Stability Analysis Of Colluvial Landslide In Relation To Variation Of Water Level In Wanzhou District,Three Gorges Reservoir

China is one of the most serious countries with geological hazards,Its main types are landslide,collapse,debris flow,ground collapse,ground fissure and land subsidence.According to the data of?the National Geological Hazard Circular?,during 2010-2019,there were 81554 landslides(excluding 2017),accounting for 71.51%of the total geological hazards.Census results show 100km² in the upper reaches of the Yangtze River 1736 landslides were found in the range,of which 64%were stacked landslides.After the first water storage in 2003,the reservoir water level fluctuates periodically between 145m-175m every year,forming a new dynamic discharge condition for the landslide groundwater in the reservoir bank accumulation layer,which induces a large number of reservoir bank accumulation layer landslides.Wanzhou District is one of the concentrated areas of reservoir bank accumulation layer landslide development,affected by the regional gentle dip angle platform bedrock,Wanzhou District reservoir bank accumulation layer landslide has the characteristics of terrain and smooth sliding surface,from the point of view of limit equilibrium,it is often difficult to start,its disaster cause,mechanism and deformation characteristics are unique.Therefore,it is of great significance to study the accumulation landslide in this kind of gentle dip stratum.On the basis of introducing the regional geological environment background of Wanzhou District,50 reservoir bank accumulation layer landslides are selected to systematically analyze their development rules and deformation evolution characteristics.At the same time,on the basis of summarizing the genetic mechanism of accumulation layer landslides,A landslide stability analysis method considering the expansive force of sliding soil is proposed.Based on the monitoring data of 30hydrological holes,the response characteristics of landslide groundwater in different geological structure accumulation layer to rainfall and reservoir water are summarized,and the prediction research of class I groundwater with double response of rainfall and reservoir water is carried out.Taking the B landslide and C landslide of Dazhou II landslide as an example,the Geo-studio software is used to simulate and analyze the variation law of landslide stability with different permeability,with the relationship of reservoir water fluctuation,and the sensitivity analysis of each influencing factor is carried out.Through the above research,the following conclusions are obtained:(1)Selecting 50 bank accumulation landslides with wading as study samples,From three aspects of distribution,geological structure and deformation evolution,the development law of reservoir bank landslide is analyzed.Elevation of landslide front edge is less than 175 m,minimum elevation is 110m,back edge elevation between185m-335m,and there is no correlation between the front and rear elevation;The slope of the landslide is small,between 5 and 25°,and landslides below 15°account for 82%of the total sample number;The profile forms are mainly convex,ladder,multi-stage,The plane shape is mainly skip shape and tongue shape;Both the longitudinal length and the transverse width of the landslide are between 200 and 600 m,the thickness is mainly middle landslide(10-25m),Large landslide(100-1000m ³)and medium landslides(10-100m³)primarily.The reservoir-bank accumulation layer affected by reservoir water in Wanzhou district has the characteristics of small slope and large flood ratio.The different geological structure characteristics of landslide can control the development of slide zone.The local relative water barrier caused by the difference of large diameter block stone and fragment stone content can change the development position of slide zone,take Sifangbei landslide CXK03?Sifangbei landslide CXK06?Malulin landslide CXK02 as an example.Based on the surface displacement monitoring curves of Sanzhouxi landslide and Tangjiao I landslide,the deformation evolution characteristics of floating support weight loss landslide and dynamic water pressure landslide induced by reservoir water fluctuation are summarized.(2)Based on the characteristics of landslide development and composition of slope body,and combination the source,the mechanism of groundwater action and the formation process of slip zone,the main influence factor of accumulation layer landslide developed on gentle dip strata in Wanzhou area is summarized as:(1)Primary environment weak intercalation(bentonite);(2)Fracture water and rainfall infiltration of bedrock at the back edge;(3)expansive sliding zone soil.The calculation method of transfer coefficient considering the expansive property of soil in slip zone is presented,the vertical expansion force(p_ey)upward of the vertical sliding surface and the lateral expansion force(p_ex)outward of the parallel sliding surface act on the strip,the two forces are affected by the degree of directional arrangement and saturation of the soil in the slip zone,therefore,it is generalized into three parameters:maximum vertical expansion force(p_eymax),lateral expansion coefficient(?),expansion degree(?),and the maximum vertical expansion force(p_eymax)and lateral expansion coefficient(?)are obtained by experiments,and the degree of expansion(?)is the ratio of the groundwater level to the highest groundwater level.For example,the landslide C of Dazhou II,through the monitoring data of deep displacement of the landslide,select the period 2016/4/15-2016/7/5 as the reservoir drops,when the deformation increases obviously,the stability coefficient is 1.343,and considering the coefficient of stability of the expansion force is 0.94,the stability calculation results and deformation characteristics of the expansion force are consistent.(3)By analyzing the data of 30 groundwater monitoring holes and summarizing the response law of landslide groundwater with rainfall and reservoir water fluctuation,four categories are classified:(1)class I groundwater monitoring holes near the reservoir bank,and affected by rainfall and reservoir water,showing the characteristics of“double response”;(2)class II groundwater monitoring holes are higher than class I,less affected by reservoir water,mainly affected by rainfall,showing the characteristics of“steep rise and slow drop”;(3)class III groundwater monitoring holes are generally located at the junction of platform sill before steep cliff and multistage platform,and the rainfall response includes double recharge of surface infiltration and bedrock fissure water,showing the characteristics of“steep rise and steep drop”;Class IV groundwater is generally located in the middle and lower part of the landslide flat,shallow groundwater depth,and not affected by reservoir water,weak by rainfall,showing irregular“jitter”characteristics.(4)A RF-LSTM model based on local search method is used to predict groundwater,and a groundwater prediction study is carried out on the class I groundwater monitoring holes affected by rainfall and reservoir water at the same time.Local search method is an algorithm based on rainfall maximum point to search local groundwater minimum point,it can effectively decompose groundwater data and obtain trend groundwater affected by long-term factors(reservoir water level,rainfall in the previous month,rainfall in the first two months),and fluctuating groundwater affected by short-term factors(rainfall,rainfall in the previous day,rainfall in the first two days,rainfall in the first three days,rainfall in the first five days).The grey correlation method is used to verify the rationality of the selected independent variables,and the validity of the local search method is also verified.Lastly,the stochastic forest model and the long-short-term memory network model are selected to predict the fluctuation term,and the total groundwater level is obtained by adding them together,the prediction model has a high accuracy,MAE is 0.2507m.(5)Establishing the water level dispatching model of reservoir in the Three Gorges Reservoir Area from 2012 to 2020 and setting the D2 steep drop section of 0.29m/d,0.6m/d,0.9m/d,1.2m/d different reservoir water level drop conditions,combined with the 50-year rainfall in the slow-down and steep-down sections,using Geo-studio software to simulate the variation characteristics of groundwater seepage field and stability of landslide with different permeability coefficient(B landslide,C landslide of Dazhou II landslide),The following conclusions are obtained:(1)As the water level of the reservoir drops,the trend of landslide stability is similar to that of reservoir water level,The larger the reservoir water level of the D2 section,the faster the stability of the landslide decreases,and the increase of the daily decline leads to the increase of the minimum stability of the landslide;(2)The effect of rainfall on landslide stability is long and has superposition effect;(3)After the reservoir water drops to the lowest water level145m,with the groundwater lag discharge,the landslide stability gradually rises,for the same slope different reservoir water drop speed,the groundwater discharge speed is the same in the lag period,for different slope the same reservoir water drop speed,the slope with large permeability coefficient is relatively large;(4)Landslides with low permeability are more sensitive to reservoir water drop and rainfall.(6)Six influencing factors including cohesion of sliding zone soil(c),internal friction angle(?),maximum vertical expansion force(P_eymax),lateral expansion coefficient(?),groundwater level(h),and reservoir water level(V)are selected,using orthogonal design method to analyze the sensitivity of Dazhou II landslide C landslide,the results of difference analysis and variance analysis are as follows:(1)The six factors affecting the stability of Dazhou II landslide C landslide from largest to smallest are:reservoir water level(V)>internal friction angle(?)>groundwater level(h)>cohesion(c)>lateral expansion coefficient(?)>maximum vertical expansion force(P_eymax),the reservoir water level(V)and internal friction angle(?)have significant influence on landslide stability;(2)The sensitivity of two important indexes of the expansion performance of the soil in the zone,lateral expansion force coefficient(?)>the maximum vertical expansion force(P_eymax),it shows that in the expansion performance experiment of expansive sliding zone soil,the determination of lateral expansion force coefficient(?)should be emphasized.