Failure Patterns and Threshold Curves for Rainfall-Triggered Loess Landslide-Debris Flows: A Combined Modeling Study

On the Loess Plateau of China, rainfall-induced loess landslides often trigger cascading landslide-debris flow hazards, posing severe threats to major engineering projects and the safety of life and property in the region. However, studies on the conditions and mechanisms underlying the transformation of landslides into debris flows under varying rainfall conditions remain relatively limited. This research, through a series of physical model tests of rainfall-induced loess landslide-debris flow processes combined with triaxial tests, investigates the mechanical response relationship between loess landslide formation and rainwater infiltration. It examines the evolution of pore-water pressure and matric suction, stress–strain behavior, and deformation–failure mechanisms during saturation and wetting processes of loess elements, thereby revealing the genetic mechanisms of loess landslides in northwestern China. Six sets of physical model tests were designed to analyze the macroscopic deformation process, hydrological response, and displacement patterns during rainfall-induced landslide-debris flow events. The influence of rainfall intensity, cumulative rainfall, and slope gradient on landslide development was explored, and threshold curves for landslide and debris flow initiation were established. The results indicate that under different rainfall conditions, loess slopes exhibit two failure modes: sliding and sliding-to-flow transition. Under short-duration intense rainfall, water fills tension cracks in fissured slopes, reducing shear strength along the slip surface and leading to block-type sliding. The sliding process is characterized by long-term creep superimposed with intermittent accelerations. Under prolonged rainfall, progressively sliding slopes develop intensely fractured zones where pore-water pressure continuously increases, resulting in liquefaction and flow of the water-rich slope mass. This flow process exhibits sustained high-velocity movement. The findings provide new experimental insights into the mechanical mechanisms governing the sliding-to-flow transition in loess landslides and offer a scientific basis for rainfall threshold-based early warning of loess landslide-debris flow hazards.