Stability Analysis Of Colluvial Landslide In Relation To Groundwater Response In Wanzhou District, Three Gorges Reservoir

China is one of the countries most seriously affected by landslides in the world.Mountainous regions and Plateaus cover 69%of the whole area of China,resulting frequent geohazards such as collapses,landslides and debris flows.Geohazards in China have the characteristics of 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.It 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 the deformation mechanism and characteristics of the landslide in the Three Gorges Reservoir.In this paper,the colluvial landslides in Wanzhou District of the Three Gorges Reservoir were studied.The landslide development regularities of Wanzhou District were analyzed based on the results of field investigation.Combined with the monitoring data of groundwater,rainfall and reservoir water level?RWL?of Dazhou landslide and Madiwan landslide,the characterization of groundwater response to rainfall and reservoir were studied using statistical method.Through comprehensive analysis,a reservoir scheduling model with high drawdown rate before non-flood season at low RWL was proposed.Then the Pearson type III distribution model was used to calculate the extreme rainfall of Wanzhou District.Last,the stability and sensitivity of Maliulin landslide were studied under the combined effect of RWL fluctuation and rainfall.Through the research above,the main conclusions are as follows:?1?The landslide development regularities of Wanzhou District were studied using statistical method.The elevation of the toe of surface rupture below 175m accounts for21.5%.The elevation of the head is consistent with the five terraces in Wanzhou District.The length and width of landslides are synchronization developed,and their Spearman rank correlation is 0.9.The slope angles are mainly concentrated at the range of 13°²8°,especially the range of 13°¹8°.Majority of plane form of landslides are dustpan shaped and tongue shaped,and the sections are mainly convex,straight and straight shaped.?2?Based on statistical method,the lag time of groundwater over rainfall are studied and forecasted.The main conclusions are as follows:?1?With the increasing of antecedent rainfall,the accumulated rainfall required to induce groundwater response decrease gradually.The combined total rainfall was obtained after summing up the two kinds of rainfall,and it is consistent with binomial distribution.The groundwater response can be induced by no more than 55 mm of the combined total rainfall in Dazhou landslide.?2?The correlation between peak rainfall time and lag time is significant.In most cases,groundwater begins to respond after peak rainfall occurred in a short time,and the lag time increases with the increasing of peak rainfall time.As the increasing of peak rainfall,the ratio between peak rainfall time and lag time increased.?3?When the hourly rainfall intensity it is greater than 10 mm,it is the main factor of lag time and the mean value of lag time is 1.15 h.There is no significant regularity when the hourly rainfall intensity is less than 10 mm.?4?The groundwater depth and the lag time are in the lower triangular distribution.With the increasing of groundwater depth,the peak value of the lag time increased.Overall,groundwater depth and the lag time are in weak binomial relation.?5?As the increasing of the distance from reservoir,the groundwater response is more sensitive to rainfall with shorter lag time,and the duration of the peak groundwater level and groundwater recession is short.When it close to reservoir,the groundwater response is less sensitive to rainfall with long lag time,and the duration of the peak groundwater level and groundwater recession is long.?6?A multiple regression forecasting model was established to predict the lag time.The combined total rainfall,hourly rainfall intensity,the ratio of peak rainfall time and lag time were chosen as independent variables.As a whole,the predicted values are in good agreement with the observation values.The amplitude regularity of groundwater response to rainfall in Dazhou landslide was studied and the main conclusions are as follows:?1?The groundwater response amplitude showed obvious characteristics of location.With the recharge from rainfall infiltration and upper groundwater,the groundwater response amplitude at toe is greater than that at slope waist.?2?The correlation between groundwater response amplitude and cumulative rainfall is significant at slope toe than that at slope waist.With the increasing of accumulated rainfall,the groundwater response amplitude at slope toe increases greatly while the groundwater response at slope waist shows no obvious correlation with rainfall.In this paper,a Characteristic Point Method was proposed to study the lag time of groundwater over reservoir fluctuation.Taken Madiwan landslide as a case,four groups of characteristic points were chosen to study the lag time.The groundwater lag time of Madiwan landslide over reservoir fluctuation is about 53h.?3?In this paper,a reservoir scheduling model with high drawdown rate was proposed before non-flood season at low RWL and the extreme rainfall was calculated.On this basis,the characteristics of seepage field and stability of Maliulin landslide were studied under the combined effect of RWL fluctuation and rainfall.The main conclusions are as follows:?1?On the whole,the reservoir scheduling curve and the variation trend of slope stability are highly similar,which means that the RWL fluctuation is the main factor affecting landslide stability.The groundwater level at the back of landslide rise easily and fall hard under the influence of rainfall,which showed a superimposed effect on landslide stability.?2?The landslide stability under constant reservoir drawdown rate is higher than that under variable reservoir drawdown rate.When the reservoir drawdown rate is constant,the landslide has sufficient free-draining surface and time to discharge groundwater.When the reservoir drawdown rate is variable,there is a package of either free-draining surface or time to discharge groundwater,resulting a lower stability.?3?The reservoir scheduling model proposed in this paper has little influence on landslide stability.When the reservoir drawdown rate is 1.5 m/d in this model,the changed ratio of factor of safety only increased for 0.79%.?4?Sensitivity analysis of Maliulin landslide stability was carried out based on orthogonal design method.Six sensitive factors,including friction angle,cohesive strength,hydraulic conductivity,immersion ratio,reservoir drawdown rate and rainfall were chosen as sensitive factors to analyze the sensitivity.The main conclusions are as follows:?1?The sensitivity factors affecting Maliulin landslide stability sorted in descending order are friction angle,hydraulic conductivity,cohesive strength,immersion ratio,reservoir drawdown rate and rainfall.The sensitivity factors affecting the changed ratio of landslide stability sorted in descending order are hydraulic conductivity,cohesive strength,immersion ratio,friction angle,reservoir drawdown rate and rainfall.?2?The Maliulin landslide is mainly influenced by internal factors,and the RWL fluctuation and rainfall have limited impact on it.Actually,the external factors often weaken the internal factors which resulting a greater impact on landslide stability.Among internal factors,the friction angle and cohesion have great influence on initial stability of landslide.The permeability coefficient has the greatest influence on the changed ratio of factors of safety locally,which is 10 times greater than other factors.For Maliulin landslide,the decrease of immersion ratio has limited positive impact on landslide stability.?3?For reservoir drawdown conditions,a critical value K₀ of hydraulic conductivity is existed that the reservoir has no influence on landslide stability.When the hydraulic conductivity is greater than the critical value,the landslide stability increases with the increasing of hydraulic conductivity.When hydraulic conductivity is less than critical value,the landslide stability decreases with the decreasing of hydraulic conductivity.In the view of geological conditions,the pipe flow system developed at the toe of reservoir landslides enhance the permeability.If the hydraulic conductivity continues to increase and exceed the critical value,the landslide stability will increase during reservoir drawdown conditions.This law can be used to explain the phenomenon that reservoir landslide deformation is slowing down after the reservoir operation normally for ages.