Study On Hydrodynamic Weakening Effect And Stability Evolution Law Of Reservoir Landslide

As the main landslide geological disaster in the Three Gorges Reservoir area,the accumulation landslide of the reservoir type accounts for 64% of the total number of landslides in this area,and most of the landslides are caused by the change of the reservoir water level.Since the impoundment of the reservoir in 2003,the water level has increased from 135 m to175m,resulting in some or all of the landslides being submerged by the reservoir water.The periodic fluctuation of the reservoir water level causes the change of the seepage field of the slope and the weakening of the soil parameters,which has a great influence on the stability of the slope.In addition,the rate,amplitude and time step of the reservoir water fluctuation are all potential factors that cause the landslide.Therefore,based on the previous dry-wet cycle test results of soil parameters,this paper summarizes the weakening law of different soil parameters.According to the monitoring data of Bazimen landslide,combined with the Seep/W module in Geo-Studio software,the seepage field under different permeability coefficients and different reservoir water lifting rates is simulated.The Sigma/W and Slope/W modules were used to simulate the displacement field and stability of different reservoir water rise and fall rates and different time steps under normal and weakening conditions.The orthogonal test was used to analyze the sensitivity of the shear strength parameters of the slope.Because there are many uncertain factors(rainstorm,earthquake,etc.)in the evaluation of slope stability,Monte Carlo method is used to analyze the reliability of Bazimen landslide.The response surface method agent model with high computational efficiency is used and calibrated.The variation law of slope instability probability under different reservoir water falling rates and different time steps under normal and weakening conditions is calculated and analyzed by the agent model.The slope instability probability and stability coefficient are compared and analyzed.Finally,the cumulative displacement of the actual monitoring data of Bazimen landslide from 2008 to 2011 is compared and verified.Corresponding relationship with stability coefficient and instability probability.The main research results are as follows :(1)Through the dispatching scheme of reservoir water in the Three Gorges area,the variation range of permeability coefficient,reservoir water lifting rate and reservoir water level is determined,and the specific deformation characteristics of Bazimen landslide are summarized.It is found that in the late stage of water level decline from May to July every year,the landslide will appear surface cracks,local collapse and cracking of roads.According to the variation of shear strength parameters of different soils under different dry-wet cycles,it is found that the shear strength parameters of different types of soils decrease with the increase of dry-wet cycles.The deterioration of cohesion is much larger than that of internal friction angle,and the deterioration of cohesion and internal friction angle is the most obvious in the early stage of dry-wet cycle.Through the Duncan-Chang model,the shear strength parameters are correlated with the deformation modulus and Poisson’s ratio,and the range of deformation parameters in different weakening stages is determined.(2)By using Geo-Studio software,the variation law of seepage field of Bazimen landslide under different permeability coefficients and reservoir water fluctuation rates is obtained.It is found that during the rise of reservoir water level,the infiltration line of the slope is in a concave curve shape as a whole,and the seepage pressure pointing to the slope is increasing.During the decline of reservoir water level,the saturation line of the slope is concave curve,and the seepage pressure pointing to the outside of the slope is increasing.Through the analysis of the seepage field under different k and v conditions,it is found that the weakening range of the hydro-fluctuation belt is negatively correlated with the rise and fall rate of the reservoir water,and positively correlated with k/v.The variation law of displacement field of Bazimen landslide under different reservoir water rise and fall rates under normal and weakening conditions is simulated.It is found that the rise and fall of reservoir water level will cause displacement of the slope under normal and weakening conditions.The displacement at the slope surface is greater than that in the slope.The cumulative displacement of the slope is inversely proportional to the increase of reservoir water,and it is proportional to the decrease of reservoir water.With the weakening of slope parameters,the displacement also increases,and the increase of slope displacement is the most obvious in the early stage of parameter weakening.The variation law of slope stability under different reservoir water fluctuation rates under normal and weakening conditions of Bazimen landslide is simulated.It is found that the stability coefficient of slope increases as a whole during the rising stage of water level,and the stability coefficient is proportional to the rising speed of reservoir water.In the stage of water level reduction,the stability coefficient of the slope shows a trend of large decrease and small increase.The stability coefficient is inversely proportional to the decrease rate and recovery rate of the reservoir water.With the weakening of the slope parameters,the stability coefficient decreases continuously,and the weakening is greatly reduced in the early stage to the weakening in the later stage.The decrease rate slows down and tends to be relatively stable.The weakening of soil parameters caused by the cyclic fluctuation of reservoir water will have an adverse effect on the stability of the slope.(3)The sensitivity of shear strength parameters of slope is determined by orthogonal test and range analysis.It is found that the sensitivity of internal friction angle is greater than that of cohesion.The sensitivity of internal friction angle changes steadily with time,while the sensitivity of cohesion changes greatly with time.The Monte Carlo method is used to analyze the reliability of the Bazimen landslide,and the surrogate model of the response surface method is constructed and verified.The surrogate model is used to calculate the change law of the instability probability of the slope under normal and weakening conditions under different reservoir water drop rates.It is found that the greater the reservoir water drop rate,the greater the instability probability corresponding to the most dangerous step 155 m of the slope,and the smaller the stability coefficient.In the process of water level rising,the instability probability of landslide will decrease with the increase of slope water level,and the stability coefficient will increase.With the increase of the number of weakening times,the probability of slope instability continues to increase,and the stability coefficient continues to decrease,but there is a trend of slowly slowing down.In the early stage of parameter weakening,the probability of slope instability increases most obviously.In addition,the decline rate of reservoir water is also an important factor affecting the stability of slope.Compared with the influence of reservoir water decline rate on the stability of reservoir bank slope,the weakening of soil parameter strength has a relatively greater impact on the stability of reservoir bank slope.(4)According to the case analysis of Bazimen landslide,it is found that the cumulative displacement is negatively correlated with the stability coefficient,and the cumulative displacement is positively correlated with the instability probability.The comparison verifies that the actual monitoring displacement is basically consistent with the stability coefficient and the instability probability.