Comprehensive Identification Of The Evolution Stage Of Typical Reservoir Bank Landslides Based On Centrifugal Model Test

China is one of the countries which is affected by landslides most seriously in the world.Mountainous regions and Plateaus cover 69% of the whole area of China,resulting geohazards such as collapses,landslides and debris flows happen frequently.Geohazards in China have the characteristics that wide coverage,complicated mechanism,great damage,high mortality and huge impact.These geohazards have caused great losses to people's lives and properties.Since 2001,geohazards in our country have caused a total of 10,705 deaths(missing),resulting nearly 70 billion RMB of direct economic losses.The geohazards in China mainly include landslides,collapses,debris flows,surface subsidence,ground settlings and ground fissures,among which landslides accounted for 71%.It is clearly that landslides are the most dangerous category.Since the impoundment of the Three Gorges Reservoir in 2003,the hydrogeological conditions of the reservoir banks have changed greatly.The cyclical fluctuation of reservoir water and rainfall have induced many landslides,majority of which are colluvial landslides.Colluvial landslide is a type of landslide,which occurred in Quaternary and modern loose colluvial.It is one of the most widely distributed,highly frequent outbreak and the most harmful geohazards,which mostly occurs in residential areas,slopes and farming areas with the characteristics of repetition and inheritance.The Three Gorges Project,a landmark project of China's water conservancy and hydropower project,has important political,economic and social status.It is of great significance to study identification of the evolution stage of the typical reservoir bank landslides in the Three Gorges Reservoir.In this paper,the Liangshuijing landslide,which is in the Yunyang county of the three gorges reservoir region,is taken as the research object.Firstly,a 1:80 physical simplified model of the landslide is established based on geological survey and sampling,the centrifuge model test of debris landslide under the combined condition of reservoir water fluctuation and rainfall is carried out,which could obtain the key experiment data in the process include laser displacement,pore water pressure and earth pressure,and discussed the destruction process and evolution model of the landslide model.Secondly,based on the centrifugal model test,the finite element numerical simulation study is carried out,and the simulation results are analyzed from the perspectives of seepage field,displacement field and stability,and compared with the centrifugal model test for verification.Then,based on the centrifugal model test and numerical simulation results of the above,the fractal parameter Hurst index R/S analysis,displacement vector angle analysis,displacement multifractal dimension analysis,energy dissipation analysis and reliability analysis,etc.of displacement data are carried out respectively.Based on the displacement vector angle-displacement multifractal dimension,the criterion of landslide evolution stage,the criterion of landslide,the criterion of slide coefficient-energy probabilistic entropy change rate and the criterion of reliability index change rate are proposed.And the multi-criteria comprehensive judgment model based on the antidistance weighting expectation is established.Lastly,taking liangshuijing landslide as an example,this paper comprehensively introduces the basic characteristics of the landslide,analyzes the monitoring data and the cause mechanism of liangshuijing landslide in detail,and summarizes the evolution model of the landslide.The numerical simulations of liangshuijing landslide under six extreme conditions are carried out,and the numerical simulation results are analyzed from three aspects of seepage field,displacement field and stability.Based on the three criteria mentioned above,three evolution stages are divided,and combined with the anti-distance weighting expectation proposed above,a multi-criteria comprehensive judgment study is conducted on the evolution stages of liangshuijing landslide under extreme working conditions.The main conclusions of this paper are as followings:(1)Centrifugal model tests were carried out: 1)pore pressure monitoring data,laser displacement monitoring data and earth pressure monitoring data show that many deformation processes occurred during the experiment.In the process of rainfall,slip signs appeared in the middle platform of the landslide model,and the whole slip signs appeared in the process of water level falling.2)The results of water content measurement after the test show that the higher water content of the model is mainly located at the foot of the front slope and the central platform of the model.In the middle and front part of the landslide model,the water content contour extends to the interior along the line parallel to the slope surface,and the water content decreases gradually with the depth.3)After the experiment,there are many transverse cracks in the landslide,and the width of the cracks is mostly 1-5 mm.Among them,there are obvious signs of collapse at the third crack in the rear edge.There are five transverse cracks in the middle and front of the model.Among them,the first three are mainly caused by traction damage,which is caused by the rise and fall of reservoir water.The latter two are caused by lapse damage and rainfall factors.4)According to the development process of cracks in the centrifugal model test,this paper divides it into four stages: the first stage is the appearance stage of micro-cracks;the second stage is the development stage of cracks;the third stage is the multi-stage sliding stage;and the fourth stage is the overall sliding stage.5)In the process of rising water level,under the influence of water level,the leading edge first appears traction failure mode.Subsequently,under the influence of rainfall,the deformation and slip of the platform in the middle part of the model resulted in a progressive failure mode in the middle and rear parts.Finally,in the process of water level decline,the deformation zones of the two modes merge gradually,and finally show a typical composite failure mode.(2)Finite element simulation of the centrifugal model test was carried out.Through numerical simulation analysis of the whole test process,the change rules of seepage field,displacement field and stability in the test process are mainly obtained.The main conclusions are as follows: 1)The difference between numerical simulation results and centrifugal test results is reflected in: in the seepage field,due to the simulation of seepage field without considering the deformation and failure of slope body,the pore water pressure in the simulation is smaller than the site test,which is one of the reasons for the significant difference in water content distribution after the test.In addition,another reason for the difference in moisture content distribution is that the actual situation of rainwater flowing down the slope is not considered.In the displacement field,the displacement field obtained by numerical simulation can only represent the displacement change trend and development process,and the absolute value cannot reflect the actual value.2)In the case of no rainfall condition,the falling water level of the reservoir has a limited impact on the deformation of the landslide,which mainly causes the large deformation of the toe area and has little impact on the platform and the rear of the platform.However,reservoir water level drops after the rainfall,the landslide is more likely to be affected by the toe deformation and occurs traction progressive failure.The rainfall before the reservoir water level drops will intensify the damage of the reservoir bank landslide caused by the reservoir water level drop.3)The stability analysis shows that the landslide stability increases during the water level rising stage.Rainfall makes the stability of landslide decrease suddenly,but with the infiltration of rainwater,the stability is restored to some extent.Falling of the water level is not conducive to slope stability.4)The deformation and failure process of landslide model can be summarized as: when the soil at the toe of the slope is immersed in water,the shear strength decreases,and the deformation occurs first.Rainfall causes the surface layer pore water pressure of the landslide to rise sharply,which directly causes the large deformation of the surface soil.With the infiltration of rainwater,the deformation gradually spreads downward,and the drop of water level makes the toe deformation area expand upward and broken through with the failure area at the rear,and eventually showed overall failure.(3)The evolution stage of the centrifugal model test was studied.The fractal parameter Hurst index R/S analysis,displacement vector angle analysis,displacement multifractal dimension analysis,energy dissipation analysis and reliability analysis,etc.of displacement data are carried out respectively,thus based on the displacement vector angle-displacement multifractal dimension,the criterion of landslide evolution stage,the criterion of landslide,the criterion of slide coefficient-energy probabilistic entropy change rate and the criterion of reliability index change rate were proposed.And the multi-criteria comprehensive judgment model based on the anti-distance weighting expectation is established.The main conclusions are as follows: 1)Based on the calculation results of displacement vector Angle and displacement multifractal dimension,the joint criterion of landslide displacement vector Angle and displacement multifractal dimension is established in this paper,and the evolution of landslide model is divided into the following four stages: inicial creep stage;steady creep stage;accelerated creep stage;overall slidding stage.2)Based on the slices method,the slide coefficient ? is defined from the angle of landslide energy conversion and dissipation.Based on the principle of virtual displacement,the analytical formula of slide coefficient with or without underground water level is derived.Based on the numerical simulation data,the energy dissipation of the landslide system was studied,the energy probabilistic entropy change rate of the landslide system was obtained,and the slide coefficient-energy probabilistic entropy change rate criterion of the landslide was established.3)Combining with the current research results of domestic and foreign scholars,this paper divides the uncertain factors of landslide analysis in the field of engineering geology into geological uncertainty,model uncertainty,and the uncertainty of geotechnical parameters.The reliability analysis of landslide model considering uncertainty factors is carried out and the change rate of landslide reliability index is established.4)Through the anti-distance weighting method to solve the weight of the three criteria,and then respectively to obtain the mathematical expectation of each interval,finally achieve the joint judgment of the three criteria.(4)The case study of liangshuijing landslide was carried out.Taking liangshuijing landslide as an example,the basic characteristics of liangshuijing landslide are comprehensively introduced,the monitoring data and the cause mechanism of liangshuijing landslide are analyzed in detail,the evolution model of liangshuijing landslide is summarized,the numerical simulation of liangshuijing landslide is carried out,and the judgment and recognition of liangshuijing landslide evolution stage is also studied.The main conclusions are as follows: 1)Liangshuijing landslide belongs to bedded type deep large,resurrection type soil old landslide.Under the action of rainstorm and reservoir water fluctuation,the shear strength of the slide surface decreases,and the anti-slide section in the front of the landslide itself effective gravity is reduced.In addition,the long-term erosion at the reservoir fluctuation causes the toe of the slide body to be denuded,which leads to the reduction of the resistance of the slide section of the landslide and the decrease of the stability of the landslide,which will cause the slide failure to the Yangtze river.2)Numerical simulation studies show that under the condition of no rainfall,Liangshuijing landslide is mainly affected by the falling speed of reservoir water level,and the deformation and failure start from the toe,presenting a relatively obvious characteristics of traction landslide.Under rainfall conditions,the toe of the slope body is still mainly affected by the fluctuation of reservoir water level,while the rear of the slope is affected by the rainfall infiltration.With the increase of rainfall,the deformation area and deformation amount of the rear edge increase simultaneously,and finally presents the evolution mode of the complex landslide.3)Based on the multi-criteria comprehensive judgment model of anti-distance weighted expectation proposed above,the comprehensive judgment was made on the evolution stages of liangshuijing landslide under 6 working conditions,and the time when liangshuijing landslide entered the overall sliding stage under 6 working conditions was respectively determined to be 105 days,95 days,93 days,88 days,86 days and 85 days.4)With the increase of the water level falling speed and rainfall recurrence period,the overall slip stage began time obviously ahead of schedule.Therefore,in the process of landslide prediction and warning,the water level falling speed and rainfall should be strictly monitored.It is suggested that the overall slip stage should be taken as the warning time.