The Stability And Control Measures Research Of A Reservoir Landslide In Qinghai Province

The landslide is located in the left bank 2#beam and 3#beam in front of a reservoir dam in Qinghai Province,only 5m～220m away from the axis of the dam.Field investigation and exploration show that the landslide is mainly composed of two groups of NE trending steep joint structural planes and rock strata,and the main sliding direction of the landslide is NE45°.The shear outlet at the front edge of the landslide lies under the riverbed,and the rear edge extends to the watershed.The maximum height difference of the landslide is about 198m.The slope is in the form of steep front to gentle back,with an average slope of 35°～40°and an average slope of about 20°at the upper part and 50°to 70°at the lower part.The total volume of landslide is about 334×10⁴m³.The sliding mass is mainly formed by unstable accumulation of slope,mainly composed of gravel soil formed by purple red and dark purplish red thick conglomerate of Cretaceous Hekou formation（K₁hk）and a small amount of Aeolian Loess at the back edge.After landslide sliding,there are a lot of tension cracks on the surface of the slope body.After the slope sliding,the front edge of the slope body moves only 20m,the landslide body is not completely disintegrated,and there is still a certain degree of integrity.The occurrence of landslide seriously affects the normal construction of the project.At the same time,once the landslide accumulation body slides again under the condition of heavy rainfall or reservoir water storage,the surge caused by the project construction and reservoir impoundment not only affects the normal use of the reservoir,at the same time,it seriously threatens the life,property and safety of downstream residents.Therefore,it is of great significance to systematically study the basic characteristics,genetic mechanism,stability and engineering treatment effect of the landslide to ensure the safe operation of the reservoir.Through field investigation and exploration,combined with indoor and outdoor tests,the morphological characteristics,material composition and structure,deformation and failure characteristics and genetic mechanism of landslide are studied,Finally,through geological macro analysis,limit equilibrium calculation and three-dimensional numerical simulation calculation,the stability of landslide under various working conditions before and after landslide treatment is systematically analyzed,which improves the geological basis for the safe operation of the reservoir.The main research contents and achievements are as follows:（1）According to the deformation degree,material composition and structure of different parts of the landslide,the landslide is divided into main sliding area（area I）and traction deformation area（area II）.The main sliding area is divided into main sliding area I₁ and I₂ according to the disintegration degree of sliding mass and free space conditions;the traction deformation area is divided into traction deformation area II₁ and II₂ according to the spatial sliding direction.（2）According to the field survey,the fault structure is not developed in the study area,and the slope rock is monoclinic structure.The lithology is thick conglomerate of Hekou formation（k1hk）of Lower Cretaceous system,and part of it is intercalated with andesitic basalt.The occurrence of rock stratum is NE50～80°NW∠20～40°and inclines outwards to the downstream.The slope structure type is inclined slope.The landslide is a wedge-shaped landslide composed of strata layer（muddy interlayer）and NE trending steep joint fissures.The upstream boundary is controlled by the structural plane with the occurrence of NE44°～60°NW/56°～76°,the downstream boundary is the structural plane with occurrence（NE4°～27°SE/60-70°）,and the occurrence of rock layer（muddy interlayer）is NE50°～80°NW∠20～40°.The back edge of the main sliding area of the landslide is a steep dip structural plane of NW277°～292°NE/52°～88°.（3）The influencing factors of landslide formation are as follows:three faces of empty terrain and landform,special slope structure,long and through structural plane cutting the sliding mass into wedge,which makes the slope sliding have space and material basis,the extra long rainfall and the front edge construction provide the inducing conditions for the landslide.The formation of landslide can be divided into the following three stages:after the formation of the slope,tension cracks are formed at the back edge of the slope under the action of shallow surface reconstruction,and the unloading rebound deformation occurs at the free position of the front edge,and the tensile cracks at the back edge expand from top to bottom and deepen until the potential slip surface;the rainfall infiltrates along the through tensile fracture surface to soften and argillaceous rock layer（muddy interlayer）,and the slope body along the rock layer（muddy interlayer）;Under the trigger action of ultra long rainfall and leading edge construction,the critical equilibrium state of the slope is broken,the sliding surface is connected and the landslide is started.（4）Based on the detailed study of the physical and mechanical parameters of the landslide rock and soil,the limit equilibrium method is used to analyze the stability of the landslide.The results show that:before the impoundment,the main sliding area I₁is in a stable state,the main sliding area to the trailing edge of the traction deformation area is in a stable state,the secondary sliding mass in the main sliding area I₁ is in an unstable and unstable state,and the stability of the main sliding area I₂is in an unstable state The stability of landslide under the condition of reservoir impoundment shows that all areas are in unstable state.As the shape of the landslide is wedge,the overall stability of the landslide is analyzed by FLAC3D.The results show that:under the current conditions,the shear strain increment of landslide mainly develops at the structural plane of the front and rear edge of the slope,and it is not connected.The maximum displacement appears in the 1-1’section,and the value is about 0.3m.After impoundment,the maximum increment of shear stress in 1-1’and2-2’sections is 1e-1 under natural,rainfall and earthquake conditions.From the landslide displacement nephogram of 1-1’and 2-2’sections,the landslide displacement under various working conditions after impoundment is large,and the displacement under seismic condition is as high as 5.2m.Therefore,if the landslide is not treated,the reservoir will store water directly and the landslide will slide again.（5）Through the comparison and selection of treatment schemes,the slope cutting and foot pressing methods are adopted to treat the landslide,and the limit equilibrium method and finite difference numerical simulation are used to calculate the stability of the treated landslide under various working conditions under water storage conditions.The calculation results of limit equilibrium method show that each area of landslide after treatment is in stable state under natural,rainfall and earthquake conditions under water storage conditions;according to the numerical simulation results,all areas of landslide after treatment are also in stable state under natural,rainfall and earthquake conditions under water storage conditions,and the deformation of main landslide area I₁ mainly occurs at the front scarp of treatment platform,and the main sliding area I₂ changes The maximum displacement is only 0.0423m.Through the calculation of landslide stability,the results show that the slope cutting and pressure foot treatment method has certain reliability.With the help of seep/W and slope/W finite element method,the stability of landslide seepage field and reservoir water level rise and fall is studied.The results show that the stability of landslide is favorable to the rise of reservoir water level,but after the rise,the stability coefficient decreases gradually due to the increase of saturated volume of rock and soil;in the process of reservoir water level falling,the stability coefficient of landslide decreases gradually,the stability coefficient increased gradually.