Research On Time-varying And Reliability Model Of Landslide On Reservoir Bank Under Water-Soil Coupling

Three Gorges Project is the world's largest water-control project nowadays, but near 2500 large landslide or slumped mass, about 500km unstable reservoir bank have been found in the reservoir region. The reservoir water level will appear rise and fall periodic between 145m-175m when the reservoir is operated based on the purpose of flood control and power generation due to be located in the area of the typical wet and heavy rain. Under reservoir water level periodic fluctuations, slope stability will be greatly affected, as a result, the possible revival of the ancient landslide has led to the generation of large-scale landslides, and poor geological conditions may also produce a large number of new landslides; At the same time the long-term cyclical changes of reservoir water level will lead to long-term cyclical fluctuations of groundwater level, leading to landslides seepage, groundwater level and soil and rock changes in physical and mechanical properties, thus affecting the stability of landslide; the physical mechanics changes of rock and soil in the landslide, especially the slip soil, have played a crucial role in the stability of landslide. Therefore, it is essential to analyse the landslide of time-varying and reliability model under normal fluctuations of reservoir water level based on water-soil coupling, that it can lay a foundation for studying long-term landslide hazard prediction and control, and provide the suggestion for disaster prevention and redution of the reservoir bank landslide in the Three Gorges Reservoir.This paper select the Badong Huangtupo riverside slump-accumulation II in the Three Gorges Reservoir of the Yangtze river as the research object, according to the theory of water-soil coupling, saturated-unsaturated seepage theory and strength criteria, combining theoretical analysis and laboratory test and studying the influence to stability of landslides as water level fluctuates cyclically, then using the finite element method set up the random time-varying and reliability model of landslide, and evaluating the landslide reliability of all and part, the specific work had been done as follows:(1) Data collection and field investigation:Select the Huangtupo riverside slump-accumulationⅡas the research object., comprehensive collection of relevant geological data(topography, geological structure, lithology and hydrogeological conditions), the various stages of investigation information, physical and mechanical properties of rock and soil test data, groundwater level monitoring data, slump-accumulation deformation monitoring data ect; and investigate and sample in 12-2009 and 9-2010 in the field,making me have a deeper understanding about the whole of Huangtupo landslide.(2)Experimental of unsaturated soil-water characteristics:for the research characteristics of Badong slump-accumulation on water-holding properties and seepage, it selected its slide soil and landslide soil as test object, reshaping the samples according its natural state, and make the test of soil-water characteristic about slide soil and landslide soil and get the curve of soil-water characteristicin order to analysis the curve's regularity; Moreover, it respectively make the test of varyinghead and get the saturated permeability coefficient.Finally,it get the relation between suction and permeability coefficient according the curve of soil-water characteristic and saturated permeability coefficient.(3) Experimental of creep on slide soil:it made of slide soil consolidated undrained triaxial creep tests under the condition of different confining pressures(200kPa,300kPa,400kpa),and ser up slide soil's relation on stress-strain-time and its long-term shear strength parameters can be calculated based on the analysis of a series of routine tests on slump-accumulation riverside No.Ⅱ(4)Transient analysis of water-soil coupling:it set up the calculation model of seepage field of slump-accumulation riverside No. II and simulated landslide seepage field and groundwater distribution in different time under reservoir water level actual running conditions based on the soil-water characteristic curve and permeability coefficient function about the slide soil and landslide soil.(5) Slope stability analysis:it calculated the tend of slope's factor of safety in different reservoir water level correspond to landslide seepage field based on the strength criteria of unsaturated soil(6) Landslide failure probability analysis:it used stochastic finite element method to build random time-varying and reliability model of landslide, and evaluated the reliability the whole of landslide under different reservoir water level, as well as reliability of part slope.Based on a comprehensive analysis of the Huangtupo Riverside No.Ⅱslump-accumulation geological data, laboratory test data, landslide geological model, numerical simulation and got following conclusions:(1) Huangtupo landslide is the long-term development of the product of geologic history evolutionunder certain geological environmental conditions and outside dynamic,is a multi-stage, multiple slump-accumulation composed of large complex plasmodium, with multi-layer overlay, complex structure, thickness and other characteristics.(2) It made respectively consolidated undrained triaxial creep tests in 200kPa,300kPa,400kPa confining pressures by the reshape of the natural water content and natural density of slide soil of Huangtupo riverside slump-accumulation No. II,and got the the slide soil's long-term strength: C∞=16.70kPa,φ∞=12.46°。(3) Saturation line distribution characteristics in different time when the reservoir water level go up and down by the landslide seepage field and found the saturation line is concave shape downward when reservoir water is rising, saturation line is convex shape upward; The changes of saturation line lag behind the changes of reservoir water level. The shape of saturation line is different due to different reservoir water level at the same reservoir water level.Saturation line influenced by the reservoir water level with increasing distance from the decrease.And also found that the pore water pressure and reservoir water level changes in the same of observation points under the reservoir water level,only the twice changes of pore water pressure when reservoir water level get through the observation point;pore water pressure influenced by the distance from the surface of the water level upon the observation points.(4) It analyzed the stability and reliability of riverside slump-accumulation II on the basis of the actual operating condition of the Three Gorges Reservoir and found:The factor of safety is gradually increase and failure probability is gradually decrease when reservoir water level is rising;The factor of safety is decrease and then increase,failure probability is increase and then increase when reservoir water level is descending. When reservoir water level remained unchanged, the factor of safety is gradually decrease and the failure probability is gradually increase in the water level of 175m, the factor of safety is gradually increase and the failure probability is gradually decrease in the water level of 145m. If reservoir water levels remain unchanged long enough, the landslide factor of safety will tend to a steady value, landslide factor of safety is bigger when reservoir water levels is rising than descending. The result of landslide factor of safety and reliability is consistent. It also found the landslide factor of safety increased by 3% when it considered the suction than not.(5) Huangtupo riverside slump-accumulation II's reliability is in low risk and medium dangerous under the normal condition of reservoir water level according slide position based on survey data, the whole of landslide is basically stable; But that does not mean any area of landslide is in the state of stability, it sought out the most dangerous area when landslide is in the most dangerous moment on the base of stability and reliability analysis, and the slide surface appear in the front landslide, cut export is under the reservoir water level, the area is in high-risk state(not considering the existing support measures).