Formation mechanism and evolution characteristics of the loess–mudstone interface landslide in Yan’an, China

Loess–mudstone interface landslides (LMILs) are among the most frequent loess landslides in China, caused by contrasting permeability and mechanical properties between loess and mudstone. In August 2020, a large LMIL occurred in Yan’an, exhibiting creeping deformation for nearly two years, severely impacting nearby infrastructure and necessitating costly remediation. This study investigates the landslide’s evolution, material properties, triggering factors, and formation mechanisms through field surveys, boreholes, UAV photogrammetry, deformation monitoring, triaxial tests, SEM scanning, and GeoStudio simulation. Results show that the landslide underwent three distinct sliding phases, triggered by combined effects of artificial excavation, rainfall, and reservoir water level rise. The landslide measured approximately 340 m wide, 230 m long, 5–49 m thick, with a total volume of 2.65 × 10⁶ m³. Deformation exhibited clear zoning (tensile–shear–extrusion), with decreasing displacement and crack density from rear to front. Loess and mudstone strength declined significantly with increased water content, as infiltrating water dissolved soluble salts and cementing agents, reducing cohesion and internal friction. SEM analysis revealed increased pore development and microstructural damage, altering stress–strain behavior. Under high water content, mudstone showed a marked drop in elastic modulus, increased Poisson’s ratio, and transitioned from brittle to ductile failure, softening the interface and promoting instability. The formation of a softened zone at the loess–mudstone interface, due to differential permeability and hydrological disturbance, was key to development of the sliding surface and ultimate failure. These findings highlight the critical role of hydro-mechanical coupling in LMIL evolution and provide insights for early warning and mitigation.