| The damage and impact on the natural and social environment caused by the construction of large scale reservoir can not be ignored. The initial impoundment and periodic water level variation may change the balance of geological environment of the reservoir area and bring abundant of geo-hazards and potential dangerous. Landslide is one kind of typical geo-hazards occurs in reservoir area susceptibly. The Three Gorges Project, locates on the main stream of Yangtze River between Yichang and Chongqing in China, is a large water conservancy and hydropower project constructed on the middle-upper stream of Yangtze River. As one of the largest engineering projects in the world, Three Gorges Project is always accompanied with great dispute for the millions of immigrants and environmental disruption since the preparation of the project. Three Gorges Research Center for Geo-hazard, Ministry of Education is a comprehensive open platform focus on the research on Geo-hazards and operated by China University of Geosciences. The underground tunnel group belongs to the research center which is built in the sliding mass of Huangtupo slope in Three Gorges Reservoir area can comprehensively expose the original features of the sliding zones in the landslide, on the other hand laboratory tests and in-situ tests for the sliding zones will bring great contribution to the study of landslide hazards. According to the existing investigation data, Huangtupo landslide is composed with4main parts, namely No.1adjacent to river sliding debris, No.2adjacent to river sliding debris, Garden Spot Landslide and Transformer Station Landslide, the total area is135×104m2and total volume is6934×104m3. As many other landslides in the reservoir area, Huangtupo landslide is a wading landslide which means the toe of the sliding body is under the highest175m water level of the reservoir. For the initial impoundment and annually water level fluctuation, parts of the sliding mass are under the drying and wetting circulation condition. The study on Huangtupo landslide has an important representative significance to the study on geo-hazards in the whole Three Gorges reservoir area.Firstly, the nature geography, geological environments and basic characteristics of the sliding mass of Huangtupo area are introduced, especially the distribution characteristics of sliding zones of adjacent to river sliding debris. Basic property of the sliding zone soils are tested by laboratory tests. From the sliding zones distribution sections, multi layers of sliding zones can be observed in the adjacent river sliding debris. The elevations of each layer of sliding zones distribute differently with the position of sections. Continuity and interconnectivity of the sliding zones are also different in different area of the sliding mass. Physico-mechanical properties and distribution characteristics of these subsistent or potential sliding zones directly dominate the stability and failure mode of the whole landslide. So, the study on sliding zones is the key points for analyze of the formation and stability of the landslides. Sliding zone soils exposed by boreholes are silty clay with gravels, in which, clay contents13%-30%, silt and sand contents15%-37%and gravel contents2.5-20%, the components of sliding zone soils have significant difference in different parts of the sliding body. Results of X-ray diffraction tests show that the main component minerals of sliding zone soils are quartz, feldspar, calcite and clay minerals such as montmorillonite, illite and chlorite, the total amount of clay minerals is40%～45%. Shear strength of sliding zone soils are tested by ring shear, peak cohesion and internal friction angle are21.38kPa and20.35。, respectively, residue cohesion and internal friction angle are8.54kPa and17.69。, respectively.As we know, soils and rocks are discontinuous materials composed by fine mineral aggregates and detrital particles. The properties and spatial arrangement of fine mineral aggregates and detrital particles directly determine the macroscopic properties of soil and rock materials. In order to quantitatively study the piratical size distribution, particle profile and surface characteristics and directionality of laminar clay mineral of sliding zone soils, particle size analysis, slice analysis and scanning electron microscope methods are employed combined with fractal theory. Particle size analyses indicate that there are obvious liner relationships between particle sizes and cumulative mass percentages in the logarithmic coordinate system, which means the particle size distribution has fractal characteristics. Higher fractal dimension of mass-size distribution represent more fine particles content and less coarse particle content, lower fractal dimension of mass-size distribution represent less fine particles content and more coarse particle content. Psephicity of sliding zone soils particles can be obtained by comparing their profile fractal dimensions and standard psephicity particles. Surface characteristics and energy spectrum analyses show the argillaceous siltstone particles has relative lower psephicity, apophysis and grooves distribute on the rough particle surfaces and form the honeycomb shapes. Argillaceous limestone particles have relative higher psephicity and smoother surfaces but a lot of linear scratches can be observed on the particle surfaces. Calcite crystals are mainly parallel developed columnar, laminal or rhombohedron shapes. Statistical results of clay particles directionality indicate that the clay mineral arrangements of siding zone soils close to shear surface have much stronger directionality. As the distances away from the shear surface, directionality of the clay mineral particles arrangements weaken gradually. The study on the structure of sliding zone soils can contribute to the analysis of physico-mechanical properties, formation and evolution process of sliding zones in the landslides, and finally invert the macroscopic deformation and failure history of the whole landslide. For example, the particle size distribution, coarse particle profiles and typical surface features can be used as quantitative analysis indexes for the evolution stage and failure mode of the sliding zones. Directionality of the clay mineral particles arrangements can be used to evaluate and predict the evolution stage and stability of the slope, and also used to distinguish and determine the main sliding surface and the position and thickness of disturbed layer of sliding zones.As the impoundment of reservoir and annually water level fluctuation, parts of areas in reservoir wading landslide are under saturated and unsaturated circulation condition, so, the study on the hydro-mechanical properties of sliding mass, and especially the sliding zones, is one of the most important problem of the study on reservoir induced landslides. Pressure plate tests are employed to test the soil water characteristic curve of remolded sliding zone soils with different pore size distributions, tests results indicate that under the same final consolidation pressure, the model parameter a related to air entrance value and model parameter n related to pore sizes distribution both decrease with the increase of particle mass-size fractal dimension. For the sample with the same particle mass-size fractal dimension, parameter a and n both decrease with the increase of final consolidation pressures. Saturated volumetric water content and residual volumetric water content both increase with the increase of particle mass-size fractal dimension or decrease of final consolidation pressures. In order to test the hysteresis of soil water characteristic curve and hydraulic conductivity function of sliding zone soils, transient water release and imbibitions method (TWRI) is employed to test both remolded and undisturbed soil samples. TWRI tests results indicate that both soil water characteristic curve and hydraulic conductivity function of sliding zone soils have obvious hysteresis, and the degree of hysteresis are affect by the particle size component of the samples. From the hydro-mechanical parameters in both drying and wetting paths of remolded sliding zone soils with different particle size distributions, we can find that the degree of hysteresis of soil water characteristic curve and hydraulic conductivity function increase with the increase of particle mass-size fractal dimension. In which, the air entrance value, saturated volumetric water content and saturated permeability appear larger hysteresis, but the hysteresis of pore size distribution parameters are relative smaller. Because of the hydraulic conductivity function is one of the determining factors of the variation of moisture condition in the soils, the large difference between permeabilities in drying and wetting paths significantly affect the moisture content of the sliding zone soils during the variation of water level, and this is the main reason for the study on hysteresis of the soil water characteristic curve and hydraulic conductivity function of sliding zone soils. There are two main influencing factors on the hysteresis of hydro-mechanical properties of soils, one is the "ink bottle effect" caused by the particle size distribution and the other one is the change of contact angle between the surface of soil particles and pore water in drying and wetting paths.Effective stresses of unsaturated soils with different moisture contents can be defined by unified effective stress principle which is based on suction stress theory. For saturated soil, the effective stress is the difference of total stress and pore water pressure. For unsaturated soil, the effective stress is the difference of total stress and suction stress. Direct shear tests results of remolded sliding zone soil samples with different matric suctions indicate that the shear strength of samples increase with the increase of matric suction under the same normal pressure. The cohesion of the sample change significantly with the matric suction but the internal friction angle do not change to much, the whole shear strength envelope rise with the increase of matric suction parallelly. There is not linear relationship between the increase of shear strength and matric suction. Comparing the direct shear tests results of remolded sliding zone soils with different saturation with the predict results on the base of suction stress theory, we can find that the predict results have acceptable accuracy. For the reservoir induced landslides, parts of the sliding mass are under periodic drying and wetting circulation condition. Analyzing this kind of problem with traditional strength theory for saturated soils can not obtain results conform to real situations. Predicting the shear strength of soils under unsaturated condition by unified Mohr-Coulomb shear strength criterion on the base of suction stress theory can adequately consider the shear strength of sliding materials under unsaturated conditions conveniently and accurately which is benefit for the application of unsaturated soil mechanics theory on the practical geotechnical engineering problems.Earth surface deformation monitoring data indicates that parts of Huangtupo slope is moving episodically into the reservoir. Such movements of the slope appear to be highly correlated to the initial and seasonal water level changes. In order to analyze the mechanisms of the reservoir induced landslides, a general conceptual model is established for landsliding triggered by initial reservoir impoundment and annual water level fluctuations. This conceptual model identifies most of the possible physical mechanisms for stabilizing or destabilizing slopes due to the reservoir water level fluctuation. These mechanisms are caused by variation of pore water pressure, suction stress, hydrostatic loading, and slope self-weight during the variation of water level. So, the conceptual model can represent the mechanisms for most of the wading landslides induced by the change of reservoir water level.Through the conceptual model, there are both increasing and decreasing in driving and resistant stress areas and regimes in wading landslides. The mechanisms for effective stress decrease are mainly the increase of pore water pressure and the disappearance of suction stress. The mechanisms for effective stress increase (thus shear resistant) are the total stress caused by the hydrostatic pressure on the slope surface and the increase in self-weight of the sliding mass. Because the geologic and hydrologic structure of the slope, the variations of effective stress in different regions are quite different. The effective stress in the toe area and the unsaturated areas that are always above the water table are less affected by the fluctuation of the water level. The effective stresses in the middle part are greatly affected by the fluctuation of the water level as the saturation in this region varies greatly with both the initial reservoir impoundment and annual water level fluctuation.In order to quantitatively analyze the relationship between the moisture conditions, pore water pressure, suction stress and effective stress in the sliding mass with the variation of water level of reservoir, a two-dimensional hydro-mechanical numerical model is constructed on the base of a typical section form No.1adjacent river sliding debris of Huangtupo landslide. The modeling period is from April2003to April2010which covers the total4stages of the impoundment of Three Gorges reservoir. For the main purpose of the numerical simulation is the comprehensive effect of the mechanisms to the wading landslide caused by the change of reservoir water level, the other factors such as precipitation and perched water are ignored in the model. Modeling results quantitatively indicate the comprehensive effect of all the mechanisms mentioned in the conceptual model on the stability of No.l adjacent river sliding debris of Huangtupo landslide. During the water level rise from68m to175m and of the seasonal fluctuation between145m and175m, it is shown that the induced variations in pore water pressure, suction stress, hydrostatic loading, and slope self-weight have different effects on the effective stress conditions and slope stability. According to above calculation and analysis results, for the No.1adjacent river sliding debris of Huangtupo landslide with particular geological structure, hydro-mechanical properties and water level boundary conditions, the dominating mechanism for the factor of safety is the hydrostatic loading by the water level fluctuation. Because of the factor of safety will decrease lower than1, this study suggests that the creep would continue seasonally in the future at Huangtupo landslide. |
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