Creep behavior and microstructural evolution of sliding zone soil in slow-moving landslides in the Three-Gorges Reservoir area

The stability of creep landslides is fundamentally governed by the time-dependent behavior of sliding zone soils. Using the Huangtupo landslide in the Three Gorges Reservoir as a representative case, this study investigates creep deformation and microstructural evolution through graded-loading direct shear creep tests, scanning electron microscopy (SEM), and laser particle-size analysis. The sliding zone soil exhibits typical three-stage creep behavior, with instantaneous and total strains increasing nonlinearly with shear stress. Isochronous stress–strain curves display pronounced normalization, indicating consistent long-term strength characteristics. Microstructurally, increased stress drives the transformation of large pores into micropores, promotes oblate pore shapes with enhanced directional ordering, and advances particle breakage toward a saturation state. Bridging these scales, quantitative relationships are established between microstructural indices—specifically pore anisotropy—and macroscopic rheological parameters. Consequently, a structure-driven creep constitutive model is proposed within a Burgers-type viscoelastic-viscoplastic framework. This model provides a physically based method for linking microstructural changes to long-term deformation, offering theoretical support for analyzing landslide mechanisms in reservoir environments.