Research On Stability And Prediction For The Colluvial Landslide In The Three Gorges Reservoir

China is one of the most serious countries, which develops six main types geological disasters including landslide, collapse, debris flow, ground collapse, ground crack and ground subsidence. Statistics data of China Geological Survey shows that only in August2012, the national total of geological disasters is5752, including4841landslides,412collapse,415debris flow,56ground collapse,25ground crack and3ground subsidence,twenty six of which caused casualties,containing that53people were killed,25people were missed,75people were injured, and direct economic loss was3.23billion yuan. According to the national geological disaster data in the official website of China geological survey, Ministry of Land and Resources, during the period from2004to2011, the main six types geological disaster every year caused a direct economic loss more than＄1.7billion, casualties more than400people, and in the six kinds of geological disaster, landslide was developed most widely, whose proportion was as high as50.60%～86.11%, accounted for more than half of the geological disasters.Annual economic loss caused by landslide disasters in China is more than＄1billion, according to the eological hazard reporting data of China Geological Survey, only in2009,16oversize and large landslide disasters were developed, and the direct economic loss was190million yuan, and in2011the major disasters which caused more than10people killed and missed or the direct economic loss exceed100million yuan were7, and the direct economic loss was599million yuan. Landslide disasters not only cause huge economic losses, but also cause serious damage to people’s life safety, in1963Vajont landslide in Italian killed more than2600people, in1983Sa Leshan landslide in Dongxiang county in Gansu province destroyed four village, killed220people and injured22people, India reported in2000YiGongCangBu high-speed giant landslide in Tibet were killed30people, more than100people were missed and caused50000homeless in India, in2003Qian Jiangping landslide in Zigui country in Hubei province killed24people and caused more than1100homeless, in2005Southern Leyte landslide disaster in Philippines caused casualties more than1800people. The colluvial landslide is a kind of hazard-affected body which is the most widely distributed and large scale developed, whose outbreak frequency is high, abruptness is strong and continuous harm is large. This kind of landslide always happens in the quaternary and modern loose accumulation, whose sliding body material is composed with the secondary deposit, such as colluvial, colluvial deposit, alluvial deposit and colluvial deposit, whose sliding surface is the contact surface between accumulation and bedrock, which distributes in the Yangtze three gorges, the Yellow River shelter-forest, and Hong Kong, Guangdong, Fujian and other places, and the most widely in the three gorges of Yangtze river. In accordance with Landslide control engineering survey specification (DZ/T0218-2006), the colluvial landslide contians landslide deposit landslide, collapse deposit landslide, avalanche deposit landslide, loess landslide, clay landslide, the diluvial layer landslide, and artificial filled soil landslide. For the three gorges reservoir area, the colluvial landslide mainly develops landslide deposit landslide, collapse deposit landslide, avalanche deposit landslide, and the diluvial layer landslide.Because of its unique geological tectonics and geological environment characteristics,a large number of landslides are developed in the Three gorges reservoir area. According to incomplete count, only in the upper Yangtze river area of one million kilometer, there has developed one thousand,seven hundred and thirty six landslides, sixty four of which are colluvial landslides, and in two and three stage control and monitoring landslides disaster or collapses in the Three gorges reservoir area, the rate of colluvial landslides is about80%. According to the statistical analysis, recently most of colluvial landslides lie a potential unstable state under rainfall and reservoir water level scheduled, and they will trend to a unstable state with time. For the three gorges reservoir area, landslide instability and failure not only causes loss of life and property, at the same time it will jam channel, more important is the damage of secondary surge disaster, Vajont landslide in Italy is the best proof. Therefore, the stability of colluvial landslides in the three gorges reservoir directly relates to the people’s life and property safety and normal operation of the reservoir,studying on the stability and prediction of colluvial landslides in the Three gorges reservoir area has an important theoretical and practical significance.This paper takes colluvial landslides in the Three gorges reservoir area as the research object, uses laboratory test, the promotion of Bayes method, deep displacement monitoring data quantitative analysis, numerical simulation and displacement monitoring back analysis, the neural network model to study selecting and predicting landslide shear strength parameters based on the statistical analysis to development law of colluvial landslides in the Three gorges reservoir area. At the same time, adopts the shear strength reduction method, Rosenblueth calculation method and Unit safety method to discuss whole and local stability of landslides by FLAC3D numerical simulation software, then employs the landslide GPS professional monitoring, landslide deep displacement monitoring, numerical analysis model and a nonlinear model to research landslide displacement prediction model, landslide time prediction model and the criterion considering he reservoir level and rainfall. Through the above research, finally get the following conclusion:(1) Get the distribution law, development characteristics and deformation failure characteristics of colluvial landslides in the Three gorges reservoir area by the statistical analysis.Colluvial landslides in the Three gorges reservoir area are developed in easy sliding formation such as mudstones of badong group in the middle Triassic and mudstone and sandstone interbedding of shaximiao group in the middle Jurassic, widely distributed in six districts and counties, zigui, badong, wushan and fengjie, YunYang and wanzhou. Most of the colluvial landslides is the super large, large landslides and deep landslides, whose profile form is convex and composition is silty clay containing gravel of residual deposit(Qel),slope deposit(Qdl), colluvial deposit(Qcol) and landslide accumulation (Qdcl) in Quaternary, and the large-scale landslide, dip slope and middle pour slope (10°～40°) is the most developed. The front and rear edge of colluvial landslides is200m elevation and725m elevation. The landslide surface cumulative displacement tracing patterns have six type, stable type, uniform type, convergence speed type, acceleration type, step type and back type, and deep displacement curves mainly contain "V" type,"D" type,"B" type,"r" type,"pendulum" type and "composite" type including three deformation failure modes,the creep-rip, slip-uplift-the overall downturn, slip-rip-cut.(2) Set up the selection and prediction methods of shear strength parameters for colluvial landslides in the Three gorges reservoir area.Through studying laboratory test, the promotion of Bayes method, numerical simulation and displacement monitoring back analysis method, deep displacement monitoring data quantitative analysis method and the BP neural network prediction method for the landslide shear strength parameters, taking typic colluvial landslides in the Three gorges reservoir area as examples, obtain the principle and applicable condition of selection and prediction methods for colluvial landslide shear strength parameters in the three gorges reservoir area:①as the result of sampling disturbance, test conditions, test error and landslide space diversity or other reasons, laboratory test results has a lot of uncertainty and its reliability has a tremendous influence, and for colluvial landslides in the three gorges reservoir area, landslide shear parameters are generally between the peak strength and residual strength, saturation and natural state coexist, test results cannot be directly applied to the landslide stability analysis etc, generally as a back analysis method based data.②the key point for the promotion of Bayes method back analyzing the distribution form of landslide shear strength parameters lies in the selection of a Prior function which generally take regional statistical rules as, and this method is mainly suitable for the landslides that lack of enough laboratory test group, and the analysis results are used to calculate the landslide failure probability for the landslide stability quantitative evaluation.③the numerical simulation and the displacement monitoring back analysis method for landslide shear strength parameters takes the laboratory test data or regional statistical rule as the foundation and the landslide monitoring displacement and displacement trend as judgment basis, and adopts the numerical simulation method to select the optimal combination of landslide shear strength parameters. This method can be applied to a large number of landslides in the three gorges reservoir area which lack of a detailed survey just have a professional monitoring, but the result is susceptible to be affected by numerical simulation software defect and the accuracy of landslide professional monitoring results.④the landslide shear strength parameter back analysis method based on quantitative analysis to the landslide deep displacement monitoring data,takes the process of landslide slip zone from full to complete destruction as the process of landslide shear strength from the peak strength to residual strength, and uses a proportional reduction for cohesion and internal friction angle. This method is mainly suitable for landslides which have a deep displacement monitoring and whose displacement monitoring curves have a typical curve characteristics, taking laboratory test data or regional statistical rule as the foundation.⑤landslide shear strength parameter prediction is a method applying the neural network model to predict landslide shear strength parameters, based on the relationship between shear strength parameters and landslide basic physical and mechanical parameters by regional statistical rule. This method is only applicable for landslides which have basic physical and mechanical test but lack of shear strength parameters test, especially for the landslides not conducting residual strength tests because of the time, test equipment and other reasons.(3) Establish a whole and local stability evaluation method for colluvial landslides taking typic ones as examples in the Three gorges reservoir area.Taking TangJiao NO.1landslide which is a typic colluvial landslide of three gorges reservoir area in WanZhou district ChongQing province as an example, firstly determine the deformation of the landslide through the landslide site investigation and professional monitoring results analysis. and establish the landslide three-dimensional geological model. Secondly adopt the shear strength reduction method and Rosenblueth calculation method to study the3-d whole stability coefficient and failure probability stability for the landslide, and compare the results with2-d stability coefficient and failure probability results to comprehensively analyze the whole stability for the landslide from the point of qualitative and quantitative view. Finally, apply numerical analysis and unit safety method to analyze and research stability coefficient distribution, displacement field, stress and strain field, and plastic zone distribution of the landslide, and make a comparison validation with field professional monitoring and macroscopic deformation survey to study the local stability of the landslide.Landslide case study results show that landslide3-d stability calculation result is bigger than those of2-d calculation results, but landslide3-d failure probability is smaller than those of2-d calculation results; Landslide calculation main section does not represent the whole landslide, and it is better to use3-d calculation method for landslide whole stability evaluation; Landslide whole stability has a big difference with local stability, the landslide stability evaluation not only has to calculate the integral stability of the landslide, but also assess landslide local stability according to analysis results of the field investigation and landslide professional monitoring, especially for large colluvial landslide.(4) From the view of considering reservoir level, rainfall or other external factors, set up the displacement prediction, time prediction model and method for colluvial landslides in the three gorges reservoir area.Taking considering the mainly external influence factors rainfall and reservoir water level of colluvial landslides in the three gorges reservoir area as a starting point, landslide whole and local stability evaluation results as a judgement, mathematical mechanics analysis and numerical simulation as a means, R/S analysis method, loading and unloading response ratio theory as the research method, establish the displacement prediction, time prediction model and method for colluvial landslides in the three gorges reservoir area:①confirm the proportion of trend displacement and cycle displacement in landslide displacement time series according to the deviation degree between Hurst index contained by R/S analysis method and0.5, then use the neural network model and polynomial fitting to predict landslide trend displacement and cycle displacement separately. This method fully consider the influence of rainfall and reservoir water level,and is suitable for predicting displacement of colluvial landslides in the three gorges reservoir area.②the landslide loading and unloading response ratio time prediction model forecasts landslide instability and failure time based on the relative size between loading and unloading response ratio and1, taking reservoir water level difference between adjacent time for the loading and unloading increment, corresponding landslide cumulative displacement acceleration difference between adjacent time for the loading and unloading response increment. This method fully considers the special human engineering activities in the three gorges reservoir area-reservior level scheduling, which has a great significance for colluvial landslide time prediction in the three gorges reservoir area.③simplify the colluvial landslide in the three gorges reservoir area as a single linear sliding surface landslide, utilize the above-mentioned relationship between landslide shear strength parameters and slip zone integrity index, adopt he residual thrust method to establish the relationship between the landslide stability coefficient and slip zone integrity index, and take slip zone integrity index while the landslide stability coefficient is equal to1as a landslide time prediction criterion. According to landslide examples test, this method considers the changing characteristics of landslide shear strength with landslide deformation and can be further discussed as an effective prediction method.④the landslide time prediction criterion research methods by numerical analysis takes the finite element calculation misconvergence, landslide plastic area cutthrough and displacement mutations of displacement monitoring key point as the landslide instability and failure criterion, and takes the landslide displacement of displacement monitoring key point when landslide is at an instability and failure state, and the rainfall size under extreme working condition as landslide instability and failure criterion, applying the numerical simulation means. This method fully considers the influence of rainfall and reservoir water level for landslides and provides a new idea to study landslide time prediction criterion. However, as a result of the certain difference among constitutive model, landslide boundary conditions, and landslide calculation parameters in numerical simulation means with landslide actual situation, lead to exist a certain access between landslides simulation results and the actual situation. Therefore, it need to discuss continuously for the practicability of this method.