Evaluation Method Of Post-earthquake Stability Of Loess Slopes

Under the effect of earthquake(main earthquake),landslides will be induced.At the same time,the interior of a large number of slopes will be damaged and in an unstable state,resulting in sliding under the effect of aftershocks or succedent rainfall.Compared with the coseismic landslides,post-earthquake landslides will also cause a lot of casualties and property losses,but the relevant research is less carried out.Therefore,the post-earthquake stability of slope needs to be solved urgently.In this paper,buzi landslide in Weixin township which damaged by Zhangxian-Minxian Ms6.6mainshock and triggered by sccedent aftershock is studied.Through field investigation,UAV mapping,High-density electrical exploration,Microtremor Measurements,indoor dynamic-static triaxial test and numerical simulation,the morphological and predominant frequency characteristics of landslide,the variation law of shear strength parameters of soil effect by the time history of Minxian mainshock ground motion with intensity of VII,VIII,IX and X,and the dynamic response law of slope under the effect of main earthquake alone or aftershock sequence or succedent rainfall are analyzed.According to the above research results,based on the limit equilibrium principle of rigid body,the stability evaluation method of loess slope under the effect of aftershock,succedent rainfall and intereffect between aftershock and rainfall after mainshock is given.The main work and achievements are as follows:(1)Through UAV mapping and high-density electrical survey,themorphological characteristics of buzi landslide are obtained.The bedrock of the main scarp is exposed.Buzi landslide is a loess-mudstone interface landslide.The total volume of landslide is about 531000 m~3,which belongs to medium landslide.There is a small landslide caused by rainfall after the earthquake.The results of H/V method of microtremor measurements show that the maximum predominant frequency of the main scarp is 4.3Hz,which is close to that of the time history of Minxian mainshock ground motion(about 4Hz).(2)Through the indoor dynamic-static triaxial test method,the shear strength of undisturbed soil under the effect of the time history of Minxian mainshock ground motion with intensity of VII,VIII,IX,X is obtained.The experimental results show that the cohesion decreases rapidly with the increase of the disturbance intensity of seismic wave,while the internal friction angle increases as a whole.The disturbance of ground motion will reduce the shear strength of soil,mainly the cohesion of loess.(3)The FLAC 3D 6.0 finite difference software is used to analyze the dynamic response of the model slope under the effect of the mainshock,the sequence of the main-after shock and the rainfall after the earthquake.(1)The numerical simulation results of the main-after shock sequence show that the amplification coefficient of the acceleration under the effect of the main-after shock sequence is about 1.01~1.25 times of that under the effect of the mainshock alone.The larger displacement area is distributed in the loess area,and the maximum permanent displacement under the main-after shock sequence is about 1.56-1.66 times of that under the single effect of the mainshock in the same intensity.When the mainshock intensity is VII and VIII,the plastic area is mainly distributed in the loess area;When the mainshock intensity is IX and X,the mudstone area at the interface produces the tensile plastic area and develops rapidly and nearly through,which has been completely through during the aftershock effect.The maximum increment of shear strain appears at the interface of loess and mudstone,and the value under the effect of main-after shock sequence is about 1.84~2.63 times of that under the single effect of the mainshock in the same intensity.(2)The results of rainfall after the earthquake numerical simulation show that the distribution of permanent displacement,plastic zone and maximum shear strain increment of the model slope are similar to that of the mainshock acting alone or main-after shock sequence.The maximum permanent displacement under the intereffect of mainshock and rainfall is about 1.0~3.8 times of that under the single effect of mainshock,and the maximum permanent displacement under the intereffect of main-after shock sequence and rainfall is about 1.1~2.8 times of that under the effect of the main-after shock in the same intensity.(4)Based on the limit equilibrium principle of rigid body,the calculation method of safety factor for post earthquake stability evaluation of slope is given.Taking buzi slope as an example,the calculation results show that:the safety factor of slope under strong aftershocks decreases gradually with the increase of intensity;the safety factor under strong aftershocks and rainfall is less than the value under strong aftershocks or rainfall alone in the same intensity.