed. The results show that under heavy rainfall conditions, the stability coefficient of the upper landslide decreased from 1.043 to 0.961. It became unstable along the contact interface between crushed stone soil and phyllite and moved toward the lower landslide. Under the dual effects of impact load and colluvial load, the stability coefficient of the lower landslide dropped from 1.121 to 0.954, triggering shear failure at the soil-rock interface and forming a landslide disaster chain. Eventually, the landslide volume reached 75.5×10⁴ m³, direct impact area 0.29 km², and residential buildings on the opposite bank of the slope also faced direct threats. This study reveals for the first time the dynamic evolution mechanism of "impact-colluvial load coupling" in spatially superimposed landslides, and simultaneously proposes a 3D quantification and stability analysis method for impact loads of such superimposed landslides. The research results contribute to deepening the understanding of similar superimposed landslides and their disaster impacts.