Research On The Instability Process Of Tower Rocks Under The Fluctuation Levels Of Water Reservoirs

The tower rock mass refers to large rock mass with length width ratio between 1 to 3 and height diameter ratio greater than 3,which is in tower columnar shape or similar to tower columnar shape.The occurrence of rock strata is horizontal or nearly horizontal,and its large height diameter ratio morphological characteristics are prone to instability,resulting in rockfall,collapse,landslide and other geological disasters.In the Three Gorges Reservoir area,dangerous rock mass is widely distributed,especially after the normal operation of the Three Gorges Reservoir area,the reservoir water level fluctuates periodically between 145 m and175m,forming a reservoir water fluctuation zone with a bank slope as high as 30 m.Periodic fluctuation of reservoir water level causes the deterioration of rock mass in the water-leveling zone,and breeds and produces a large number of new tower pillar like dangerous rock mass,such as the JDRM(jianchuandong dangerous rock mass),the rock mass in the water fluctuation zone guanmuling dangerous rock mass and longmenzhai dangerous rock mass.Under the influence of reservoir water fluctuation,the tower pillar like dangerous rock mass is easy to cause large-scale rockfall,collapse,landslide and other geological disasters,and then generate surge secondary disasters to form a disaster chain,which will pose a great threat to the life safety of river ships,mountain towns and residents,and often lead to serious casualties and huge economic losses.It is a typical hidden danger of geological disasters in the Three Gorges Reservoir area,It is also one of the hidden dangers of geological disasters that the safe operation of the Three Gorges Reservoir area focuses on.Based on the field geological survey and laboratory analysis,the deterioration characteristics of the tower pillar dangerous rock mass under reservoir water level fluctuation are systematically studied by using engineering geological analysis,large-scale physical model test and numerical simulation,This paper analyzes the deformation characteristics and influencing factors of the tower column dangerous rock mass under the periodic reservoir water fluctuation,explores the instability mechanism of the tower column dangerous rock mass under the periodic reservoir water fluctuation,and preliminarily puts forward the instability mode of the tower column dangerous rock mass under the periodic reservoir water fluctuation.The research results can provide scientific basis for the prevention and control of the same type of geological disasters and the safe operation of the reservoir area,It is of great practical significance to the disaster prevention and reduction in the reservoir area and to ensure the safety of people's lives and property.The main research contents and conclusions of this paper are as follows:(1)Based on field geological survey,long-term in-situ monitoring data collection and indoor analysis,under the condition of reservoir water level fluctuation,there are many fractures and the opening degree of the dangerous rock body increases continuously at the back edge,especially the deformation of dangerous rock body along the downstream of Yangtze River increases year by year,and the cumulative horizontal displacement from 2012 to 2018 reaches 31.27 mm,and the cumulative settlement displacement reaches 31mm;The ground stress of the base of dangerous rock body is also increasing,and it gradually increases to 8MPa from 2013 to 2018.The base rock mass is constantly deteriorated and damaged,and the fracture and crushing zone is gradually formed.Through data analysis,the deformation of the rock mass is closely related to the fluctuation of reservoir water level,and the fluctuation of reservoir water level accelerates the deformation of the dangerous rock mass.(2)Taking JDRM as the prototype,based on the similarity theory,the optimal proportion of structural plane similar materials for geomechanical model physical test of jianchuandong dangerous rock mass is determined.Using barite powder,quartz sand,gypsum and water as raw materials to prepare similar materials of structural plane,the experimental study on the proportion of similar materials was carried out based on orthogonal experimental design method.Taking bone cement ratio(aggregate / binder),water cement ratio(water / gypsum)and heavy bone ratio(barite / aggregate)as three control factors,the triaxial compression test was carried out.The mechanical indexes of similar materials with different proportions were obtained in II.The optimal proportions of similar materials in structural plane were 1.68,0.6and 20.8,and the optimal proportions of similar materials in base deteriorated rock mass were1.7,1.6 and 20.9.(3)Taking JDRM as an example,a large-scale physical and mechanical model of dangerous rock body is constructed based on the similar structure,similar materials and different rock deterioration rate.The results of physical simulation experiments show that the collapse failure of the base will occur when the proportion of the base deterioration area is 100%under the condition of water level fluctuation;When the proportion of base deterioration area is 50%,the base deterioration of dangerous rock body forms a cavity,which has the tendency of collapse failure;When the proportion of base deterioration area is 33%,the whole situation of dangerous rock body is stable.When the proportion of the deteriorated area is less than 33%,the dangerous rock body is stable.If the proportion of the deteriorated area is less than 33%,the dangerous rock body is likely to lose stability and failure if it is more than 33%.(4)Taking JDRM as an example,the numerical simulation of the dangerous rock mass under periodic reservoir water level fluctuation is carried out based on the discrete element method.The numerical simulation results show that: with the increase of the period of reservoir water level fluctuation,the base is gradually destroyed and deformed under the pressure of overlying rock mass.The deformation of the dangerous rock mass can be divided into two stages: the initial stage and the accelerated deformation stage.In the initial stage,the deformation is mainly manifested as the gradual destruction of the base rock mass and the slow subsidence of the dangerous rock mass;In the accelerated deformation stage,the main deformation is that the plastic failure zone develops upward from the base,the displacement deformation of the dangerous rock body increases,the local sliding of the base occurs,and the overall subsidence deformation is the main deformation.(5)Through the comparison of physical model test,numerical analysis and survey monitoring data,it is clarified that the main influencing factors of typical tower column dangerous rock mass are reservoir water level fluctuation and base rock mass deterioration.The instability process is divided into three stages: start-up,base accelerated failure and disintegration failure.The deformation of JDRM is triggered by its own gravity and reservoir water level fluctuation,in which the reservoir water level fluctuation accelerates the deterioration of the base rock mass,resulting in the collapse of the base rock mass,leading to the instability of the dangerous rock mass,and gradual deformation and disintegration,which eventually leads to the collapse disaster and the formation of debris flow sliding into the water.(6)Through large-scale physical model test and numerical simulation analysis,the instability modes of tower column dangerous rock mass can be summarized into two types:base fragmentation collapse sliding type and base fragmentation collapse toppling type.According to the development characteristics of JDRM and the deterioration status of the base rock mass,the instability mode can be attributed to the base crushing sliding type.