| Super-large cataclastic bedding rock landslides are widely distributed in the Three Gorges Reservoir area.Until now,such landslides have been continuously displaced and deformed under the action of external forces such as rainfall and reservoir water level fluctuation,posing a great threat to the residents on the landslides,the Yangtze River channel and even the Three Gorges Dam.This paper expounds the geological and geomorphic conditions,deformation and failure modes of Tanjiahe landslide,Qianjiangping landslide and Jiuxianping landslide in the Three Gorges Reservoir area,and summarizes the common characteristics of super-large cataclastic bedding rock landslides.On this basis,with the help of GPS monitoring data,hydrometeorological data and investigation report of Tanjiahe landslide,the characteristics and mechanism of landslide deformation under the dynamic action of rainfall,reservoir water and groundwater are analyzed by means of multi-factor comprehensive process analysis,qualitative and quantitative combination and numerical simulation analysis,and the characteristics of groundwater charged by reservoir water and rainfall are studied.Point out the key warning index and criterion of landslide deformation.Finally,a two-dimensional UDEC numerical calculation model for Tanjiahe landslide was established to simulate the landslide deformation under the coupling of rainfall and reservoir water,and to analyze the stress characteristics,seepage characteristics and displacement characteristics of the landslide.The main conclusions and achievements are as follows:(1)The cumulative displacement of the Tanjiahe landslide presents a steady-step increasing trend,and the pushing slip section is constantly squeezed and the blocking section has the peristaltic deformation.From 2008 to 2014,the landslide deformation response characteristics of "weight reduction and soaking softening effect" were presented.From 2015 to 2020,the composite landslide deformation response characteristics of "weight reduction effect of floating support weakened and hydrodynamic pressure effect strengthened" were presented,and the reservoir water level rise induced deformation,requiring the compensation of continuous heavy rainfall.The first storage of reservoir water at the end of 2008 promoted the landslide deformation.After 2015,part of the rock and soil mass at the leading edge was soaked by reservoir water for a long time,and the soaking softening effect of reservoir water gradually weakened.Under the action of gravity,the sliding-promoting section slides down the compression-resisting section,and the original open structural plane in the cataclastic rock mass gradually closes,improving the continuity and integrity of rock and soil mass and thus reducing its permeability.On the one hand,the increase of water head difference increases the seepage pressure of groundwater.On the other hand,each rock block is closely together to the greatest extent,which will weaken the floating support force of reservoir water on the front rock mass.(2)The response law of landslide deformation under the action of rainfall and reservoir water in 2016-2018:(1)When the reservoir water level drops from 175 m to 158 m at a low speed,the landslide deformation rate is relatively low;When the water level of the reservoir drops from 158 m to 145 m,the higher the water level fall rate and rainfall intensity,the more obvious the deformation of the landslide.(2)The rapid subsidence of reservoir water level has a certain hysteresis effect on the landslide deformation.In the operation stage of 145 m low water level,the landslide body keeps "inertia" sliding deformation.(3)When the water level of the reservoir rises from 145 m to 175 m,there is no obvious deformation of the landslide,and the rise of the water level of the reservoir can inhibit the deformation of the landslide.(3)Characteristics of groundwater recharge by rainfall and reservoir water infiltration:the increase of rainfall intensity is conducive to the rise of QSK2 groundwater level,which will change from a downward trend to an upward trend;After the rainfall intensity weakened,the groundwater level changed from rising to falling.When a rainfall of more than 20 mm acts on the landslide,the rainfall in the infiltration vadicle zone drops under the action of gravity and pushes the older rain under it,and raises the groundwater level of QSK1 and QSK2 with a lag and delay of about 1 day.The decrease rate of groundwater level in QSK2 in the middle of the landslide is lower than that in front QSK1,and the downward seepage of groundwater in QSK2 is slow,which is related to the compression of the front cataclastic rock mass in the landslide and the reduction of its permeability and the distribution of water bodies.During the slow rise and fall of reservoir water level,the groundwater level of QSK1 has no obvious delay.When the rise and fall rate of the reservoir water level further increases,the groundwater level of QSK1 lags behind the fluctuation of the reservoir water level.In addition,reservoir water,regional rainfall,and groundwater in the middle and back of slope all infiltrate and recharge the front groundwater,which will lead to accelerated rise of the front groundwater level.(4)The hydraulic action of groundwater fluctuation will stimulate the landslide deformation,and groundwater is an important factor affecting the stability of landslide.If the heavy rainfall did not cause the displacement and deformation of ZGX289,the landslide displacement and deformation would be induced by the rise of QSK2 groundwater level to the critical value(≥242.4 m)2 days after the rainfall infiltration recharge slope,and the landslide deformation trend would be significantly weakened after the local groundwater level dropped from the peak value(about 245 m)to the critical value(≥241 m).At the same time,during the fluctuation of QSK2 groundwater table,the rise height of QSK2 groundwater table(≥2.85 m),the peak value of groundwater table(≥244.01 m),the higher the value of relevant parameters of groundwater table,the more severe the landslide deformation tends to be;otherwise,the lower the groundwater table,the more stable the landslide body usually is.(5)The numerical simulation results show that when the landslide reaches the stress equilibrium state under the action of gravity,the reservoir water level fluctuation and concentrated rainfall will aggravate the deformation of the landslide.During the slow decline of reservoir water level,the displacement and deformation of the landslide body are weak,and the rock mass is mainly affected by gravity and downward seepage of groundwater,resulting in the direction of motion velocity pointing vertically down or outward slope.During the period of rapid subsidence and rise of reservoir water level,under the action of concentrated rainfall,the rock mass in the sliding section is promoted to produce the same movement velocity direction as that of bedding sliding or pointing out the slope,and the shallow and deep rock mass in the blocking section is extruded to produce rebound deformation.When there is no concentrated rainfall during this period,the movement velocity of the rock mass in the sliding section points to the inner side of the slope,and the rise of reservoir water level inhibits the landslide deformation.(6)According to the numerical simulation results,the evolution mechanism of landslide deformation is deduced:(1)Stress concentration at the junction of steep slope;(2)The slope slides along the soft sliding surface,and the rear edge cracks;(3)Shear failure occurs on the front slope and the final landslide becomes unstable.The shear failure of landslide body depends on the anti-sliding ability of the landslide front arresting section.The current edge slope body has high stability and the front sliding surface is not connected.In this paper,it is numerically simulated that the resistance section of landslide may shear out at the junction of steep and gentle under the sliding compression of the promoting section for a long time,and when external factors increase the sliding force of the promoting section,this deformation and failure process will be accelerated. |
