Study On Deformation And Failure Mechanism Of Typical Reverse Rock Bank Slope Under Rock Mass Deterioration In Drawdown Zone Of Reservoir Area

The geological structure of the Three Gorges Reservoir area is complex,geological disasters occur frequently,and a large number of high and steep bank slopes are bred.After the normal operation of the Three Gorges Reservoir area,the reservoir water level fluctuates periodically between 145～175m,and the water storage and water level rise and fall periodically,forming a water fluctuation zone of up to 30 m.The complex hydrodynamic conditions make the rock mass in the drawdown zone deteriorate strongly,and a large number of geological disasters are formed or accelerated.This paper collects the typical reverse rock bank slope cases in the Three Gorges,Daning River and other trunk and tributary canyons where the deterioration of limestone rock mass is concentrated,and analyzes its common geological characteristics and rock mass deterioration trend.Taking the limestone reverse rock bank slope as the main research object and Gongjiafang landslide as the prototype,through field investigation and indoor analysis,by means of engineering geological analysis,physical model test and numerical simulation calculation,this paper explores the structural characteristics of slope rock mass in Gongjiafang area,and analyzes the deterioration type and trend of typical limestone reverse rock bank slope,The influence of rock mass deterioration in Drawdown zone on the deformation and failure of reverse rock bank slope is systematically studied.At the same time,the deformation failure mechanism and instability mode of typical reverse rock bank slope with rock mass deterioration in the drawdown zone of the reservoir area are explored.The results of this study can provide calculation support for the identification of hidden points of the same type of geological disasters and disaster prevention and reduction in the Three Gorges Reservoir area,and have great practical significance to ensure the smooth operation of the golden waterway and the safety of people’s property in the region.The research results are as follows:(1)Through the field investigation and indoor analysis of the engineering geological conditions of limestone reverse rock bank slope,a common engineering geological model is summarized: this kind of bank slope is limestone karst bank slope with reverse layered structure.Moreover,long-term toppling deformation is caused by many factors such as valley undercutting,slope toe deterioration and damage.At the same time,the investigation shows that the slope toe is an obvious anti sliding section,which is the key to the stability of bank slope.(2)The deterioration trend of typical reverse rock bank slope is analyzed,and the deterioration trend of typical reverse rock bank slope is explored.According to the long-term survey from 2009 to 2013,the annual erosion rate of Gongjiafang slope is 0.18m/y and the erosion depth is 2.2m.The annual erosion rate of Maocao slope is 3.5m/y,the erosion depth is0.29 M,and the value of deterioration variable de increases year by year.In 2019,the cumulative horizontal displacement of the deformed slope from Xiaodongcao to Zhengjiadagou also reached 254.5mm,and the cumulative settlement displacement was-140.0mm.(3)Taking Gongjiafang landslide as the prototype,based on the similarity theory,the similarity ratio n = 200 is taken.On the basis of previous studies,the material selection and proportioning tests of the model are carried out,and the best similar materials in the model test are selected through comparison.The mixed materials of limestone strip,polyvinyl acetate glue and gypsum are selected.The ratio of gypsum: quartz powder = 2.5:2.5:1:0.55.(4)Taking Gongjiafang landslide as the prototype,a large-scale physical model test based on similarity theory is carried out to study the deformation and failure mechanism of typical reverse rock bank,and explore the failure mode of this kind of landslide.That is,after the rock mass at the foot of the slope is deteriorated,the rock mass at the foot of the slope is loose and the bank collapses,due to its own gravity,the trailing edge sinks,resulting in tensile cracks,shear creep at the front,bending deformation from the layer to the free surface,extrusion forms a bending zone inside the slope,and multi-level sliding surfaces appear.When the shear stress exceeds the shear strength of the rock mass,the bank slope is bent and toppled.(5)Through a series of tests,the influence of rock mass deterioration in the drawdown zone of the reservoir area on the slope deformation of the reservoir bank is analyzed under multiple working conditions.The deformation and failure characteristics of bank slope under different deterioration degrees can be reflected according to the size of erosion area.In working condition 1,after impoundment,the erosion area deteriorates,cracks expand,only the shallow surface collapses,the rock stratum has no large deformation,and the overall deformation of the bank slope is small,which also reflects that the bank slope in the reservoir area can remain stable after the deterioration of the shallow surface.Under condition 2,no obvious damage can be seen before water storage,but after water storage,the rock mass begins to deteriorate and the structure begins to degrade.Under the pushing of the overlying rock mass load,shallow landslide finally occurs,but the bank slope has large bending deformation.In condition 3,there are a large number of fractures before water storage,which also shows that many dangerous rock fractures in the reservoir area are developed and have not been damaged before the operation of the reservoir area.It shows that under this working condition,creep occurs in the bank slope,the structure is changing,and it is still in a stable state as a whole.After impoundment,deep landslide occurs on the bank slope,with large rock mass deformation and obvious bending and sliding phenomenon.(6)Combined with the discrete element software,the numerical model of reverse bank slope is constructed,and the deformation and failure process of reverse rock bank slope is inversed.The numerical simulation results are basically consistent with the physical model results,and the apparent phenomena are also basically consistent with the field investigation results.At the same time,the displacement field and plastic deformation zone in the process of model deformation are analyzed.With the erosion and bank collapse of the slope toe,the displacement increases step by step,the failure area expands from the surface to the inside,the range of plastic area increases from bottom to top,the failure unit is widened,lengthened and then connected,and multiple failure surfaces run through,resulting in bending and sliding failure.