| After the reservoir is impounded,the bank slope often has geological disasters such as bank collapse or landslide,and the main factors that induce these disasters are closely related to the transient water level in the bank slope.The upper reaches of the Lancang River Miaowei hydropower station Large Chute Ditch-Gully slope in the water level steep zone,the transient change of water level and stability of bank slope under the influence of external factors such as reservoir storage and rainfall have become the engineering and technical issues concerned by the construction department.In this paper,the large chute-gully bank slope as the research object,based on a large number of field work to identify the structural characteristics of bank slope;The rock and soil parameters of the bank slope are obtained by summarizing the rock mechanics test of the right bank of the Miao Wei Hydropower Station and the supplementary shear strength test,the particle analysis test and the soil water characteristic curve about accumulation body;Based on the saturated unsaturated seepage theory,the development law of transient saturation zone and the magnitude of transient water load are calculated;Combined with the three-dimensional finite difference method,the deformation response of the bank slope due to the transient water level is calculated,and the deformation mechanism of the bank slope is analyzed;The stability of bank slope is evaluated by using limit equilibrium theory.The main results are as follows:(1).The large chute-gully bank slope is located on the right bank of the Miaowei hydropower station reservoir area,which is monoclinic terrain slope,and the landform type is the deep mountain valley.The slope bedrock is in Jurassic huakaizuo lower segment(J2h1)metamorphic quartz sandstone,slate,phyllite,overlying quaternary talus base terrace.Influence of slope in the valley incision under the formation of the slope is 45 degrees ~50 degrees,the middle upper part is 25~35 degrees,and the groove is steep.In the study area,there is no regional fault,and the three direction stress state is dominated by potential strike slip,and the stress concentration is low.(2)By means of high density electrical prospecting,the undulating forms of the interface between the accumulation and the bedrock have been investigated.Based on the investigation of the data of bank slope facies,the structural grading system of bank slope is put forward,the structural plane of the slope rock mass includes class II structural plane(Compressive fracture zone F146),class ?2 structural plane(Interlayer dislocation belt),class ? structural plane(Bedding surface),class ? structural plane(joint J1 and J2).According to the variation of rock mass structure with depth,a classification system of toppling rock mass is put forward,and the toppling rock mass of the slope is divided into strong toppling rock and weak toppling rock with horizontal depth of 10~30m and 50~70m respectively.Based on the results of the previous survey and the results of the supplementary engineering geological survey,the geological concept model composed of accumulation body,strong dumping rock,weak dumping rock,unconsolidated rock mass and fractured fault zone F146 was constructed.(3)The rock shear strength index of bank slope rock and soil was obtained through the rock mechanics test of the right bank of the dam site and the accumulation direct shear test,based on the test data of borehole pressure water,the seepage characteristics of dumped rock mass are evaluated,and the water holding capacity and unsaturated permeability of the accumulation body are evaluated by the SWCC test of the accumulation.According to the above experimental results,the calculation parameters of rock and soil on the bank slope are studied.(4)The transient seepage field of the slope is calculated according to the saturated-unsaturated seepage theory,the results show that the influence of reservoir water level decline on the transient water level is mainly the decline of transient phreatic water level,and the water pressure is the hydrostatic pressure between the transient water level and the steady water level after the decline;In the flood season,the transient saturation zone of the bank slope is developed in the form of "water film" on the surface of the accumulation body and the cutting slope.After the rainfall intensity is increased to the occasional rain intensity,the transient saturation zone is rapidly expanding to "saturated shell",The saturated water pressure value of the saturated area is also increased,and the transient saturation zone after rainfall is shifted down and gradually dissipated under the control of gravity.The transient water load before and after the rainfall stop can be equivalent to the uniform load and the hydrostatic load after the reduction,but the value of the equivalent load is related to the rainfall intensity,the saturated permeability and the water holding capacity.(5)Based on the three-dimensional finite difference numerical simulation,the natural stress field of the bank slope and the deformation response under the condition of water storage and long term heavy rainfall are calculated.The deformation response caused by water storage is mainly distributed under the water level.The maximum amount of displacement is 0.3m,and the plastic deformation is occurred in this area,the bank slope is easy to produce bank collapse under the action of river erosion;When the long term heavy rainfall causes the bank slope to be saturated,the deformation response of the accumulation body is remarkable,the front and back boundaries are respectively slope surface elevation about 1440 m and the back edge of accumulation body,the maximum deformation is about 14.2m,and the additional displacement contour is circular.Based on the geomechanical theory of plate beam,the fracture depth of the toppling rock mass under natural and saturated conditions is calculated to be 31.77 m and 16.10 m respectively.The slope deformation response as the foundation,combined with the deformation theory of plate and beam geological mechanics theory to analyze the deformation and failure mechanism of slope,slope deformation potential failure mode for sliding along the base cover interface and strong tensile fracture rock dumping.(6)The stability of the dumping block with a large geometric dimension on the cutting slope is less affected by the transient water pressure,but the stability of the block due to the structural cutting of the surface layer ? and ? is weak,which is the causes of local shallow rupture.The stability coefficient of the bank slope based on the limit equilibrium theory shows that the stability of the natural condition of the bank slope is better,Water storage and heavy rain are difficult to cause the overall sliding of the bank slope in service life,the stability of waterlogging and reservoir water level is obviously weakened for the stability of the No.3 slip surface,and the stability coefficient is only 1.087 at the sudden drop of water level,and it is in the critical stable state;The rainstorm has the biggest disturbance to the stability of No.2 slip surface,so there are many cracks in the cutting slope during the flood season... |
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