Study On The Deformation And Failure Mechanism Of Outang Large Landslide In The Three Gorges Reservoir Region, China

The annual reservoir operation since the completion and initial impounding of the Three Gorges Project has formed water level fluctuations nearly 30 m high, which disturbs the inherent geological environment of the reservoir bank. The complex geological conditions in the reservoir region further bring great challenges to the prevention and control of geological hazards such as landslide. This has become a key geological environment problem that affects the social and economic sustainable development in the Yangtze River Economic Zone. Under the influence of regional geological structure and geomorphology, bedding landslides are widely developed in the Three Gorges Reservoir Region. Triggered by external forces such as river erosion, rainfall and fluctuation of reservoir water level, overall or local revival instability of large or giant ancient bedding rockslide can take place easily. Therefore, the research on the revival instability mechanism and stability of landslide should not be ignored. Besides, the formation conditions for development, the deformation evolution mechanism and the failure mode of bedding landslide at reservoir bank as well as the evaluation of landslide stability under the effect of various inherent geological structure control factors and external force factors have been key issues and hot topics of research that attract experts, scholars, engineers and technicians at home and abroad since a long time ago. Especially, the research on the deformation instability mechanism of large and huge bedding rockslides at reservoir bank has been one of the cutting-edge subjects in the engineering and academic circles.In this paper, Outang landslide, a giant consequent bedding rockslide of the largest scale among those recently taking place in the Three Gorges Reservoir Region, was chosen as the research object. Based on a lot of field engineering geological survey work, and adit, trench and borehole exposure, the special geological structure characteristics of Outang landslide were defined, including multi-order and multi-stage sliding and three-dimensional lateral anti-slide effect. In combination with the geological environmental background in the Yangtze River Three Gorges Reservoir Region and the results of ESR dating of slide zone soil, the theoretical methods of geomorphology and engineering geomechanics were used for systematic analysis of the cause of landslide formation and the evaluation process. Through deep mining of field monitoring data, a research was conducted on how rainfall and fluctuation of reservoir water level is correlated with displacement deformation of the geological complex of Outang landslide. With advanced laboratory facilities and instruments and taking landslide rock-soil mass as the research object, indoor and outdoor physical and mechanical tests, such as water chemical experiment, X-ray diffraction experiment, scanning electron microscopy of microstructure, and large scale direct shear test of slide zone soil, were performed to discuss the inherent correlation between the physical and mechanical properties of landslide rock-soil mass and the landslide revival instability. The unsteady unsaturated seepage and unsaturated soil shear strength theory was used to investigate the rule of slide mass seepage field and stability change at different fluctuation rates of water level and different permeability coefficients for the generalized geomechanical model of landslide; meanwhile, the monitoring data of actual rainfall and reservoir water level were combined to analyze the whole process of dynamic change in landslide stability. Three-dimensional numerical simulation was employed to analyze the lateral anti-slide effect of the giant consequent bedding rockslide, and a comprehensive prevention and control scheme was proposed based on the result of early landslide control engineering. This paper aims to provide a certain theoretical basis for rational evaluation of landslide stability and forecasting of landslide, offer scientific guidance for implementation of subsequent control engineering measures, and also provide theoretical and technical support for scientific decision-making on prevention and reduction of hazards similar to consequent bedding rockslide in the Three Gorges Reservoir Region. Therefore, it is of great significance in theoretical guidance and important value in engineering application. The main research findings and conclusions are summarized as follows:(1) Outang landslide is a typical giant ancient consequent bedding rockslide in the Three Gorges Reservoir Region. Its plane shape is characterized by multi-order and multi-stage sliding, with mainly three orders of sliding. Its spatial form features lateral anti-slide from three-dimensional perspective, which is under control of the stable mountain on the west side.(2) The analysis on the result of ESR dating of slide zone soil shows that the landslide exhibits a certain law in terms of geological historical time span of multi-order and multi-stage sliding. Specifically, the time span of third-order landslide is 47,000~49,000 years, second-order landslide 49,000~68,000 years and first-order landslide 121,000~130,000 years. The temporal distribution of third-order sliding is completely within the period of high frequency development of the ancient landslide, i.e.,(5~17)×104 a. Also, the landslide development time series well coincides with the distribution of warm and wet paleoclimate.(3) The research on the cause of landslide formation shows that the consequent bedding monoclinic structure with interbedded soft and hard rocks having developmental joint fissure is a factor controlling landslide formation. The river valley development and evolution process caused by downcutting of the Yangtze River and terrace rise provides a dynamic foundation and motion space for Outang landslide. Concentrated rainfall and reservoir impoundment are the main factors that induce revival of Outang landslide.(4) Formation mechanism and evolution pattern of multi-order and multi-stage sliding of Outang landslide: The first-order landslide mass exhibits a “tension fracture- slip(bending)-shear” pattern; the second-order landslide mass exhibits a “planar slip” pattern; and the third-order landslide mass exhibits a “slip- shear” pattern.(5) Significant difference in surface displacement is observed between different areas of the landslide; the cumulative displacement in the severe deformation zones on the east and west sides is much bigger than that of deep landslide mass. Among the three orders of landslide, the third-order landslide has the biggest cumulative displacement, followed by the second-order and first-order landslides in sequence. The landslide cumulative displacement-time curve shows deformation characterized by staged “leap” growth, which well coincides with the temporal distribution of concentrated rainfall and fast decline of reservoir water level. The landslide displacement rate is characterized by “wave crest” change, which is negatively correlated with the fluctuation rate of reservoir water level. The maximum peak of displacement deformation rate somewhat lags behind the maximum decline value of reservoir water level.(6) Water-rock coupling of certain strength exists when rainfall infiltrates into the landslide rock-soil mass, which generates a series of complex water ion chemical reactions, causing water quality changes. The slide zone soil contains a large amount of hydrophilic clay minerals, featuring strong water absorption and softening property. Since the microscopic aggregate particles of slide zone soil are highly oriented, its engineering mechanical properties are obviously anisotropic. The shear test suggests that the lighter the overburden load is, the lower the shear stress will be when slope deformation or structural failure of particles in slide zone soil occurs; this is also the theoretical basis for explaining why the deformation sign of the third-order landslide mass is the most obvious among all deep landslide masses.(7) At different fluctuation rates of reservoir water level, the underground water level in the slide mass always fluctuates with the fluctuation of reservoir water level, but the former always lags behind the latter. During decline of water level, the phreatic line in the front of the slide mass is “convex upward”; the water head difference, with the characteristic of inside higher and outside lower, resulted in hydrodynamic pressure, which was adverse to the landslide stability. During rise of the reservoir water level, the phreatic line in the front of the slide mass is “convex downward”, and the main force that reservoir water applies to the landslide is seepage pressure. As the permeability coefficient increases, the “convex upward” or “convex downward” phenomenon of the phreatic line in the front of the slide mass becomes less obvious during fluctuation of reservoir water level. At the stage when the reservoir water level declines from high to low, as the water level declines faster, the landslide stability coefficient reduces to the minimum value more quickly and the extremum becomes smaller. At the stage when the reservoir water level rises from low to high, as the water level rises faster, the landslide stability coefficient increases to the maximum value more quickly and the extremum becomes bigger.(8) The stability change curves of the landslide at different fluctuation rates of reservoir water level when the permeability coefficient is the same show some similarity. At the same fluctuation rate of reservoir water level, the weaker the landslide permeability is, the greater the influence of fluctuation of reservoir water level on the landslide stability will be. At the same permeability coefficient, when the reservoir water level fluctuates to a certain water level and keeps this level for a long term, the degree of landslide stability increase(decrease) is positively correlated with the decline(rise) rate of reservoir water level.(9) Under the combined effect of rainfall and fluctuation of reservoir water level, the landslide stability coefficient is generally smaller than that only taking into account the effect of fluctuation of reservoir water level. Especially, during fast decline of water level and continuous heavy rainfall, the stability coefficient is significantly reduced.(10) Under the long-term effect of geostatic stress, Outang landslide is subjected to creep deformation mainly along the true dip direction; an obvious high speed value area is formed between the slide mass and the stable mountain, which has a significant lateral anti-slide effect. According to experience in early control, and based on the principle of “putting prevention first and combining prevention with control”, it is suggested to take the comprehensive prevention and control measure that combines rear-edge cutting and load shedding and front-edge backfill for slope toe pressing of the landslide, slope protection with lattice frame, and surface and underground drainage systems.