Using continuum numerical method, this paper investigates triggering mechanism, controlling conditions and evolution of deep-seated gravitational slope deformation (DSGSD) in a shale-dominated dip slope located in Kii-mountains. The simulation results demonstrate that due to weathering and stress-induced damage of rock, long-term strength degradation of rock mass is a significant controlling factor for the initiation and development of DSGSD. Moreover, river incision causes stress concentration, stress redistribution and stress release-related rock damage within the slope, triggering the formation of different plastic shear bands at different river incision stages. The results taking into account the influence of bedding planes suggest that the DSGSD in a dip slope involves both shear slip along bedding planes and brittle fracture of intact rock mass across the bedding planes. The bedding planes constitute a structural control on DSGSD evolution due to their weaker strengths. Numerical simulations reproduce internal disintegrated weak zones of the slope observed from high-quality drill cores and topographic features very well, which allows a more indepth understanding of the relationships between interior deformation of the slope, exterior geomorphology and associated controlling mechanisms As an integrated mass movement process, DSGSD in the Kii-mountains is controlled by slope topography, geological structures, lithology, and triggering factors. Differing from traditionally geomorphological and macroscopic structural analysis for DSGSD, this study suggests that numerical simulation provides a significant convenience for studying the driving mechanism and development process of DSGSD.

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