Rainfall-induced landslide mechanisms in loess–paleosol slopes: combined effects of preferential flow pathways and low-permeability layers

In the Chinese Loess Plateau, interbedded loess–paleosol sequences constitute a typical geological setting for rainfall-induced landslides. The contrast in seepage behavior between loess and paleosol is an important triggering factor for landslides. This study develops a hydro-mechanical coupling framework for loess–paleosol slopes characterized by layered heterogeneity in permeability structure. Within this framework, a unified dual–single permeability moisture-migration model is established to capture nonequilibrium seepage in loess and the single-permeability behavior of the paleosol. The coupled analysis incorporates the hydro-mechanical effects of pore-water pressure and soil unit weight. A loess–paleosol slope in Yan'an City, China, is taken as the study site to investigate the hydraulic response and stability evolution under two regionally representative rainfall scenarios. The results are further discussed through comparative analysis and actual landslide cases. The paleosol layer markedly impedes preferential flow in the overlying loess, facilitating the rapid development of perched water along the loess–paleosol interface. This results in spatial discontinuities in pore-water pressure across the interface, thereby inducing strain concentration at the interface. Under high-intensity, short-duration rainfall, the factor of safety decreases more substantially, and the interface-controlled failure mode is more pronounced than under low-intensity, long-duration rainfall.