Study On Rainfall Threshold And Early Warning And Prediction Of Wangjiapo Landslide In Three Gorges Reservoir Area

The key to the early warning and prediction of rainfall-type landslides is to determine the rainfall threshold which can induce landslides to deform or failure.At present,there are two main research directions on the rainfall threshold:one is empirical rainfall threshold based on mathematical statistics,the other is landslide numerical simulation or indoor model test combined with infiltration mechanism analysis.Both methods have achieved fruitful research results,and play an important role in landslide early warning and prediction research.This paper takes Wangjiapo landslide in the Three Gorges Reservoir area as the research object.Based on the GPS monitoring data analysis,the deformation threshold which induces the cumulative displacement of the landslide to take a step is analyzed.Based on the numerical analysis,the damage threshold which induces the landslide to lose stability and slide is determined.On this basis,based on the stage of landslide development and evolution,a four-stage early warning and prediction model of landslide is established and quantified.research results can be revealed as follows:1)Wangiiapo landslide is a typical rainfall induced landslide,when the rainfall intensity is high,the cumulative displacement curves of landslide steps,and when the rainfall intensity is low,the cumulative displacement curves of landslide tend to be flat,therefore,the cumulative displacement curves present a step-like deformation characteristic as a whole.And the beginning and end time of deformation lag behind the beginning and end time of rainfall,which is the process of increasing and dissipating pore water pressure.2)Wangiiapo landslide experienced two stages movement.Firstly,due to the uplift of crust and channel degradation,the free surface of the front landslide is enlarged,Wangjiapo landslide finally occurred cutting-bed and accumulated on the alluvial-diluvial layer of the river as a result of the cutting of dominant joints.Secondly,owing to the recent human engineering activities such as excavation of the front edge of landslide,loading of soil in the middle and rear part,and tunnel blasting construction,as well as the influence of heavy rainfall in flood season,the landslide of Wangjiapo revived again.3)In the aspect of deformation threshold analysis,firstly,based on the slope single change-point analysis,the similar failure points of Wangiiapo landslide's ground accumulated displacement are calculated.Taking the antecedent rainfall into consideration,different observation periods are selected to analyse the antecedent rainfall and rainfall slope of each similar failure point,and the slope threshold is eventually obtained through weighted average.The results reveal that average cumulative rainfall slope increases firstly and then decreases with the increasement of observation period,meanwhile,in the same observation period,the slope of accumulated rainfall is negative with the time interval.This paper chooses four observation periods n=2,5,10,15,calculating the slope threshold of each observation period under the condition of time interval d=1,and the observation period n=10,d=1 is choosed to verify and analyze.If the rainfall slope is greater than the slope threshold,the landslide is likely to have a step displacement.The results show that the prediction effect is good and the accuracy is high.4)In the aspect of failure threshold analysis,the finite element numerical analysis model of Wangjiapo landslide is established under the assumption of normal distribution of physical and mechanical parameter of rock and soil,and the stability of landslide is calculated under different rainfall conditions.Taking the complexity of actual rainfall conditions,the concept of global average is adopted to analyze the time D needed for landslide under different rainfall intensity for shore-term rainfall.For long-term rainfall,the time D needed for landslide sliding is obtained by decomposing the long-term rainfall process into the antecedent rainfall A+the current rainfall I.In order to determine the effective time of early rainfall for current rainfall,a rainfall process is introduced.By analyzing the time of landslide cumulative displacement falling during an independent rainfall process,the efrective time of early rainfall T=10 days is determined.Finally,according to the conditions of different early rainfall,the time D needed for landslide failure under different rainfall intensity is analyzed,and A-I-D threshold surface for landslide failure is established.5)In the research of early warning and prediction,according to the deficiencies of three-level early warning and prediction established by deformation threshold and failure threshold,and based on the stage division of landslide development and evolution under rainfall conditions,a four-level early warning and prediction model of landslide is established through average daily rainfall intensity and rainfall duration:?Blue warning of grade ?:I<16.53(0<D ?2),I<10.69(2<D? 5),I<10.95(5<D?10),I<5.98(10<D?15);?Yellow warning of grade ?:exceeding Class I blue early warning,but satisfying:I<157.192*D-0584;?Orange warning of grade ?:beyond yellow warning of grade ?,if it is short-term rainfall:I<181.066*D-0.703,if it is long-term rainfall:I*D<500(D?10);?Red warning of grade ?:beyond orange warning of grade ?,if it is short-term rainfall:I>181.066*D-0.703?703,if it is long-term rainfall:I*D>500(D?10).Taking the rainfall-type landslide as an example,this paper focuses on the analysis of the deformation rainfall threshold which induces the cumulative displacement step of landslide,abbreviated as the deformation threshold,and the destructive rainfall threshold which induces the instability and sliding of landslide,abbreviated as the destructive threshold.According to the division of the development and evolution of landslide,a four-stage early warning and prediction model of landslide is established.Its innovation lies in the unification and quantification of the early warning and prediction of landslide.Based on this index,advance geological prediction of landslide displacement step and instability sliding can be carried out,It has certain engineering significance for the early warning and prediction of rainfall-type landslides.